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Data-driven 3D super resolution imaging of turbulentjet flame using generative adversarial network

Wenjiang Xu, Weiyi Luo, Yu Wang, and Yancheng You

DOI: 10.1364/AO.392803 Received 17 Mar 2020; Accepted 29 May 2020; Posted 29 May 2020  View: PDF

Abstract: Three-dimensional (3D) computed tomography (CT) is becoming a well-established tool for turbulent combustiondiagnostics. However, the 3D CT technique suffers from contradictory demands of spatial resolution and domainsize. This work therefore reports a data-driven 3D super resolution approach to enhance the spatial resolution bytwo times along each spatial direction. The approach, named 3D super resolution generative adversarial network(3D-SR-GAN), builds a generator and a discriminator network to learn the topographic information and infer highresolution 3D turbulent flame structure with a given low resolution counterpart. This work uses numericallysimulated 3D turbulent jet flame structures as training data to update model parameters of the GAN network.Extensive performance evaluations are then conducted to show the superiority of the proposed 3D-SR-GANnetwork, compared with other direct interpolation methods. The results show that a convincing super resolution(SR) operation with the overall error of ~4% and the peak signal-to-noise ratio (PSNR) of 37 dB can be reachedwith an upscaling factor of two, representing eight times enhancement of the total voxel number. Moreover, thetrained network is capable of predicting the SR structure of the jet flame with a different Reynolds number withoutretraining the network parameters.

Mantis: An all-sky VNIR hyper-angularspectropolarimeter

Robert Foster, Deric Gray, Jeffrey Bowles, Dan Korwan, Ilya Slutsker, Mikhail Sorokin, Michael Roche, Adam Smith, and Larry Pezzaniti

DOI: 10.1364/AO.393822 Received 26 Mar 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: In this paper, we introduce and present first results from Mantis, a pushbroom type spectropolarimeter recently acquired by the Naval Research Laboratory and built by Polaris SensorTechnologies, Inc. The instrument is designed for high-spatial and spectral resolution polarimetricimaging of downwelling skylight. Linear Stokes vectors are acquired over the spectral rangeof 382-1017nm, with ≈0.64nm channel spacing, and each line scan consists of 2226 pixelsover a 72◦ field-of-view (0.75 milliradian instantaneous). Measurement of full sky dome isachieved through the use of a high precision motorized pan-tilt unit and systematic scanning. Anautomated Sun-shade allows for data collection in the main solar plane without saturation of thefocal plane. The uncertainty in the degree of linear polarization varies between 0.07% and 0.5%depending on incidence angle and wavelength. The total radiometric uncertainty is 2.07% to2.5%, of which 2% is absolute calibration error. Preliminary data analysis reveals the instrumenthas a large potential for remote sensing applications.

Constrained pupil balance compensation for specificlithographic illuminator settings

Dawei Rui and Huaijiang Yang

DOI: 10.1364/AO.394424 Received 07 Apr 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: Pupil energy balances have always been a significant performance for emersion lithography generally due to largeangle of incidence and offset imaging field. Those imbalances impact on exposure uniformity and decay patternresolution. To overcome such shortcomings, a study on pupil compensation is discussed in this manuscript. Acomputational method based on constrained optimization solution is proposed. By using a self-designed opticalmodel of zoomed system incorporating axicon lenses, a series of computations are developed and discussed. Also,the validity of our compensation method has been fully verified by the simulations under multiple illuminationsettings.

Dual-view integral imaging display using polarizer

Fei Wu, BaiChuan Zhao, ZESHENG LIU, and Guo-Jiao Lv

DOI: 10.1364/AO.394532 Received 08 Apr 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: We propose a dual-view integral imaging display using a polarizer. It consists of a display panel, a polarizer, amicro-lens array and two pairs of polarizer glasses. The polarizer is composed of the left and right sub-polarizerswhose polarization directions are orthogonal. Two kinds of elemental images are captured from different threedimensional scenes and located on the left and right half of the display panel, respectively. The lights emitting fromtwo kinds of elemental images are respectively polarized by the left and right sub-polarizers. The polarizationdirections of two pairs of polarizer glasses used in the left and right viewing zones are the same as those of the rightand left sub-polarizers, respectively. Two different three-dimensional images are simultaneously viewed in the leftand right viewing directions by wearing two pairs of polarizer glasses. A prototype of the proposed dual-viewintegral imaging display is developed, and the experimental results verifies the hypothesis.

Nonstationary angular distribution of optical fieldradiance from an isotropic source in sea water

Alexander Luchinin, Mikhail Kirillin, and Lev Dolin

DOI: 10.1364/AO.395561 Received 22 Apr 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: The spatial-angular and temporal characteristics of the radiance of the light field emitted by a nonstationary pointisotropic source in sea water are studied. Using the Monte Carlo method, we calculated the pulse transfer functionsand frequency responses of the angular radiance distributions at various distances from the source. Particularintegral characteristics of the angular radiance distributions are estimated. It is shown that with an increase in thedelay time, measured from the time of arrival of ballistic photons, the angular radiance distribution asymptoticallytends to isotropic. The frequency and phase responses of the alternating radiance component from a sourcemodulated by power at a high frequency are studied. It is shown that with an increase in the modulation frequency,the angular distribution of the alternating radiance component is concentrated close the direction to the source.

Design of infrared microspectrometers based onphase-modulated axilenses

Yuyao Chen, Wesley Britton, and Luca Dal Negro

DOI: 10.1364/AO.390610 Received 14 Feb 2020; Accepted 28 May 2020; Posted 28 May 2020  View: PDF

Abstract: We design and characterize a novel axilens-baseddiffractive optics platform that flexibly combines efficient point focusing and grating selectivity and is compatible with scalable top-down fabrication based on a4-level phase mask configuration. This is achieved using phase-modulated compact axilens devices that simultaneously focus incident radiation of selected wavelengths at predefined locations with larger focal depthscompared to traditional Fresnel lenses. In addition,the proposed devices are polarization insensitive andmaintain a large focusing efficiency over a broad spectral band. Specifically, here we discuss and characterize modulated axilens configurations designed for longwavelength infrared (LWIR) in the 6 µm–12 µm wavelength range and in the 4 µm–6 µm mid-wavelength infrared (MWIR) range. These devices are ideally suitedfor monolithic integration atop the substrate layers ofinfrared focal plane arrays (IR-FPAs) and for use ascompact microspectrometers. We systematically studytheir focusing efficiency, spectral response, and crosstalk ratio, and we demonstrate linear control of multiwavelength focusing on a single plane. Our designmethod leverages Rayleigh-Sommerfeld (RS) diffraction theory and is validated numerically using the Finite Element Method (FEM). Finally, we demonstratethe application of spatially modulated axilenses to therealization of compact, single-lens spectrometer. Byoptimizing our devices, we achieve a minimum distinguishable wavelength interval of ∆λ = 240nm atλc = 8µm and ∆λ = 165nm at λc = 5µm. The proposed devices add fundamental spectroscopic capabilities to compact imaging devices for a number of applications ranging from spectral sorting to LWIR andMWIR phase contrast imaging and detection.

Design of a panoramic annular lens with ultrawide angle and small blind area

kun zhang, Xing Zhong, Lei Zhang, and Tianqing Zhang

DOI: 10.1364/AO.395598 Received 21 Apr 2020; Accepted 28 May 2020; Posted 28 May 2020  View: PDF

Abstract: A panoramic annular lens (PAL) is composed of a panoramic annular head unit(PAHU) and a relay lens group. Due to the properties of annular imaging, single-channel PALsystems contain a blind area in the central field of view (FOV), which is difficult to reduce ina traditional optical system structure. Therefore, we propose a novel reflective PAHUstructure, using two mirrors to replace the traditional single-lens or multi-lens cementedPAHU, to achieve a smaller blind area and ultra-wide-angle imaging. In this paper, wepresent the design of an ultra-wide-angle PAL with a FOV of (22° ~ 120°) × 360° using ourproposed method. The resulting optical system achieves a blind area ratio of less than 3.67%.Compared with the traditional PAL system with the lowest blind area ratio, the blind arearatio of our system is reduced by half, demonstrating the feasibility of the proposed method.

Digital lensless holographic microscopy: numericalsimulation and reconstruction with ImageJ

Carlos Trujillo, Juan Pablo Piedrahita Quintero, and Jorge Garcia-Sucerquia

DOI: 10.1364/AO.395672 Received 23 Apr 2020; Accepted 28 May 2020; Posted 28 May 2020  View: PDF

Abstract: The description and validation of an ImageJ open source plugin to numerically simulate and reconstruct digitallensless holographic microscopy (DLHM) holograms are presented. Two modules compose the presented plugin;the simulation module implements a discrete version of the Rayleigh-Somerfield diffraction formula that allowsthe user to directly build a simulated hologram from a known phase and/or amplitude object by just introducingthe geometry parameters of the simulated setup. The reconstruction module of the plugin implements a discreteversion of the Kirchhoff-Helmholtz diffraction integral allowing the user to reconstruct DLHM holograms by justintroducing the parameters of the acquisition setup and the desired reconstruction distance. The plugin offers thetwo said modules within the robust environment provided by the complete set of built-in tools for imageprocessing available in ImageJ. While the simulation module has been validated through the evaluation of theforecasted lateral resolution of a DLHM setup in terms of the numerical aperture, the reconstruction module istested by means of reconstructing experimental DLHM holograms of biological samples.

Multi-focus Image Fusion using Fractal Dimension

Chinmaya Panigrahy, Ayan Seal, Nihar Mahato, Ondrej Krejcar, and ENRIQUE HERRERA-VIEDMA

DOI: 10.1364/AO.391234 Received 24 Feb 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: Multi-focus image fusion is defined as “the combination of a group of partially focused images of a samescene with the objective of producing a fully focused image”. Normally, transform domain based imagefusion methods preserve the textures and edges in the blend image but many are translation variant. Thetranslation invariant transforms produce same sized approximation and detail images, which are moreconvenient to device the fusion rules. In this work, a translation invariant multi-focus image fusion approach using À-trous wavelet transform is introduced, which uses fractal dimension as a clarity measurefor the approximation coefficients and Otsu’s threshold to fuse the detail coefficients. The subjective assessment of the proposed method is carried out using the fusion results of nine state-of-the-art methods.On the other hand, eight fusion quality metrics are considered for the objective assessment. The resultsof subjective and objective assessment on gray-scale and color multi-focus image pairs illustrate that theproposed method is competitive and even better than some of the existing methods.

Digital Holographic Interferometry Investigation ofLiquid Hydrocarbons Vapor Cloud Above a Circular Well

Digvijay Shukla and Pradipta Panigrahi

DOI: 10.1364/AO.394874 Received 14 Apr 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: The current study investigates evaporation of liquid hydrocarbons from a circular well cavity of small depth.Gravimetric analysis is performed to measure the evaporation rate and digital holographic interferometry isused for the measurement of normalized mole fraction profile inside the vapor cloud above the well. Phaseunwrapping has been implemented to obtain continuous phase distribution in the image plane. Fourier-Hankeltomographic inversion algorithm is implemented to obtain the refractive index change distribution inside theobject plane i.e. vapor cloud. Four liquid hydrocarbons i.e. pentane, hexane, cyclohexane, and heptane arestudied. The radius of circular well cavities is varied in the range of 1.5 mm to 12.5 mm. Results using quasisteady, diffusion-controlled model is compared with the experimental evaporation rate. Measured evaporationrates are higher than the diffusion-limited model calculation for all working fluids and well size. This differenceis attributed to natural convection occurring inside the vapor cloud due to the density difference between gasvapor mixture and the surrounding air. Holographic analysis confirms the presence of natural convection byrevealing the formation of flat disk-shaped vapor cloud above the well surface. Experimentally obtained vaporcloud shape is different from the hemispherical vapor cloud obtained using pure diffusion-limited evaporationmodel. The gradient of vapor mole fraction at the liquid-vapor interface is higher compared to that of thediffusion-limited model because of the additional transport mechanism due to natural convection. Transientanalysis of vapor cloud reveals time invariant overall shape of vapor cloud with reduction in average magnitudeof vapor concentration inside the vapor cloud during evaporation. The existing correlation for sessile dropletcannot successfully predict the evaporation rate from a liquid well. A new correlation is proposed forevaporation rate prediction which can predict the evaporation rate within root mean square (RMS) error of 5.6% for a broad size range of well cavity.

Optical design using image distortion for orthorectification

Tim Johnson, Jose Sasian, and Lacy Cook

DOI: 10.1364/AO.392302 Received 11 Mar 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: Imaging the curved earth from above typically results in a distorted image with reduced spatial resolution near the edgeof field. The effect is proportional to field of view (FOV) and altitude. It is similar to the negative (barrel) distortioncommon in fish-eye lenses, but is due here to the convex object and not the optical system. Although image processingmethods exist to partially correct for negative distortion, the reduced spatial resolution near the edge of field isunrecoverable. Instead this can be corrected for optically by inducing the right amount of positive distortion into theoptical design. The amount of positive distortion required to counter the negative distortion from the curved earth iscalculated as a function of FOV and altitude. An optical system with positive distortion is more challenging to designthan with negative distortion as this increases the FOV in image space and requires a larger focal plane array (FPA). Anoff-axis, all-reflective design with f/2.5, 2.5 inch effective focal length (EFL), 70° x 4° FOV, +13% distortion is shownwhich could be used as a push-broom satellite sensor in polar orbit with constant spatial resolution.

Geometry effects on luminescence solarconcentrator efficiency: analytical treatment

Ilya Sychugov

DOI: 10.1364/AO.393521 Received 25 Mar 2020; Accepted 26 May 2020; Posted 26 May 2020  View: PDF

Abstract: Luminescence solar concentrators act as semi-transparent photovoltaic cells of interest formodern urban environment. Here their efficiencies were analytically derived for differentregular unit shapes as simple, integral-free expressions. This allowed analysis of the shape andsize effect on the device performance. All regular shapes appear to have a similar efficiency asrevealed by optical path distribution formulas, despite differences in the perimeter length.Rectangles of the same area feature a higher efficiency due reduced average optical path. Itcomes with the cost of a longer perimeter, and the relation between these two is provided. Anexplicit formula for the critical size of an LSC unit, above which its inner part becomesinactive, has been obtained. For square geometry with matrix absorption coefficientߙ thiscritical size is ~ 2.7/ߙ ,corresponding to 70-90 cm for common polymer materials. Obtainedresults can be used for treatment of individual units as well as for the analysis of tiling forlarge areas.

Novel Fe-based metallic glass for improvedresolution, maskless phase-changephotolithography

Chao He, Zhe Yang, Chao Chen, Hao Tong, and Xiangshui Miao

DOI: 10.1364/AO.393682 Received 06 Apr 2020; Accepted 26 May 2020; Posted 26 May 2020  View: PDF

Abstract: Phase change lithography (PCL) is expected to become the next generation of mainstreamlithography for its high efficiency, non-toxicity and maskless. However, the low resolution ofcurrent PCL limits its practical application. We propose to use FeMoCrCBY metallic glassfilm as a high-resolution photoresist in phase-change lithography, which can greatly improvethe resolution than other metallic glass for its high heat capacity and crystallize temperature.Thermal distribution in FeMoCrCBY indicates that the feature size of phase transition regioncan controllably be made smaller. For example, the size of micro-patterns observed by SEM isaround 1.3 μm. The resolution can be further improved by reducing the laser spot size. Ourresults show that metallic glass film FeMoCrCBY is a promising photoresist forhigh-resolution phase-change lithography.

Fiber Directional Position Sensor byMultimode Interference Imaging and MachineLearning

Kai Sun, Zhenming Ding, and Ziyang Zhang

DOI: 10.1364/AO.394280 Received 08 Apr 2020; Accepted 26 May 2020; Posted 27 May 2020  View: PDF

Abstract: Fiber directional position sensor based on multimode interference and imageprocessing by machine learning is presented. Upon single-mode injection, light in multimodefiber generates a multi-ring-shaped interference pattern at the end facet, which is susceptibleto the amplitude and direction of the fiber distortions. The fiber is mounted on an automatictranslation stage with repeating movement in four directions. The images are captured froman infrared camera and fed to a machine-learning program to train, validate and test the fiberconditions. As a result, accuracy over 97% is achieved in recognizing fiber positions in thesefour directions, each with 10 classes totaling an 8-mm span. The number of images taken foreach class is merely 320. Detailed investigation reveals that the system can achieve over 60%accuracy in recognizing positions on a 5-μm resolution with a larger dataset, approaching thelimit of the chosen translation stage.

Photon quantification in Ho3+/Yb3+ co-doped opto-thermalsensitive fluotellurite glass phosphor

J. X. Yang, Desheng Li, Li Gang, Edwin Yue Bun Pun, and Lin Hai

DOI: 10.1364/AO.396393 Received 28 Apr 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: Multi-photon-excited thermal-correlated green and red up-conversion (UC) emissions havebeen quantified in Ho3+/Yb3+ co-doped fluotellurite (BZLFT) glass phosphor under the 978 nmlaser excitation. The temperature dependence of the fluorescence intensity ratio (FIR) originatedfrom UC emissions bands centered at 550 nm and 661 nm has been verified in the range of 303−543K. The net emission photon numbers of 5F4+5S2→5I8 and 5F5→5I8 transition emissions are up to40.08×1012 and 68.51×1012 cps in 0.4wt% Ho2O3-0.4wt% Yb2O3 co-doped BZLFT case under the6.95 W/mm2 laser power density. Furthermore, the quantum yield (QY) and luminous flux aredetermined to be dependent on pumping power. When the excitation power increases 874 mW, theQY values for 550 nm and 661 nm emissions are as high as 0.94×10−5 and 1.60×10−5. In addition,the high photon producing efficiency is conducive to ensuring high feedback to thermosensitiveperformance. The temperature thermal sensor can be manipulated steadily in medium temperaturerange, and the relative sensitivity reaches 0.4%K−1 at 303 K, which is one order of magnitude largerthan those in several rare-earth doped materials. Efficient photon-conversion ability and hightemperature sensitivity indicate that the rare-earth ions doped fluotellurite material has a prospectiveapplication in the construction of optical temperature sensors based on the FIR technique allowingfor self-referenced temperature determination.

Probing the reciprocal lattice associated witha triangular slit to determine the orbitalangular momentum for a photon

Willamys Cristiano Soares Silva, Andre de Lima Moura, Askery Canabarro, Emerson de Lima, José Henrique de Andrade, Eduardo Fonseca, Marcelo de Lima, Bertúlio Bernardo, Jandir Hickmann, and Sabino Chavez-Cerda

DOI: 10.1364/AO.394745 Received 09 Apr 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: The orbital angular momentum conservation of light reveals different diffractionpatterns univocally dependent on the topological charge of the incident light beam whenpassing through a triangular aperture. It is demonstrated that these patterns, which areaccessed by observing the far field measurement of the diffracted light, can also be obtainedusing few photon sources. In order to explain the observed patterns, we introduce an analogyof this optical phenomenon with the study of diffraction for the characterization of the crystalstructure of solids. We demonstrate that the finite pattern can be associated to the reciprocallattice obtained from the direct lattice generated by the primitive vectors composing any twoof the sides of the equilateral triangular slit responsible for the diffraction. Using the relationthat exists between the direct and reciprocal lattices, we provide a conclusive explanation ofwhy the diffraction pattern of the main maxima is finite. This can shed a new light on theinvestigation of crystallographic systems.

An optical method for micron-scale tracer-lessvisualization of ultra-fast laser induced gas flowat a water/air interface

Dashdeleg Baasanjav, Javier Hernandez-Rueda, Allard Mosk, and Dries van Oosten

DOI: 10.1364/AO.389542 Received 18 Feb 2020; Accepted 24 May 2020; Posted 26 May 2020  View: PDF

Abstract: We study femtosecond laser-induced flows of air at a water/air interface, at micrometerlength scales. To visualize the flow velocity field, we simultaneously induce two flow frontsusing two adjacent laser pump spots. Where the flows meet, a stationary shockwave is produced,the length of which is a measure of the local flow velocity at a given radial position. By changingthe distance between the spots using a spatial light modulator, we map out the flow velocityaround the pump spots. We find gas front velocities near the speed of sound in air vs for twolaser excitation energies. We find an energy scaling that is inconsistent with the Sedov-Taylormodel. Due to the flexibility offered by spatial beam shaping, our method can be applied to studysubsonic laser-induced gas flow fronts in more complicated geometries.

Challenges in multiphysics modeling of dual-bandHgCdTe infrared detectors

Marco Vallone, Michele Goano, Alberto Tibaldi, Stefan Hanna, Detlef Eich, Alexander Sieck, Heinrich Figgemeier, Giovanni Ghione, and Francesco Bertazzi

DOI: 10.1364/AO.394197 Received 01 Apr 2020; Accepted 24 May 2020; Posted 26 May 2020  View: PDF

Abstract: We present three-dimensional simulations of HgCdTe-based focal plane arrays (FPAs) with two-color and dualband sequential infrared pixels having realistic truncated-pyramid shape, taking into account also the presenceof compositionally-graded transition layers. After a validation against the spectral responsivity of two-color, midwavelength infrared detectors from the literature, the method has been employed for a simulation campaign ondual-band, mid- and long-wavelength infrared FPAs illuminated by a Gaussian beam. Simulation results underscore the importance of a full-wave approach to the electromagnetic problem, since multiple internal reflectionsdue to metallizations and slanted sidewalls produce non-negligible features in the quantum efficiency spectra, especially in the long-wavelength band. Evaluations of the optical and diffusive contribution to inter-pixel crosstalkindicate the effectiveness of deep trenches to prevent diffusive crosstalk in both wavebands. In its present form,the detector seems to be subject to significant optical crosstalk in the long wavelength infrared band, which couldbe addressed through pixel shape optimization.

Optomechanical Inertial Sensors

Adam Hines, Logan Richardson, Hayden Wisniewski, and Felipe Guzman

DOI: 10.1364/AO.393061 Received 16 Mar 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: We present a performance analysis of compact monolithic optomechanical inertial sensors that describestheir key fundamental limits and overall acceleration noise floor. Performance simulations for lowfrequency gravity-sensitive inertial sensors show attainable acceleration noise floors of the order of1 × 10−11 m/s2/√Hz. Furthermore, from our performance models, we devised an optimization approachfor our sensor designs, sensitivity, and bandwidth trade space. We conducted characterization measurements of these compact mechanical resonators, demonstrating mQ-products at levels of 250 kg, whichhighlight their exquisite acceleration sensitivity.

6.5m telescope for multi-object spectroscopyover a 3° field of view

Ryker Eads and Roger Angel

DOI: 10.1364/AO.393365 Received 24 Mar 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: Multi-object spectroscopy via independently positioned optical fibers is of growing importance in many researchareas in astronomy. Currently the most powerful instrument of this type is the Dark Energy SpectroscopicInstrument (DESI), now being commissioned at the 3.8m Mayall telescope. It has a 3.2° field of view where spectraof 5000 different objects may be recorded simultaneously. Here we present an optical design for a two-mirror6.5m telescope with 3.0° field of view, for an etendue 2.6 times larger than for DESI. The images are at f/3.7 for fibermatching, and averaged over the field, and elevations down to 40°, have a diameter of 0.53 arcsec for 80%encircled energy. We outline methods capable of polishing and testing the 1.56m diameter gull-wing lens of thewide field corrector.If a 2m diameter lens could be made, the same design could be scaled up to an 8.4m primaryfor a 4.3-fold etendue advantage.


Lev Dolin and Dmitry Turlaev

DOI: 10.1364/AO.394082 Received 30 Mar 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: The possibility to improve imaging of the bottom of a water body by suppressing aninhomogeneous light background that occurs as a result of sky light reflection from the watersurface and is superimposed on the bottom image is theoretically studied. It is shown that theradiance of this background can be reduced by a polarization filter and correct choice of theimaging direction. It has been determined how the optimal directions of the bottom imagingwith a polaroid and without it depend on the Sun position. The possibility of expanding thebottom swath by about two times due to joint use of its imaging facilities in polarized andnonpolarized light is proved. Photographs that demonstrate the effect of improving the bottomimaging with a polaroid are presented.

Microfabrication of a color filter array utilizing coloredSU-8 photoresists

Linan Jiang, Kyung-Jo Kim, FRANCIS REININGER, SEBASTIEN JIGUET, and Stanley Pau

DOI: 10.1364/AO.391579 Received 27 Feb 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: Patterned color filter array is an important component in digital cameras, camcorders, scanners, and multispectral detection and imaging instruments. In addition to the rapid and continuous progress to improve cameraresolution and efficiency of imaging sensors, research in novel designs of color filter array are important in orderto extend the imaging capability beyond conventional applications. This paper reports utilization of colored SU-8photoresists as a material to fabricate color filter arrays. Optical properties, fabrication parameters, and patternspatial resolution are systematically studied for five color photoresists of violet, blue, green, yellow, and red colors.An end-to-end fabrication process is developed to realize a five-color filter array designed for a wide anglemultiband artificial compound eye camera system for pentachromatic and polarization imaging. Colored SU-8photoresists present notable advantages, including patternability, color tunability, low-temperature compatibility,and process simplicity. The results regarding the colored SU-8 photoresist optical properties and their fabricationprocess provide significant insights into their utilization as an optical material to investigate non-conventionalcolor filter designs.

Sensor for dynamic focus control of a deformablemirror

Orestis Kazasidis, Sven Verpoort, and Ulrich Wittrock

DOI: 10.1364/AO.392970 Received 17 Mar 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: We recently presented a novel unimorph deformable mirror which allows for dynamic focus shift with anactuation rate of 2 kHz. Such mirrors suffer from hysteresis and creep. Therefore, they have to be operatedin closed-loop. For this purpose, we developed a defocus sensor, based on an astigmatic detection system.In this paper, we present the sensor design and discuss its performance.

Developement of a stabilized Fabry-Pérot etalon basedcalibrator for Hanle Echelle Spectrograph (HESP)

Tanya Das, Ravinder Banyal, Sivarani Thirupathi, and Ravindra B.

DOI: 10.1364/AO.384713 Received 29 Nov 2019; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: Accurate wavelength calibration is an important factor for any measurement with high resolution spectrographs. Stellar spectrum comprises of discrete absorption or emission lines whose position is precisely determined by calibrating the spectrograph using known reference lines generated from laboratory sources.For the spectrograph to measure small variations in Doppler shift, the wavelength calibration must besufficiently stable during observation time. Instrument instability, mainly due to environmental factorslike temperature and pressure variations, limitations of traditional calibration methods, for example ThAr lamps, are the main challenges which high precision spectroscopy. Through proper environmentalcontrol, by maintaining pressure at few mbar and temperature fluctuations within ±0.05 ◦C, Fabry Pérotetalon (FP) can yield a velocity precision of 1-10 m/s, when used for wavelength calibration. We havedeveloped a passively stabilized FP based wavelength calibrator for Hanle Echelle Spectrograph (HESP)installed on Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory (IAO), Hanle, India. The etalon has been characterized using Fourier Transform Spectrograph (FTS) and initial test runshave been performed with HESP. In this paper we present the design and construction of the instrumentalong with preliminary test results obtained from HESP.

D-Shaped Silicon Core Fiber based SurfacePlasmon-Resonance Refractive Index Sensorin 2-μm

Yi-Lin Yu, Hiroki Kishikawa, Nobuo Goto, and Shien-Kuei Liaw

DOI: 10.1364/AO.387832 Received 10 Jan 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: In this article, we propose a concept which is the D-shaped silicon core fiber sensorbased on surface plasmon resonance within the 2-μm range. Au layer coating on the D-shapedside-face of silicon core fiber acts as a plasmonic active metal. By using the limited elementmethods, the sensing properties of the proposed sensor are investigated, and an averagesensitivity up to 2650 nm/RIU with the resolution of 3.776×10-5 RIU is obtained for theanalyte of different refractive indices varies from 1.25 to 1.6. In the investigated simulation,we used real parameter of liquid (e.g. CH4 liquid and CHBr3) in our simulations for beingcloser to reality. In addition, the Au coating layer and analyte are placed on the D-shapedside-face of the proposed fiber sensor; hence, this structure does not need to fill the voids.Thus, it is easy to fabricate and put to use. Also, the wavelength band in 2-μm is far fromoptical communication band, which means the proposed sensor could be connected with fibernetwork without any influence and crosstalk.

Modulation Format Identification of Optical Signals: AnApproach Based on Singular Value Decomposition ofStokes Space Projections

Rania El Taieb, HossamEl-din Ahmed, Ahmed Farghal, Waddah Saif, Amr Ragheb, Saleh Alshebeili, Hossam Shalaby, and Fathi Abd El-Samie

DOI: 10.1364/AO.388890 Received 23 Jan 2020; Accepted 21 May 2020; Posted 26 May 2020  View: PDF

Abstract: In this paper, two Stokes space (SS) analysis schemes, for modulation format identification (MFI), areproposed. These schemes are based on singular value decomposition (SVD) and Radon transform (RT)for feature extraction. The singular values (SVs) are extracted from the SS projections for different modulation formats to discriminate between them. The SS projections are obtained at different optical signalto-noise ratios (OSNRs) ranging from 11 to 30 dB for seven dual-polarized modulation formats. Thefirst scheme depends on the SVD of the SS projections on three planes. On the other hand, the secondscheme depends on the SVD of the RTs of the SS projections. Different classifiers including supportvector machine, decision tree and k-nearest neighbor for MFI based on the obtained features are used.Both simulation and experimental setups are conducted for proof-of-concept of the proposed schemes forthe MFI task. Complexity reduction is studied for the SVD scheme by applying the decimation of theprojections by 2 and 4 to achieve an acceptable classification rate, while reducing the computation time.Besides, the effect of the variation of phase noise and state of polarization on the accuracy of the MFI isconsidered at all OSNRs. The two proposed schemes are capable of identifying the polarization multiplexed modulation formats blindly with high accuracy levels up to 98% even at low OSNR values of 12dB, high phase noise levels up to 10 MHz, and state of polarization up to 45◦.

Design of diffractive optical elements to producearbitrary axial distributions of light by a Fourier-liketransformation

Victor Rico Botero and Pedro Negrete-Regagnon

DOI: 10.1364/AO.394833 Received 09 Apr 2020; Accepted 21 May 2020; Posted 21 May 2020  View: PDF

Abstract: We present a method to produce a customized distribution of light along the optical axis using a radiallysymmetric diffractive optical element calculated without the need of iterative algorithms. We propose thatadding spherical waves generated from each point of the required axial distribution of light and using a2 f lens array, a diffractive optical element can be calculated easily. We show that intensity and phase ofthe light field could be controlled because the transmittance function of the diffractive element, definedin radial coordinates, and the axial distribution of the light generated, are a Fourier-like transform pair.The proof of concept from the theoretical, numerical and experimental approaches is shown.

A common-path phase-shift microscope based onmeasurement of the Stokes parameters S2 and S3 for3D phase extraction

Dahi Abdelsalam

DOI: 10.1364/AO.395722 Received 23 Apr 2020; Accepted 21 May 2020; Posted 26 May 2020  View: PDF

Abstract: In this paper, we report a common-path, phase-shift optical microscope based on measurement of the Stokesparameters S2 and S3 to extract the three-dimensional (3D) phase map of transparent objects with highprecision. The microscope employs three polarizers and two identical quarter wave plates (QWPs) to extract theS2 and S3. The reference phase in the absence of the object is subtracted from the total phase in the presence ofthe object to extract the 3D phase of the object. The microscope is tested on imaging a USAF resolution testtarget and a reticle test pattern with excellent results. To the best of our knowledge, this is the first report of acommon-path phase-shift optical microscope for 3D phase extraction based on measurement of the Stokesparameters S2 and S3.

Designing Miniature X-ray Optics for the SmallSatLunar Science Mission Concept CubeX

Vinay Kashyap, Jaesub Hong, suzanne romaine, Leandra Sethares, Vincenzo Cotroneo, Daniele Spiga, and Larry Nittler

DOI: 10.1364/AO.393554 Received 26 Mar 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: Planetary remote-sensing instruments are often required to cover a relatively large field of view, ideallywith a uniform angular resolution over the field, due to relatively large apparent sizes of planetary targetsat close proximities. They also have to comply with relatively tight mass and volume constraints. Forthese reasons, planetary X-ray telescopes in the past were mainly collimation-based X-ray spectrometerswithout focusing optics. Recent advances in X-ray optics technology now enable compact focusing X-raytelescopes suitable for planetary science (e.g., BepiColombo). We present two design options for compactWolter-I X-ray optics for a SmallSat lunar mission concept - the CubeSat X-ray telescope (CubeX). Theprimary objectives of CubeX are to map surface elemental abundances of selected lunar impact cratersand to assess the feasibility of millisecond X-ray pulsar timing navigation in realistic deep space navigation environments. The Miniature X-ray Optics (MiXO) in CubeX utilizes electroformed NiCo alloyreplication (ENR) technology, which provides many advantages over micro-pore optics (MPO) employedin BepiColombo. We carry out extensive raytraces over a grid of mirror parameters and explore a noveltapered shaped design of tightly nested shells, where both shell length and focal plane offsets vary withshell diameter. One of the two configurations is optimized for large effective areas at low energies, whilethe other for lower mass and high-energy response. We compare their performances with those of conventional designs through the spatial resolution and effective area estimated by raytraces.

Investigation of CsPbBr3 films with controllablemorphology and its influence on photovoltaicproperties for carbon-based planar perovskitesolar cells

Fei Zhao, yixin guo, Jiahua Tao, Zeng Li, Jinchun Jiang, and Junhao Chu

DOI: 10.1364/AO.392404 Received 09 Mar 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: Recently, inorganic CsPbBr3 perovskites have obtained much researchinterest in photovoltaic field owing to its excellent stability. However, it is stillchallenging to prepare high-performance CsPbBr3 films with good crystallinity andsuperior morphology at moderate temperatures. In this work, we report a novelmultistep spin-coating deposition method to prepare CsPbBr3 perovskite film. Throughoptimizing the number of deposition cycles of a CsBr solution, the CsPbBr3 filmscontaining high phase purity, good crystallinity and strong absorption are obtained. Theinorganic planar perovskite solar cell with optimized CsPbBr3 film realizes a maximumpower conversion efficiency (PCE) of 4.70 % under standard solar illuminationconditions. Moreover, our devices with FTO/TiO2/CsPbBr3/CuPc/C exhibit fabulousstability when stored in ambient atmosphere with ~40 % humidity for 60 days. Ourwork provides new opportunities for facilitating practical application of CsPbBr3 planarperovskite solar cells.

Indoor Daylighting using Fresnel lens solar concentratorbased hybrid cylindrical luminaire for illumination andwater heating

Mayank Gupta, Atul Dubey, Virendra Kumar, and Dalip Mehta

DOI: 10.1364/AO.389044 Received 23 Jan 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: Abundant availability of sunlight during daytime and broad spectrum of solar energy has attracted great attentionof researchers. The photometric parameters of sunlight such as color coordinates, color rendering index and colortemperature are most appropriate for human vision as compared to artificial light sources. This is due to the factthat visible portion of sunlight is almost uniform and matches perfectly with human eye sensitivity curve. Further,it is well known that sunlight is also having great health benefits. To exploit these advantages, we have developeda solar concentrator system based on a large Fresnel lens and a light guide to transport sunlight indoor. Infraredportion of solar energy is utilized for water heating and visible portion of sunlight is transmitted via plastic opticalfiber (POF) bundle which guides sunlight into the rooms. One end of the POF is coupled with a light guide andanother end is coupled with a cylindrical rod shape luminaire made up of acrylic. POF’s are low cost, flexible andeasily available compared to glass fibers, therefore they are generally used for transporting sunlight indoor. Butthe spectral profile of transmitted sunlight does not remain uniform in the visible portion while propagating vialong POF. To achieve optimum spectral profile a blue led is ingrooved in the cylindrical luminaire. The design ofthe system, experimental details, thermal efficiency and photometric parameters such as color coordinates,illuminance, optical efficiency and spectrum of indoor lighting are reported. The proposed hybrid system willreduce the requirement of electricity consumption during the daytime, improve indoor illumination quality anduseful for sustainable development of the country.

Moiré Effect in Double-Layered CoaxialCylinders

Vladimir Saveljev, Woojun Han, Hakcheol Lee, Jaewan Kim, and Jaisoon Kim

DOI: 10.1364/AO.392223 Received 06 Mar 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: The moiré effect in multi-layered cylindrical objects was studied theoretically andexperimentally. In theory, the central projection of double-layered convex and concave halfcylinders was considered. Based on the projected period expressed analytically in terms of theincidence angles, the period of the moiré patterns on the axis was obtained as the closed-formexpression; the off-axis effect was analyzed qualitatively. The distance was discovered, wherethe period as the function of the angle is nearly constant. The experiments performed withtwo cylindrical devices confirm the theory. The results can be applied to nanoparticles andcurved/flexible displays.

An Ultra-Compact Polarization Rotator Based onMode Coupling in a Groove-like WaveguideAssisted by Subwavelength Grating

Chen Yuan and Di Gao

DOI: 10.1364/AO.394651 Received 08 Apr 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: A novel scheme of an ultra-compact polarization rotator (PR) is based on an optical axis rotated section of groove-likewaveguide with a subwavelength grating (SWG) structure used to replace one of the silicon strip waveguides in thepolarization conversion section. In the proposed scheme, the mode coupling occurs and the mode conversions (TM0/TE0-to-TE0/TM0) is realized. At the operating wavelength of 1.55μm, the simulated extinction ratio (ER) of ~41dB, insertionloss (IL)<1dB and reflection loss (RL)<-22dB for TM0-to-TE0 conversion with total conversion length of 5.4μm have beenobtained. The simulated transmission spectra also show relatively broad bandwidth of 53nm(41nm) for TM0-to-TE0(TE0-to-TM0) conversion to keep the ER>25dB as well as high polarization conversion ratio of 99.95%(99.31%) for TM0-toTE0(TE0-to-TM0) process at 1.55μm. The presence of SWG greatly enhances the device performance and the tolerances tofabrication uncertainties have also been investigated.

Extension of time window into nanoseconds insingle-shot ultrafast burst imaging by spectrallysweeping pulses

Hirofumi Nemoto, Takakazu Suzuki, and Fumihiko Kannari

DOI: 10.1364/AO.392676 Received 08 Apr 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: We achieved single-shot 2D-burst imaging with a ~22-ps temporal resolution in a nanosecond time window using asequentially timed all-optical mapping photography with a spectral filtering (SF-STAMP) scheme, where a single snapshot ofspectral images measured with a linear frequency chirped laser pulse forms time-resolved snapshots. We combined a pulsestretching scheme of a free-space angular-chirp-enhanced delay (FACED) composed of a pair of tilted mirrors and a 4fsystem. With a 4f-FACED system, we generated colinearly propagating burst laser pulses with a different center wavelengthand a tunable time interval and demonstrated single-shot burst imaging with a 303-ps interval in a 1.5-ns time window bySF-STAMP with spectrally sweeping probe pulses.

An Integrated Optical Time-Domain Reflectometerwith Low Overhead

Thomas Mauldin, Zhenyu Xu, Zheyi Yao, and Tao Wei

DOI: 10.1364/AO.393072 Received 19 Mar 2020; Accepted 20 May 2020; Posted 21 May 2020  View: PDF

Abstract: This paper introduces an integrated Optical Time-Domain Reflectometry (iOTDR) design with low-overhead basedon several innovations, such as analog-to-probability conversion (APC) and probability density modulation (PDM),as well as well-known techniques. Due to the small form factor, low-overhead, and CMOS-compatibility of theproposed system, it holds the promise to be integrated with any optical communication transceiver. Theprototyping system clearly demonstrates the feasibility of the proposed instrumentation by comparing itsperformance to a conventional OTDR setup that is based on an analog-to-digital converter. In addition, the systemis integrated onto a custom printed circuit board to demonstrate the promise of future integration into opticaltransceivers. The experimental results demonstrate a spatial resolution (SR) of cm, a sensitivity of -60dB, and adynamic range (DR) of 24.15dB.

Noise reduction in supercontinuum sources for OCT bysingle-pulse spectral normalization

Ryan Niemeier, Zach Simmons, and Jeremy Rogers

DOI: 10.1364/AO.393141 Received 24 Mar 2020; Accepted 20 May 2020; Posted 21 May 2020  View: PDF

Abstract: Supercontinuum (SC) sources offer high illumination power from a single mode fiber with large spectralbandwidth including the visible spectrum, a growing application area for Optical Coherence Tomography (OCT). However, SC spectra suffer from pulse-to-pulse variations, increasing noise in the resultingimages. By simultaneously collecting a normalization spectrum, OCT image noise can be reduced bymore than half (7 dB) for single pulses without any pulse averaging using only simple optical components.

Analysis of surface changes of burning nanopowdersusing digital processing of laser monitor images

Fyodor Gubarev, Sergey Kim, Lin Li, Andrei Mostovshchikov, and Alexander Ilyin

DOI: 10.1364/AO.392897 Received 16 Mar 2020; Accepted 19 May 2020; Posted 20 May 2020  View: PDF

Abstract: An imaging laser projection system, a laser monitor, is applied for real-time monitoring of the surface ofnanopowders during high-temperature combustion. The mirror-based scheme of laser monitor is used in theexperiments to increase the range of observation. For the analysis of surface changes during combustion, thecorrelation coefficient together with average brightness of the images of laser monitor obtained by digitalprocessing are proposed to use. The results of calculating the correlation coefficient demonstrates compliance withthe change in the intensity of the images of the laser monitor during combustion and visual observation of thecombustion process. The distortions introduced by the instability of the brightness amplifier into the measurementresults are estimated. When observing a static test object, the variation of correlation coefficient is three orders ofmagnitude less than fluctuation in the correlation coefficient during combustion, the variation of average imagebrightness is less than 5%. The obtained results indicate the possibility of usage of correlation coefficient andaverage brightness of images as informative parameters in analysis of surface changes of burning nanopowders instudies using laser monitors both with conventional and mirror-based schemes.

Direct estimation of NIR reflection spectrautilizing snapshot-type spectrometer withphotonic crystal multi-spectral filter array

Yasuo Ohtera, Nao Ikeda, Tomohisa Takaya, and Kazuma Shinoda

DOI: 10.1364/AO.384820 Received 05 Dec 2019; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We fabricated 16-, 25-, 36-, and 64-channel distributed passband-type multi-spectralfilter arrays by utilizing a multi-layer-type photonic crystal and integrated them onto a CCD toform a snapshot-type spectroscopic sensor. Reflection spectra from target objects (fruits) under abroad-band light illumination were directly estimated using the Wiener estimation method. Aroot mean square error of the reflectivity on the order of 2∼5% was obtained under optical shotnoise with 6×6 pixel binning. A number of constituent filters of 36 was sufficient for such typeof fruit spectral measurement. We also visualized reflection images at specified wavelengths byapplying the estimation method to a multiple filter region on the sensor.

Ring-shaped Microstructured ChalcogenideOptical Fiber for Octave-spanning Flat-topMid-infrared Supercontinuum Generation

Chunlei Huang, wanjun bi, biao zheng, Cheng Zhang, Jun Wang, and Shupei Zheng

DOI: 10.1364/AO.391564 Received 10 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: Broadband flat-top mid-infrared supercontinuum (SC) generation in optical fibersis of great interest for many applications. Here, we designed a microstructured chalcogenideoptical fiber composed of hexagonal rings. By optimizing the structure parameters, it isdemonstrated that the fiber is characterized by an all-normal dispersion profile. Numericalsimulation results show that the generated SC spectrum with intensity above -3 dB rangesfrom 3450 nm to 8015 nm corresponding to more than one octave when the fiber is pumpedwith 150 fs pulses at 5 μm. By solving the generalized nonlinear Schrödinger equation, weinvestigated the influence of pump pulse parameters including pump wavelength, peak power,and pulse duration on the spectral broadening. The results turn out that the optimal pumpwavelength is around 5 μm for obtaining SC spectra with sufficient flatness. High peak powerand short duration enhance the spectral flatness and bandwidth. We also investigated SCgeneration in the optimized fiber pumped at 3 μm. The generated SC at -6 dB level coverswavelength from 1970 nm to 4370 nm, corresponding to more than one octave.

Transformable Reflective Telescope for Optical Testingand Education

Woojin Park, Soojong Pak, Geon-Hee Kim, Sunwoo Lee, Seunghyuk Chang, Sanghyuk Kim, Byeongjoon Jeong, Trenton Brendel, and Dae Wook Kim

DOI: 10.1364/AO.392304 Received 12 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We propose and experimentally demonstrate the Transformable Reflective Telescope (TRT) Kit for educational purposes and for performing various optical tests with a single kit. The TRT Kit is a portableoptical bench setup suitable for interferometry, spectroscopy, measuring stray light, and developing adaptive optics, among other uses. Supplementary modules may be integrated easily thanks to the modulardesign of the TRT Kit. The Kit consists of five units; a primary mirror module, a secondary mirror module, a mounting base module, a baffle module, and an alignment module. Precise alignment and focusingare achieved using a precision optical rail on the alignment module. The TRT Kit transforms into threetelescope configurations: Newtonian, Cassegrain, and Gregorian. Students change telescope configurations by exchanging the secondary mirror. The portable design and the aluminum primary mirror ofthe TRT Kit enable students to perform experiments in various environments. The minimized baffle design utilizes commercial telescope tubes, allowing users to look directly into the optical system whilesuppressing stray light down to ∼10−8 point source transmittance (PST). The TRT Kit was tested usinga point source and field images. Point source measurement of the Newtonian telescope configurationresulted in an 80% encircled energy diameter (EED) of .8 µm.

Polarizing interferometer for the unambiguousdetermination of the ellipsometric parameters

Arnulf Röseler and Ulrich Schade

DOI: 10.1364/AO.392538 Received 11 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We report on a polarizing interferometer-ellipsometer arrangement whichovercomes the need for additional measurements with a retarder for the unambiguousdetermination of the ellipsometric parameters in the far infrared spectral range. It consists of aMartin-Puplett interferometer and a wire-grid polarizer as analyzer. The application of suchinterferometer-ellipsometer is experimentally demonstrated on a polyethylene samplederiving the refractive index and the thickness in the spectral range between 15 and 35 cm-1.Based on these results a similar solution without retarder for the mid-infrared spectral regionis additionally proposed.

Multiple spatial and wavelength conversionoperations based on a frequency-degeneratedintermodal four-wave-mixing process in a gradedindex 6-LP few mode fiber

Haisu Zhang, Marianne Bigot, Pierre Sillard, Guy Millot, Bertrand Kibler, and Julien Fatome

DOI: 10.1364/AO.393299 Received 20 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We report on the experimental observation of a simultaneous threefold wavelength and spatial conversionprocess at telecommunication wavelengths taking place in a 6-LP-mode graded-index few-mode fiber. Thephysical mechanism is based on parallel and phase-matched frequency-degenerated inter-modal four-wavemixing (FD-IFWM) phenomena occurring between the fundamental mode and higher-order spatial modes.More precisely, a single high-power frequency-degenerated pump wave is simultaneously injected in the fourspatial modes LP01, LP11, LP02 and LP31 of a 1.8-km long graded-index few-mode fiber together with threeindependent signals in the fundamental mode. By means of three parallel phase-matched FD-IFWMinteractions, these initial signals are then simultaneously spatially and frequency converted from thefundamental mode to specific high-order modes. The influence of the differential modal group delay is alsoinvestigated and shows that the walk-off between the spatially multiplexed signals significantly limits thebandwidth of the conversion process for telecom applications.

Sub-terahertz refractive flat-top beam shapingvia 3D printed aspheric lens combination

Brad Price, Seth Lowry, Ian Hartley, and Matthew Reid

DOI: 10.1364/AO.393857 Received 27 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: The viability of 3D printed aspheric lenses for the purpose of frequency-scalablesub-terahertz Gaussian to flat-top beam shaping is evaluated. A cylindrical one-dimensionalFresnel-Kirchhoff diffraction equation was implemented in Matlab® and used to design a pairof aspheric lenses with customized vertex radius of curvature and conic constant. The lenseswere printed at maximum possible resolution in acrylonitrile butadiene styrene (n = 1.6) andtested with a 102 GHz continuous wave sub-terahertz source. The aspheric lens combinationproduced a flat-top profile from a low-quality incident Gaussian beam at the minimal cost of 3Dprinting substrate. The flat-top profile exhibited small (< 14% root-mean-square deviation overflat region) intensity fluctuations and is expected to prove useful in future terahertz applicationsthat require a high degree of beam uniformity.

Metallic metamaterial terahertz sensors forsimultaneous measurement of temperature andrefractive index

Gangqi Wang, Tingting Lang, and Zhi Hong

DOI: 10.1364/AO.394764 Received 08 Apr 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: A new type of sensor based on metamaterials, consisting of an array of metal ringson a quartz substrate, is proposed to measure temperature and refractive indexsimultaneously. Simulations of the transmission spectrum show two dips, both of which varywith the refractive index and temperature of the environment. The sensitivities of the two dipsto refractive index are 67.9 GHz/RIU and 142.76 GHz/RIU, respectively, and to temperatureare 6.35 MHz/K and 7.14 MHz/K, respectively. These small, highly sensitive sensors havewide application prospects as physical, biological, and chemical sensors. They simultaneouslymeasure temperature and refractive index in the terahertz band.

Spatially polarization-modulated ellipsometry (SPME)based on vectorial optical field and image processing

Chao Gao and Bing Lei

DOI: 10.1364/AO.395234 Received 16 Apr 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We report a new ellipsometric measurement scheme called spatially polarization-modulated ellipsometry(SPME) based on the vectorial optical field and digital image processing. A zero-order vortex half-waveretarder (ZVHR) is employed to generate the vectorial optical field and analyze the elliptically polarizedlight reflected by a thin film, and an analyzer is set after the ZVHR to form an hourglass intensity patterndue to the spatially polarization modulation, and then the film’s ellipsometric angles can be obtained byprocessing the hourglass intensity image. By analyzing the working principle of SPME, we have foundthat the film’s ellipsometric angles are determined by the bright areas’ azimuth angle and contrast of themodulated images, and their mathematical relationships have been deduced and provided. To reduce theadverse effects of image noise and improve the measurement precision, an improved operation mode ofSPME is presented by processing the modulated images with their bright areas’ azimuth locating at near45◦or 135◦ and 0◦ or 90◦. The numerical analyzing studies have been carried out on the SiO2 films tovalidate the feasibility of SPME, and the simulation experiments indicate that the SPME can operate welleven though obvious noise has been added to the modulated intensity image and the measuring error offilm thickness and refractive index were less than 0.1nm and 0.001.

Multi-channel Opto-mechanical Switch and LockingSystem for Wavemeters

Mojtaba Ghadimi, Elizabeth Bridge, Jordan Scarabel, Steven Connell, Kenji Shimizu, Erik Streed, and Mirko Lobino

DOI: 10.1364/AO.390881 Received 20 Feb 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: Here we present a cost effective multi-channelopto-mechanical switch and software PID (proportional–integral–derivative controller) system for locking multiple lasers to a single channel commercialwavemeter. The switch is based on a rotating cylinderthat selectively transmits one laser beam at a time to thewavemeter, the wavelength is read by the computer andan error signal is output to the lasers to correct wavelength drifts every millisecond. We use this systemto stabilise 740 nm (subsequently frequency doubledto 370 nm), 399 nm and 935 nm lasers for trapping andcooling different isotopes of Yb+ion. We characterisethe frequency stability of the three lasers by usinga second, more precise, commercial wavemeter. Wealso characterise the absolute frequency stability ofthe 740 nm laser using the fluorescence drift rate of atrapped 174Yb+ion. For the 740 nm laser we demonstrate an Allan deviation, σy, of 3 × 10−10 (at 20 sintegration time), equivalent to sub-200 kHz stability.

Dynamically tunable reflecting near infrared band-passfilter based on hybrid graphene-nanometallic structure

Seyededriss Mirniaharikandi and Brian Lail

DOI: 10.1364/AO.391030 Received 21 Feb 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: A dynamically tunable reflecting near-infrared band-pass filter based on hybrid graphene-nanometallicstructure is demonstrated by numerical simulation. The proposed filter is constructed by unit cellswith graphene monolayer embedded into the nanometallic grating structure. The gradual transition ofgraphene monolayer from very thin metal to dielectric plays the key role in tuning the reflection spectrumof the structure. Spectral analysis clearly shows a passband blueshift with increasing graphene Fermi energy. The filter parameters are investigated by varying graphene Fermi energy through external voltagegates. The modulation depth, center frequency, bandwidth and quality factor of the filter could be tuned.We achieved stable modulation depth as high as 0.735, and quality factor as high as 3.4. The center frequency can be tuned in a broad range from 210 to 0 THz and with bandwidth tuning from 60 to 95THz. The effect of nanogap size and environment refractive index is also numerically investigated. Theseresults are very promising for the future use and integration of the proposed filters as a key element ofoptical communication systems and infrared sensing.

Optical Bistability of Partial Reflection Coated Thin Film of OilRed O

quy ho, Mai Luu, Thai Thanh, KIEN BUI XUAN, Thang Nguyen, and QUANG HO DINH

DOI: 10.1364/AO.391634 Received 28 Feb 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: In this paper, a thin film of oil red O coated by partial dielectric mirrors (PRFORO) operating as an optical bistabledevice is proposed. The equation of the output-input intensity relation considering nonlinear absorption insidePRFORO is derived. The optical bistability (OB) of PRFORO is shown. The influence of the nonlinear refractive,absorptive coefficients of oil red O (ORO), reflective coefficient of mirrors, and thickness of thin film on the OB isnumerically investigated and discussed. As a result, the threshold switching power is lower than 19 mW, theminimum power of output up-state is larger than 40μW for the optimal PRFORO designed with ORO concentrationlarger than 0.1mM, thickness of thin film is longer than 0.1mm and reflective coefficient of mirrors is lower than93%. The proposed model is appreciable for the all-optical switch, optical image and signal processing.

Two-photon laser-induced fluorescence ofsodium in multiphase combustion

Keke Zhu, James Michael, Chloe Dedic, Stuart Barkley, and Travis Sippel

DOI: 10.1364/AO.392710 Received 13 Mar 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: Alkali metals are prevalent in coal combustion, biomass thermal conversion to fuels,energetic material combustion, and have applications as dopants for combustion diagnosticsand plasma-based combustion control. Recently, the dynamic control of propellant burningrate and pyrotechnic luminosity have been demonstrated through combined microwave fieldradiation and incorporation of alkali-dopants. Understanding of the alkali distribution withinthe flame is essential to predicting field-flame interaction. However, the multiphase propellantcombustion environment exhibits significant particle scattering, broadband emission background,and high optical density, which complicates optical measurements. Here, multiple two-photonlaser-induced fluorescence (LIF) schemes for atomic sodium were compared in gas-phase flamesand sodium-doped solid propellant combustion. For the two-photon Na LIF scheme, a rateequation model incorporating fluorescence, amplified spontaneous emission and ionization showsgood agreement with experimental characterization in a gas phase flame. The technique wasthen extended to sodium-doped solid propellant flames, a multiphase combustion environmentcharacterized by strong particle scattering. The 3s–3d two-photon excitation LIF achieved bettersignal-to-noise ratio of ∼100 in the propellant flame and allowed imaging of gradients in the gasphase sodium distribution at the burning surface.

Simple and fast field curvature measurement by depth from defocus using electrowetting liquid lens

Gyu Suk Jung and Yong H. Won

DOI: 10.1364/AO.394565 Received 06 Apr 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: Field curvature, also called “Petzval field curvature”, is a defect in a lens in which the object of a flat plane is not focused on the image surface of the flat plane. Field curvature measurement is very important in lens performance testing. This paper presents an electrowetting liquid lens-based depth from defocus (DfD) method for measuring field curvature. This method uses only a pair of defocused images for a patterned flat object, which are captured using the focus-tuning function of the electrowetting liquid lens. Image processing for calculating field curvature is carried out using the MATLAB program. The results of the measurement experiment showed high accuracy of a 12-um root-mean-square error between the captured image surface and fitted curved image surface. There is no need for complex equipment, such as lasers, microscopes, and telecentric systems. Because this system has no mechanical movement for focus tuning, it is simple and shows fast measurement time compared to other conventional methods.

Thermal compensation of molded siliconeoptics

Nicholas Lyons, Sifang Cui, Remington Ketchum, Kyung-Jo Kim, and Robert Norwood

DOI: 10.1364/AO.392233 Received 05 Mar 2020; Accepted 17 May 2020; Posted 18 May 2020  View: PDF

Abstract: Optical grade silicone has various properties that make it attractive for solarconcentrators, such as excellent transmission across the solar spectrum and flexiblemoldability for freeform profiles. In this study, a glass-silicone lens structure is proposed toreduce the opto-thermal effect on the silicone lens. Experimental measurements andsimulation modeling results demonstrate that the focal length sensitivity of the glass-siliconelens with respect to temperature can be reduced by a factor of 10 when compared to a siliconelens alone. This model has been extended to the simulation of a proposed two-stage siliconesolar concentrator, consisting of an array of acylindrical lenslets and rows of waveguides thatfocus light onto micro-photovoltaic cells. The optical efficiency of the solar concentrationsystem showed a change of less than 10 % compared to the efficiency at room temperature fortemperature changes from -10 oC to 70 oC.

Consideration of temperature-dependent emissivityof selective emitters in thermophotovoltaic systems

Rajendra Bhatt, Ivan Kravchenko, and Mool Gupta

DOI: 10.1364/AO.394326 Received 03 Apr 2020; Accepted 17 May 2020; Posted 18 May 2020  View: PDF

Abstract: Spectral emissivity control is paramount for designing a high-efficiency selective emitter surface required forthermophotovoltaic (TPV) applications. Owing to the temperature dependency of materials optical constants,the spectral properties of a selective emitter surface changes with the emitter temperature. This paperpresents the fabrication of a multilayer metal-dielectric (Si3N4/W/Si3N4) coated tungsten selective emitter aimedfor GaSb-based TPV systems and studies the dependence of its surface spectral emissivity, ࢿሺࣅሻ, upon temperatureranging from 300 K to 1500 K. Both simulation and experimental methods were used to characterize ࢿሺࣅሻ as afunction of temperature. For wavelengths less than 1.4 µm, ࢿሺࣅሻ was found to have minimal dependence ontemperature. Beyond 1.4 µm, ࢿሺࣅሻ increases with temperature. At 1.55 µm, the simulation and experimental dataestimated a ~4% greater emissivity at 1500 K than at room temperature. At 1500 K, the increased ࢿሺࣅሻ at longerwavelengths lowered the spectral conversion efficiency of the selective emitter from 58% to 47%. The outputpower density, sub-bandgap loss, and TPV conversion efficiency (ηTPV) for a GaSb cell illuminated by the selectivethermal emitter at 1500 K were estimated. ηTPV drops from 13.7% to 11% due to the increased sub-bandgapemission at 1500 K. Essential approaches for mitigating the sub-bandgap losses to further improve ηTPV are alsodiscussed.

Displacement measurement in Sagnac interferometerwith thermal light second-order correlation

De-Qin Xu, Lingyu Dou, Dezhong Cao, and Xinbing Song

DOI: 10.1364/AO.393599 Received 06 Apr 2020; Accepted 17 May 2020; Posted 19 May 2020  View: PDF

Abstract: We report a displacement measurement experiment insecond-order correlation and subwavelength correlation with thermal light. Our target is a single slit,which is inserted into a Sagnac interferometer to mimica double-slit apparatus. According to the correlateddiffraction pattern, we can predict the excursion of theslit from the optical axis. When the slit is shifted,we can predict the displacement accurately. We alsopresent the mathematical presentations of beam propagation and reflection in comprehensive deduction.

Reflective temperature sensor based on fiber Bragggrating combined with fiber loop ring down technique

Bangning Mao, Wen Wei Chen, Chunliu Zhao, Yan Shi, Yu Shen, and Shangzhong Jin

DOI: 10.1364/AO.392423 Received 13 Mar 2020; Accepted 17 May 2020; Posted 28 May 2020  View: PDF

Abstract: In this paper, a reflective fiber sensing system combined with fiber loop ring down (FLRD) techniquefor temperature detection is proposed. A Fiber Bragg grating (FBG) with an initial center wavelength of1543.33 nm was set as the sensor head. The experimental results showed that the average sensitivity oftemperature was 1.003 μs/°C in the temperature range of 30-38 °C, and the resolution is 0.007 °Cwhich is 14 times better than current FBG sensing demodulator. In addition, the system has theadvantages of simple structure, low cost and insensitive to optical power fluctuation.

High-precision miniaturized low-cost reflectivegrating laser encoder with nanometric accuracy

Mohammad Hossein Goudarzi Khouygani and Jeng-Ywan Jeng

DOI: 10.1364/AO.393951 Received 07 Apr 2020; Accepted 16 May 2020; Posted 26 May 2020  View: PDF

Abstract: This paper presents a novel optical encoder, M-encoder, which can be used withinhigh-precision metrology systems based on the technology of customized prism and homodynedetection. M-encoder is an exposed encoder measuring reflective glass and metal scale which istargeting the biggest area of encoder market; 20 휇푚 pitch and having resolution under 100 푛푚.The error of 푛푚 for 100 푚푚 traveling distance has been measured. The applied technologyhas successfully improved alignment tolerances which the tolerance of installation achieved to±0.5◦, ±0.5◦, and ±1◦for pitch, yaw, and roll angle, respectively.The accuracy of results has been verified by comparing them with one of the commercial encodersand also by using HP interferometer. The results show the resolution and accuracy can becompatible with the market products. In addition, a new method of grating calibration has beendescribed based on the Littrow configuration and the pitch of the fabricated scale by femtosecondlaser has been measured with the accuracy of 2 푛푚.

Radiation-hardened configuration context realizationfor field programmable gate arrays

Hiroki Shinba and Minoru Watanabe

DOI: 10.1364/AO.396525 Received 30 Apr 2020; Accepted 16 May 2020; Posted 26 May 2020  View: PDF

Abstract: In space environments and at nuclear power plants, data can be destroyed by radiation, even data recordedon flash memories. To realize a radiation-hardened configuration context for field programmable gate arrays (FPGAs) under such radiation environments, this paper presents a proposal of a method to increasethe radiation tolerance of configuration contexts used for FPGAs by introducing a holographic memorytechnology and a new FPGA architecture. When reading a configuration context from holographic memory, the context robustness depends on the number of bright bits on the configuration context. Our proposed method exploits that holographic memory property and uses a new FPGA architecture to fit theproperty to increase the radiation tolerance of configuration contexts from holographic memory. Thispaper presents simulation results and an experimental demonstration result.

Quantum hybrid optomechanical inertial sensing

Logan Richardson, Adam Hines, Andrew Schaffer, Brian Anderson, and Felipe Guzman

DOI: 10.1364/AO.393060 Received 17 Mar 2020; Accepted 16 May 2020; Posted 19 May 2020  View: PDF

Abstract: We discuss the design of quantum hybrid inertial sensor that combines an optomechanical inertial sensorwith the retro-reflector of a cold atom interferometer. This sensor fusion approach provides absolute andhigh accuracy measurements with cold atom interferometers, while utilizing the optomechanical inertialsensor at frequencies above the repetition rate of the atom interferometer. This improves the overallmeasurement bandwidth as well as the robustness and field deployment capabilities of these systems.We evaluate which parameters yield an optimal acceleration sensitivity, from which we anticipate a noisefloor at nano-g levels from DC to 1 kHz.

A transfer function replacement ofphenomenological single-mode equations insemiconductor microcavity modeling

Rolf Binder, Mario Carcamo, and Stefan Schumacher

DOI: 10.1364/AO.392014 Received 05 Mar 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: Semiconductor microcavities are frequently studied in the context of semiconductorlasers and in application-oriented fundamental research on topics such as linear and nonlinearpolariton systems, polariton lasers, polariton pattern formation, and polaritonic Bose-Einsteincondensates. A commonly used approach to describe theoretical properties includes a phenomenological single-mode equation that complements the equation for the nonlinear opticalresponse (interband polarization) of the semiconductor. Here we show how to replace thesingle-mode equation by a fully predictive transfer function method that, in contrast to thesingle-mode equation, accounts for propagation, retardation and pulse filtering effects of theincident light field traversing the distributed Bragg reflector (DBR) mirrors, without substantiallyincreasing the numerical complexity of the solution. As examples we use cavities containingGaAs quantum wells and transition-metal-dichalcogenides (TMDs), respectively.

Research on quantitative method forthree-dimensional computed tomography ofchemiluminescence

Gongxi Zhou, Fei Li, Kuanliang Wang, Xin Lin, and Xilong Yu

DOI: 10.1364/AO.393225 Received 19 Mar 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: To develop a more advanced 3D computed tomography of chemiluminescence (3D-CTC) method, the firstquantitative 3D diagnosis was realized. The nonlinearity coefficient, the nonuniformity coefficient of the cameraresponse, and various optical fiber attenuation coefficients were obtained through correction experiments. Theconversion relationship between the number of photons released by the target object per unit time and the cameragray value at a specified solid angle is also calibrated. To verify the quantitative reconstruction equation, 3Dreconstructions of a methane-air flat flame and a simulated phantom were performed for comparison. The methodcan overcome artificial distortions caused by uncorrected reconstruction.

Evolving Expertise For Automated LensOptimization

Caleb Gannon and Rongguang Liang

DOI: 10.1364/AO.391888 Received 03 Mar 2020; Accepted 15 May 2020; Posted 18 May 2020  View: PDF

Abstract: We present a process for locating the desired local optimum of high dimensionaldesign problems such as the optimization of freeform mirror systems. By encoding activedesign variables into a binary vector imitating DNA sequences, we are able to perform a geneticoptimization of the optimization process itself. The end result is an optimization route that iseffectively able to sidestep local minima by warping the variable space around them in a way thatmimics the expertise of veteran designers. The generality of the approach is validated through theautomated generation of high-performance designs for off-axis 3 and 4 mirror freeform systems.

Photo-induced Hole Dipoles’ Mechanism of LiquidCrystal Photoalignment

Alexander Muravsky, Anatoli Murauski, and Iryna Kukhta

DOI: 10.1364/AO.392068 Received 04 Mar 2020; Accepted 15 May 2020; Posted 18 May 2020  View: PDF

Abstract: Polarized light absorption in photoalignment material induces anisotropic long-range interactions that orientliquid crystals. The main physical mechanisms standing behind anisotropic interaction nature are photo crosslinking and photo destruction of polymers, photo-isomerization and photo-rotation of azo-dyes. Investigation ofAtA-2 azo-dye azimuthal anchoring vs. exposure dose revealed the presence of unusual strong anchoring peak atlow doses, which is outside understanding of the known mechanisms. Here we explain the observations and showthe existence of the new photoalignment mechanism based on photo-induced hole dipole moments in azo-dyelayer. Strong azimuthal anchoring energy >2x10-4 J/m2 is obtained within <0.5 J/cm2 exposure dose.

Compressed Single-Shot HyperspectralImaging for Combustion Diagnostics

Zichen He, Nathan Williamson, Cary Smith, Mark Gragston, and Zhili Zhang

DOI: 10.1364/AO.390335 Received 17 Feb 2020; Accepted 14 May 2020; Posted 18 May 2020  View: PDF

Abstract: This paper demonstrates a compressed sensing-based single-shot hyperspectralimaging system for combustion diagnostics. The hyperspectral system can capture wellresolved spectra in a 2-D plane through a single shot, i.e. converting a 3-D data cube of 2-Dspatial and 1-D spectral information into a compressed 2-D hyperspectral image.Experimentally, the light emissions are first coded by a random binary pattern to generate thehyperspectral content, which is then sent through a spectrometer. The resulting compressedhyperspectral image is computationally analyzed to recover original 2-D spatial and 1-Dspectral information. C2* and CH* chemiluminescence emissions of a methane/air flame atvarious equivalence ratios are measured using the compressed hyperspectral imagingtechnique. Comparison to traditional measurements shows good agreement in the correlationof emission ratio to equivalence ratio. The technique can be further applied to other laserbased combustion diagnostics.

Blue Laser Diode-Based High CRI Lighting and HighSpeed Visible Light Communication Using NarrowBand Green/Red-Emitting Composite Phosphor Film

Amjad Amjad, Riffat Amjad, Mithilesh M, Zhang Chao, Syeda Agha Hassnain, Xiao Chen, y xq, Faizan ur Rehman, Wenmin Ge, Ying Ye, and Jing Xu

DOI: 10.1364/AO.392340 Received 10 Mar 2020; Accepted 14 May 2020; Posted 14 May 2020  View: PDF

Abstract: We experimentally demonstrate high-speed visible light communication (VLC) and high-quality solid-state lighting (SSL)using polymethyl methacrylate (PMMA) doped phosphor film based on cesium lead bromide quantum dot (۱܊۾ܛ۰ܚ-૜QD)and potassium fluorosilicate ࡷ૛ࡲ࢏ࡿ :૟࢔ࡹ૝ା(KSF) which is excited by a blue gallium nitride (GaN) laser diode (LD). 1.6-Gbps data rate is achieved by employing a non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. Themeasured bit error rate (BER) of 2.7× ૚૙ି૜ adheres to the standard threshold (૜. ૡ × ૚૙ି૜) of forward error correction(FEC). Moreover, the generated white light source has a high color rendering index of 93.8, a correlated colortemperature of 4435 K, and exhibits a Commission Internationale de l’Eclairage (CIE) 1931 chromaticity coordinate at(0.3556, 0.3520), which is close to the ideal CIE value of white light (0.3333, 0.3333). This work opens up excitingpossibilities for future high-speed indoor VLC and high-quality SSL.

The effect of laser pulse duration and fluence onDKDP crystal laser conditioning

Zhichao Liu, Feng Geng, Xiang-Yang Lei, Yaguo Li, Jian Cheng, yi zheng, Jian Wang, and Qiao Xu

DOI: 10.1364/AO.393097 Received 14 Apr 2020; Accepted 14 May 2020; Posted 14 May 2020  View: PDF

Abstract: The impact of laser conditioning (LC) fluence and pulse duration on nanosecond laser damage performance ofdeuterated potassium dihydrogen phosphate (DKDP) crystal is studied. The result shows that higher LC fluenceleads to a better damage resistance. In general, the sub-nanosecond LC effect is better than the nanosecond LC.However, in the range of 0.3 ns to 0.8 ns, the pulse duration has no obvious impact on the LC effect. An ultra-fastprocess characterization technology is employed to demonstrate that the cleaning effect of the protuberancedefects on the surface is one of sub-ns LC mechanism. Eventually, a couple of optimized LC parameters doubled themaximum damage threshold of DKDP crystal is proposed.

Invariant correlated optical fields driven by multiplicative noise

Mayra Morales, elizabeth Saldivia-Gomez, Juan Carlos Atenco Cuautle, Geovanni Arenas Munoz, Andrea Garcia Guzman, Patricia Martinez Vara, and Gabriel Martinez-Niconoff

DOI: 10.1364/AO.391029 Received 19 Feb 2020; Accepted 14 May 2020; Posted 15 May 2020  View: PDF

Abstract: We describe the evolution of a linear transmittance when it is perturbed withmultiplicative noise; the evolution is approximated through an ensemble of random transmittances which are used to generate diffraction fields. The randomness induces a competition mechanism between noise and transmittance and it is identified through the self-correlation function. Weshow that the geometry of the self-correlation function is a single peak that is preserved in the diffraction field which can be matched with like-localization effects. To corroborate the theoretical predictions, we perform an experiment using a linear grating where the noise is approximated by stochastic Markov chain. Experimental results are shown.

Wavelength Widths of the Optical Filters for OptimumSINR in WDM-VLC Systems

Luis Carlos Mathias, ALVARO CASTRO E SOUZA, and Taufik Abrao

DOI: 10.1364/AO.391219 Received 25 Feb 2020; Accepted 14 May 2020; Posted 19 May 2020  View: PDF

Abstract: Wavelength Division Multiplexing (WDM) permits the light spectrum to be divided into several parallelchannels allowing the capacity of visible light communication (VLC) systems to be increased. However,the spectral overlap due to the emission spread of the different colored LEDs generates crosstalk interference that can degrade the WDM-VLC. In this context, the adequate design of the optical filters (OFs) canmitigate this problem by limiting the spectral capture of the signals from the adjacent WDM channelsThus, a specific WDM-VLC model is developed to analyze the effects of the width of OFs on the signalinterference-plus-noise ratio (SINR). After confirming the quasi-concavity of the signal-to-interferenceplus noise ratio (SINR), the optimal widths of the OFs are determined by the SINR maximization usingiterative search (IS) algorithms considering fixed and mobile WDM-VLC systems. In the mobile communication case, it was confirmed that the maximum SINR points shift, implying in the variation of theoptimal OFs width according to the optical powers captured, which vary conforming to the distance andthe orientation of the LEDs, and/or receivers. Finally, a statistical approach is presented that allows thedesign of non-adaptive OFs.

Picometer-differential twice-exposed elementfor three-dimensional measurement withextremely long depth of field

Chao Li, Changhe Zhou, Lu Yunkai, Chaofeng Miao, Junjie Yu, Zhengkun Yin, and Jin Ye

DOI: 10.1364/AO.392306 Received 09 Mar 2020; Accepted 13 May 2020; Posted 14 May 2020  View: PDF

Abstract: The accuracy of optical three-dimensional (3D) shape measurement is alwaysinfluenced by the defocusing of projection or imaging system. In this paper, a novel opticalelement made by picometer-differential twice exposed, called picometer comb, is proposed togenerate the projection pattern for 3D shape measurement. Two interference fields withpicometer-scale period difference are recorded on a substrate to fabricate picometer comb bytwice-exposed laser holography, this element reconstructs the diffraction field which isessentially the interference between the holograms of two object waves with slight angle. Thispicometer comb has the advantage of generation of a stable light field distribution withextremely-long depth of field and small divergence angle. We demonstrate that thisdiffraction field provides a solution for non-defocusing 3D shape measurement.

Non-Null testing of aspheric surface using aquadriwave lateral shearing interferometer

Rui Zhang, Yongying Yang, Hongyang Zhao, Zijian Liang, Shengan Liu, and Jian Bai

DOI: 10.1364/AO.391263 Received 21 Feb 2020; Accepted 13 May 2020; Posted 14 May 2020  View: PDF

Abstract: A non-null aspheric surface testing system using a quadriwave lateral shearinginterferometer (QWLSI) based on the randomly encoded hybrid grating is proposed. Thesystem uses a non-null lens to partially compensate the longitude aberration of the asphericsurface, which is more versatile than the null configuration. QWLSI is a common-path andself-reference system, and requires no reference surface. Compared with the conventionalTwyman-Green interferometer (TGI), QWLSI has higher dynamic range, better anti-vibrationability and higher stability. For non-null testing system based on the QWLSI and TGI, thecompensation ranges of different aspheric surface and the allowable range of aspheric axialposition are analyzed under the condition that the interference fringe satisfies the Nyquistsampling theorem. The iterative reverse optimization retrace error correction algorithm basedon the exactly system modeling and the localization algorithm of aspheric axial position arediscussed. Simulation results show that the root mean square value of the aspheric surfacereconstruction residual is less than 1/200 λ, which indicates that the system has extremelyhigh theoretical testing accuracy. In the experiment, the QWLSI non-null system and theautocollimation null method with ZYGO interferometer agree with the testing results of thesame aspheric surface, which demonstrates the feasibility and accuracy of the QWLSIaspheric non-null testing method.

Scattering characteristics of a metasurface covered chiral sphere

zeeshan awan and Divitha SEETHARAMDOO

DOI: 10.1364/AO.392709 Received 13 Mar 2020; Accepted 13 May 2020; Posted 14 May 2020  View: PDF

Abstract: An analysis about the scattering characteristics of a chiral sphere covered with a metasurface has been presented. The electromagnetic fields inside and outside of a metasurface covered chiral sphere have been expanded in terms of spherical vector wave functions. The influences of chirality and surface impedance of a metasurface upon the scattering cross section have been investigated. It is studied that a specific type of metasurface can be used to significantly enhance and diminish the back scattering cross section from a chiral sphere having fixed value of chirality but different radii. It is also observed that a chiral sphere having chirality close to unity can not be apparently cloaked using lossless metasurface as compared to a dielectric sphere both having same electromagnetic and geometrical parameters. A comparative study of the scattering characteristics of a metasurface covered realistic chiral and low-loss Teflon dielectric spheres has also been developed. It is found that a metasurface covered realistic chiral sphere can be used to significantly reduce the scattering as compared to scattering from a metasurface covered Teflon sphere at a specific angle provided that both spheres having same radii and surface reactance.

Degree of Polarization of Luminescence from III-V Materials as a Function of Strain

Daniel T. Cassidy and Jean-Pierre Landesman

DOI: 10.1364/AO.394624 Received 07 Apr 2020; Accepted 13 May 2020; Posted 14 May 2020  View: PDF

Abstract: Experimentally it is known that the degree of polarization (DOP) of luminescence is a sensitive function of strain in III-V materials. It has been assumed that DOP = $-K_e\,(e_1-e_2)$ and that the rotated degree of polarization (ROP) = $2\,K_e\,e_6$, where $K_e$ is a positive calibration constant, $e_1$ and $e_2$ are the normal components of strain along perpendicular `1' and `2' directions, and $e_6 = e_{12}$ is the tensor shear strain. $K_e$ has been measured experimentally for GaAs and InP.In this paper, the results of a simple analytic determination of expressions for DOP as a function of strain are presented. Given the wide ranges reported for the strain deformation potentials $b$ and $d$, it is not possible to give definitive and meaningful numerical values for expressions for DOP and $K_e$. However, the sensitivity of DOP to strain suggests that it might be possible to design simple experiments to provide accurate values for the deformation potentials. The $b$ and $d$ deformation potentials might not be independent. For the results presented here and in the limit of isotropic material, an isotropic result for the DOP is found if $d=\sqrt{3}\,b$.

Output characteristics of Pr:YAlO3 and Pr:YAG laserspumped by high-power GaN laser diodes

Shogo Fujita, Hiroki Tanaka, and Fumihiko Kannari

DOI: 10.1364/AO.394792 Received 08 Apr 2020; Accepted 12 May 2020; Posted 12 May 2020  View: PDF

Abstract: We investigated whether Pr:YAlO3 and Pr:Y3Al5O12 (YAG) can work as gain media for high-power visible lasers andreplace Trivalent Praseodymium (Pr)-doped fluoride crystals, with particular focus on thermal loading resistivity.Pr:YAlO3 exhibits a high laser gain at 747 nm, and we obtained a maximum output power of 1.2 W and a slopeefficiency of 26.7 % with high-power GaN laser diode pumping. Excited state absorption and large phonon energyhinder laser oscillation of Pr:YAG at room temperature. We obtained 616-nm laser oscillation of Pr:YAG at 40 K.Furthermore, we achieved a visible laser with Pr:YAG ceramics for the first time. The maximum output power is~30 mW with a slope efficiency of ~0.7 %.

IR characterization of plant leaves, endemic to semi-tropicalregions, in two senescent states

Roberto Barragan-Campos, Jesus Castrellon-Uribe, Guillermo García-Torales, and Antonio Rodríguez Rivas

DOI: 10.1364/AO.389005 Received 22 Jan 2020; Accepted 12 May 2020; Posted 18 May 2020  View: PDF

Abstract: We are developing a robust and economic electro-optical remote sensing methodology to monitor the state of healthand hydration of trees, endemic to subtropical regions. We measured reflectance spectra with FTIR of three samplesof two different oak trees. We find that spectral bands suitable for monitoring the state of the health and senescence ofthe oak include intervals around 0.9 μm and 1.8 μm. The easiest and the most cost-effective strategy would be toimplement an electro-optical remote sensing radiometric system featuring a commercial camera incorporating atraditional CCD/CMOS detector, and a wide-band transmission filter, from about 0.8 to 1 μm.

Multi-planar Full-Field Blur Correction Methodfor Infrared Microscopy Imaging

Anselmo Jara, Sergio Torres, and Guillermo Machuca

DOI: 10.1364/AO.387120 Received 29 Jan 2020; Accepted 12 May 2020; Posted 21 May 2020  View: PDF

Abstract: We propose a 3-D full-field focusing method for microscopic mid-wave infrared(MWIR) imagery. The method is based on the experimental estimation of a confinedvolumetric vision microscope Point Spread Functions (PSFs). The technique employs ourwell-known Constant Range based non-uniformity correction algorithm, as a pre-processingstep, and then, an iterative in z axis Fourier-based de-convolution. The technique’s ability tocompensate for localized blur is demonstrated using two different real MWIR microscopicvideo sequences, captured from two microscopic living organisms using a Janos-SofradirMWIR microscopy setup. The performance of the proposed algorithm is assessed on real andsimulated noisy infrared data by computing the root-mean-square error and the roughnesslaplacian pattern indexes, which is specifically developed for the present work.

Broadband second-harmonic and sum-frequency generation with a long-wave infrared laser in AgGaGe5Se12

Yi Chen, baoquan yao, Haixin Wu, Youbao NI, gaoyou liu, Tongyu Dai, Xiaoming Duan, and Youlun Ju

DOI: 10.1364/AO.394970 Received 10 Apr 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: Using an 8 μm long-wave infrared laser as the fundamental wave, we achieved second harmonic generation (SHG) and sum-frequency generation (SFG) simultaneously in AgGaGe5Se12, and obtained a 4 μm laser output. Among them, SHG was achieved in the 173 nm spectral range of the fundamental wave, which was consistent with theoretical calculations. The average power of the obtained 4 μm laser was 41 mW, corresponding to an optical-to-optical conversion efficiency of 3.2%. The measured temperature acceptance bandwidth (LδT) (FWHM) was 50 K·cm, the angular acceptance bandwidth (Lδθ) (FWHM) was 13.3 mrad·cm, and the average absorption coefficient in the wavelength range of 2.5-11.3 μm was 0.07cm-1. Besides, the spectral acceptance bandwidth (Lδλ) of fundamental wave in AgGaGe5Se12 SHG and the spectral gain bandwidth of frequency down-conversion in AgGaGe5Se12 were calculated. In view of the small absorption coefficient, the large temperature acceptance bandwidth, and the large spectral gain bandwidth, we conclude that AgGaGe5Se12 is a suitable nonlinear crystal for high-power mid/long-wave infrared lasers and frequency conversions of nanosecond-femtosecond infrared lasers. These results are conducive to the further development of AgGaGe5Se12 lasers.

Flight model characterization of the wide-field off-axistelescope for the MATS satellite

Woojin Park, Arvid Hammar, Soojong Pak, Seunghyuk Chang, Joerg Gumbel, Linda Megner, Ole Christensen, Jordan Rouse, and Dae Wook Kim

DOI: 10.1364/AO.392187 Received 17 Mar 2020; Accepted 11 May 2020; Posted 14 May 2020  View: PDF

Abstract: We present optical characterization, calibration, and performance tests of the Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS) satellite, which for the first time for a satellite appliesa linear-astigmatism-free confocal off-axis reflective optical design. Mechanical tolerances of the telescope were investigated using Monte-Carlo methods and single-element perturbations. The sensitivityanalysis results indicate that tilt errors of the tertiary mirror and a surface RMS error of the secondarymirror mainly degrade optical performance. From the Monte-Carlo simulation, the tolerance limits werecalculated to ±0.5 mm, ±1 mm, and ±0.15° for decenter, despace, and tilt, respectively. We performed characterization measurements and optical tests with the flight model of the satellite. Multi-channel relativepointing, total optical system throughput, and distortion of each channel were characterized for end-users.Optical performance was evaluated by measuring modulation transfer function (MTF) and point spreadfunction (PSF). The final MTF performance is 0.25 MTF at 20 lp/mm for the ultraviolet channel (304.5 nm),and 0.25 - 0.54 MTF at 10 lp/mm for infrared channels. The salient fact of the PSF measurement of thissystem is that there is no noticeable linear astigmatism detected over wide field of view (5.67° × 0.91°).All things considered, the design method showed great advantages in wide field of view observationswith satellite-level optical performance.

Aerial Infrared Target Tracking Method Based on KCFfor Frequency-Domain Scale Estimation

Kai Zhang, Gaole Wei, Xi Yang, Shaoyi lI, and Jie Yan

DOI: 10.1364/AO.390619 Received 18 Feb 2020; Accepted 11 May 2020; Posted 11 May 2020  View: PDF

Abstract: The kernel correlation filter (KCF) tracking algorithm encounters the issue of tracking accuracy degradation due to large changes in scale and rotation of aerial infrared targets. Therefore, this paper proposesa new scale estimation KCF-based aerial infrared target tracking method, which can extract scale featureinformation of images in the frequency domain based on the distribution characteristics and change lawsof frequency domain energy. In addition, the proposed method can improve the accuracy of target scaleinformation estimation. First, the KCF tracking algorithm is used to obtain the target position. Then,spectral eigenvalues are calculated as eigenvectors, and the frequency-domain rotation scale invariance isadopted to extract the eigenvector between two frames as the target rotation change information. Reverserotation is performed on the current frame spectrum map for isolating the effects of target rotation onscale information estimation. Then, the current target scale is estimated on the basis of the eigenvectorsbetween the adjacent frames. Finally, the length-to-width ratio and the scale of the tracking box are updated on the basis of the target rotation information, which improves the adaptability of the tracking boxto changes in the target scale and rotation. The results indicate that the proposed algorithm is suitable forstable tracking of target scales and rapid changes in attitudes. The average tracking accuracy and the average success rate of the algorithm are 0.954 and 0.782, which represent improvements of 5.3% and 18.9%,respectively, compared with the KCF algorithm. The average tracking success rate is improved by 4.1%compared with the discriminative scale space tracker algorithm, and the average tracking performance isbetter than that of related filter tracking algorithms based on other scale estimation methods.

Full-polarization wavefront shaping for imagingthrough scattering media

Runze Li, Tong Peng, Meiling Zhou, Xianghua Yu, Junwei Min, Yan long Yang, and Baoli Yao

DOI: 10.1364/AO.391909 Received 04 Mar 2020; Accepted 11 May 2020; Posted 11 May 2020  View: PDF

Abstract: The scattering effect when lights pass through refractive-index inhomogeneous media such as atmosphere orbiological tissues will scramble the light wavefront into speckles and impede the optical imaging. Wavefront shapingis an emerging technique for imaging through scattering media by addressing a correction on the disturbedwavefront. Besides the phase and amplitude, the polarization of the output scattered light will also become spatiallyrandomized in some cases. The recovered image quality and fidelity benefit from correcting as much distortions ofscattered lights as possible. Liquid crystal spatial light modulators (LC-SLM) are widely used in the wavefrontshaping technique since they can provide exceedingly number of controlled modes and thereby high-precisionwavefront correction. However, due to the working principle of LC-SLMs, the wavefront correction is restricted inonly one certain linear polarization state, resulting in only image information in the right polarization retrieved whilethe information in the orthogonal polarization lost. In this paper, we developed a full-polarization wavefrontcorrection system to shape the scattered light wavefront in two orthogonal polarizations with a single LC-SLM. Thelight speckles in both polarizations are corrected for retrieving the whole polarization information and the faithfulimages of objects. As demonstrated in the experiments, the focusing intensity can be increased by full-polarizationwavefront correction, and the images of objects in arbitrary polarization states can be retrieved and the polarizationstate of object’s light can also be recognized.

Radial-shearing Interferometric imaging with TheonKepler bifocal telescope

Yaocun Li, LIU HUIYA, Jing Xie, and Junyong Zhang

DOI: 10.1364/AO.392574 Received 11 Mar 2020; Accepted 11 May 2020; Posted 11 May 2020  View: PDF

Abstract: A method of radial-shearing interferometric imaging based on Theon-Kepler bifocal telescope is proposed. TheTheon-Kepler bifocal telescope system consists of two identical Theon photon sieves with bifocal spots. The shortfocal length of the first photon sieve coincides with the long focal length of the second photon sieve. At the sametime, the first focal length coincides with the second short focal length. This setup naturally constitutes two sets of4f systems. When a plane wave is incident on the bifocal telescope, two plane waves of different sizes are emitted.These two beams undergo radial-shearing interference. The common-path setup of this new type of radialshearing interferometer is very simple, with low requirements for the experimental environment and strong antiinterference ability.

640-nm Pr:YLF regenerative amplifier seeded by gainswitched laser diode pulses

Yusaku Hara, Shogo Fujita, Yuta Shioya, and Fumihiko Kannari

DOI: 10.1364/AO.395801 Received 21 Apr 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: We demonstrated the regenerative amplification of picosecond laser pulses generated by a gain-switched laser diode at 640 nmwith a Pr:YLF crystal that was continuously pumped by a multimode blue laser diode. A 0.7-pJ seed pulse energy sufficientlysuppressed the self-oscillation in the amplifier. The amplified pulse energy reached 33 μJ at a repetition rate of 10 kHz. The spatialbeam quality was nearly TEM00. We also demonstrated second and third harmonic generation and obtained 320- and 213-nm pulseenergies of 18 and 0.83 μJ at 10 kHz.

Aerogel scattering filters for cosmic microwave background observations

Thomas Essinger-Hileman, Charles Bennett, Lance Corbett, Haiquan Guo, Kyle Helson, Tobias Marriage, Mary Ann Meador, Karwan Rostem, and Edward Wollack

DOI: 10.1364/AO.390408 Received 25 Feb 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and sub-millimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultra-low-density (typically < 150 mg/cm³) aerogel substrate provides an index of refraction as low as 1.05, removing the need for anti-reflection coatings and allowing for broadband operation from DC to above 1 THz. The size distribution of the scattering particles can be tuned to provide a variable cutoff frequency. Aerogel filters with embedded high-resistivity silicon powder are being produced at 40-cm diameter to enable large-aperture cryogenic receivers for cosmic microwave background polarimeters, which require large arrays of sub-Kelvin detectors in their search for the signature of an inflationary gravitational-wave background.

Explanation of the Competition between O- and E-Wave induced Stimulated Raman and Supercontinuum in Calcite under Ultrafast Laser Excitation

SHAH FAISAL MAZHAR, Henry Meyer, Tyler Samuels, Mikhail Sharonov, Lingyan Shi, and Robert Alfano

DOI: 10.1364/AO.393932 Received 03 Apr 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: Key optical properties of calcite were measured to unravel the difference between Stimulated Raman Scattering (SRS) and Self-Phase Modulation (SPM) for Supercontinuum (SC) for Ordinary (O) wave and Extraordinary (E) wave. These properties are Group velocity dispersion (GVD), walk-off, Spontaneous Raman Spectra and cross-section, optical 1086 cm^(-1) phonon linewidth, nonlinear susceptibility (χ3), steady-state and transient SRS, and SC caused from SPM. These are investigated for O-waves and E-waves from a 2.7 cm thick Calcite crystal. Using 390 fs pulses (~ 0.8 μJ pulse energy) at 517 nm, the O-wave produced a stronger sharp SRS peak at 1086 cm^(-1) and a weaker SC spectrum in the visible range than E-wave. The salient difference found between the O- and E-waves for SRS and SPM in calcite is attributed to the larger Raman cross-section and the size of nonlinear susceptibility (χ3) for O-waves as compared to E-waves.

Fabrication of infrared wire-grid polarizer viaanisotropical Si etching in KOH solution

Itsunari Yamada and Rei Yoshida

DOI: 10.1364/AO.392883 Received 17 Mar 2020; Accepted 10 May 2020; Posted 19 May 2020  View: PDF

Abstract: An infrared wire-grid polarizer was fabricated using two-beam interference lithography, anisotropical etching of Siin KOH solution, and thermal evaporation of Al. Consequently, an infrared wire-grid polarizer with a 1-µm-pitchand 195-nm-thick Al grating could be fabricated on the Si grating. The transmittance of transverse magnetic (TM)polarization was greater than 48% at 10-μm wavelength, and the extinction ratio was over 22 dB at thiswavelength. This polarizer can be fabricated at a much lower cost and using simpler fabrication processescompared to those for conventional infrared polarizers fabricated using dry etching.

Towards cavity-free ground state cooling of anacoustic-frequency silicon nitride membrane

Christian Pluchar, Aman Agrawal, Edward Schenk, and Dalziel Wilson

DOI: 10.1364/AO.394388 Received 03 Apr 2020; Accepted 10 May 2020; Posted 20 May 2020  View: PDF

Abstract: We demonstrate feedback cooling of a millimeter-scale,40 kHz SiN membrane from room temperature to 5 mK(3000 phonons) using a Michelson interferometer, anddiscuss the challenges to ground state cooling withoutan optical cavity. This advance appears within reach ofcurrent membrane technology, positioning it as a compelling alternative to levitated systems for quantumsensing and fundamental weak force measurements.

Measuring the topological charge of orbitalangular momentum radiation in single-shot bymeans of wavefront intrinsic curvature

Bruno Paroli, Mirko Siano, and Marco Potenza

DOI: 10.1364/AO.392341 Received 10 Mar 2020; Accepted 09 May 2020; Posted 11 May 2020  View: PDF

Abstract: We show a method to measure the topological charge of orbital angular momentumradiation in single-shot by exploiting the intrinsic local curvature of the helicoidal wavefront.The method is based on oriented Hartmann cells in a suitable detection scheme. We showexperimental results and propose a Shack-Hartmann configuration with sectored photodiodesto improve resolution and detection time. The method can be applied for telecommunicationapplications in the far field of the radiation beam and more in general to measure the topologicalcharge from a small portion of the radiation wavefront.

Ultra-fast all-optical 2-to-4 decoder based on thephotonic crystal structure

Mohammad-Javad Maleki, Mohammad Soroosh, and Ali Mir

DOI: 10.1364/AO.392933 Received 16 Mar 2020; Accepted 09 May 2020; Posted 11 May 2020  View: PDF

Abstract: In this paper, a photonic crystal structure based on nonlinear cavities has been proposed for improvingthe time response of the 2-to-4 decoder. The structure includes an array of chalcogenide rods with the air gap inwhich the spatial period of rods is 500 nm. The radius of the fundamental rods is assumed to be 125 nm whichresults in a photonic bandgap of 1092-1724 nm at TM mode. Three cavities including the nonlinear rods with aKerr coefficient of 10-14 m2/W drop the incoming waves concerning the amount of optical intensity. The finitedifference time domain method was used to calculate the components of electric and magnetic fields throughoutthe structure. The time analysis of the device shows the rise time is equal to 200 fs which is less than one for theprevious structures. The area of 110 µm2 and the margins of 4% and 88% for logics 0 and 1 are other advantages ofthe proposed structure. Based on the obtained results, it was proved that the performance of the 2-to-4 photoniccrystal-based decoder has been improved in this work.

Improved multi-resolution foveated laparoscope withreal-time digital transverse chromatic correction

Jeremy Katz, Sangyoon Lee, and Hong Hua

DOI: 10.1364/AO.393088 Received 17 Mar 2020; Accepted 09 May 2020; Posted 11 May 2020  View: PDF

Abstract: A multi-resolution foveated laparoscope (MRFL) with autofocus and zooming capabilities was previously designedto address the limiting trade-off between spatial resolution and field of view during laparoscopic minimallyinvasive surgery. The MRFL splits incoming light into two paths enabling simultaneous capture of the full surgicalfield and a zoomed-in view of the local surgical site. A fully functional prototype was constructed to demonstrateand test the autofocus, zooming capabilities, and clinical utility of this new laparoscope. The test of the prototype inboth dry lab and animal models was successful, but it also revealed several major limitations of the prototype. Inthis paper, we present a brief overview of the aforementioned MRFL prototype design and results, and theshortcomings associated with its optical and mechanical designs. We then present several methods to address theshortcomings of the existing prototype with a modified optical layout and redesigned mechanics. Theperformances of the new and old system prototypes are comparatively analyzed in accordance with the designgoals of the new MRFL. Finally, we present and demonstrate a real-time digital method for correcting transversechromatic aberration to further improve the overall image quality, which can be adapted to future MRFL systems.

Experimental Characterization of a ModeSeparating Photonic Lantern for ImagingApplications

Mary Salit, Jeffrey Klein, and Lisa Lust

DOI: 10.1364/AO.390715 Received 17 Feb 2020; Accepted 08 May 2020; Posted 15 May 2020  View: PDF

Abstract: The point spread function (PSF) of an imaging system has a minimum size, a“diffraction limit,” determined by the size of the limiting aperture. Image features smallerthan this PSF can be, in a conventional imaging system, resolved only if the intensity noise islow enough to permit deconvolution. Measuring image-plane intensity as a function of spatialmode rather than position has the potential to reduce the quantum noise and thus enablesubdiffraction resolution at lower light levels or in shorter measurement times than can betolerated with conventional imaging. Here we examine experimental measurements ofintensity and intensity noise as a function of spatial mode. We characterize the impulseresponse of a spatial mode coupling measurement at the focal plane of an imaging system tothe position of a far-field point source. Our measured intensity noise scales with power in away that suggests photon shot noise is a significant contributor, and we find that the signal-tonoise ratio of our modal-basis measurement of point source position exceeds that of aconventional image plane pixel array for subdiffraction objects imaged against darkbackgrounds. The mode coupling is measured with a custom mode-separating fiber photoniclantern. Photonic lanterns and equivalent structures constructed from rigid waveguides aresimple, passive devices which lend themselves to real-world implementations of thismeasurement scheme with minimal size, weight, power, and cost.

Design and experiment of the soft-edgeaperture with high light energy utilizationefficiency and uniformity

Liwei Liu, Lifang Shi, Axiu Cao, pang hui, Wei Yan, Yingfei Pang, Li Xue, wen Liu, and Qiling Deng

DOI: 10.1364/AO.389992 Received 07 Feb 2020; Accepted 08 May 2020; Posted 08 May 2020  View: PDF

Abstract: The soft-edge aperture can be used to suppress the diffraction effect to achievehigh beam quality. In this study, an optimal design theory was proposed to design a serratedsoft-edge aperture (SSA) that could produce a beam with high light energy utilizationefficiency and uniformity. Two parameters of the peak-to-average ratio (PAR) and the lightenergy utilization efficiency were introduced as the criteria for evaluating the soft-edgeaperture performance. By analyzing the influence of the shape, number and height of theserrated structure on the beam propagation performance, the relationship between the PAR,light energy utilization efficiency and the structure parameters of the aperture wereestablished. After that, an optimal design method of SSA was proposed based on the theory ofFresnel diffraction. The designed SSA was fabricated by lithography, and an opticalconfiguration was constructed to test the effect of the aperture. In the experiment, the lightenergy utilization efficiency is greater than 92%, the PAR is less than 1.4, and the root meansquare error of PAR curve is 0.3502, indicating that the beam maintains high light energyutilization efficiency and uniformity during propagation.

A Sapphire Fabry–Perot Interferometer for HighTemperature Pressure Sensing

Zhifeng Wang, Jintao Chen, Heming Wei, Huanhuan Liu, Zhangwei Ma, Na Chen, Zhenyi Chen, Tingyun Wang, and Fufei Pang

DOI: 10.1364/AO.393353 Received 20 Mar 2020; Accepted 08 May 2020; Posted 08 May 2020  View: PDF

Abstract: An adhesive-free encapsulation sapphire Fabry–Perot interferometer (FPI) is proposed and demonstrated forhigh-temperature pressure measurements. The sapphire FPI sensor is packaged by zirconia ferrules and a zirconiasleeve, which is easy to be configured and low in cost. Owing to this packaging technology, the sapphire FPI sensorpresents good stability and high temperature resistance. The pressure and temperature properties of the sapphireFPI sensor are investigated within a temperature range from -50 °C to 1200 °C and a pressure range from 0.4 to 4.0MPa. Experimental results show the FPI have a temperature sensitivity of pm/°C and still works as thetemperature is up to 1200 °C. Meanwhile, the wavelength shift of the sapphire FPI versus the applied pressure islinear at each tested temperature. The pressure sensitivity is measured to be 1.20 nm/MPa at 1200 °C, and thelinear response shows the proposed sensor has good repeatability within 0.4-4.0 MPa. Such a sapphire FPI sensorhas potential applications in engineering areas, such as oil industry, and gas boiler.

Silicon nitride waveguide devices based on GRIN lensesimplemented by subwavelength silicon grating metamaterials

Seyed Hadi Badri and Mohsen Mohammadzadeh Gilarlue

DOI: 10.1364/AO.393501 Received 24 Mar 2020; Accepted 07 May 2020; Posted 11 May 2020  View: PDF

Abstract: The rapid development of photonic integrated circuits demands the design of efficient and compactwaveguide devices such as waveguide tapers and crossings. Some components in the silicon nitride (SiN)waveguide platform are superior to their counterparts in the silicon waveguide platform. Designingcompact SiN waveguide taper and crossing is crucial to reduce the size of SiN photonic components. Inthis paper, we utilize the focusing property of the Luneburg lens to design an SiN taper connecting a 10µm-wide waveguide to a 1 µm-wide waveguide. Three-dimensional full-wave simulations indicate thatthe designed 13 µm-long taper has an average transmission efficiency of 92% in the wavelength range of1500-1600 nm. We also present an in-plane SiN waveguide crossing based on the imaging property of thesquare Maxwell’s fisheye lens designed with quasi-conformal transformation optics. The designedwaveguide crossing occupies a compact footprint of 5.65 µm × 5.65 µm while its average insertion loss is0.46 dB in the bandwidth of 1500-1600 nm. To the best our knowledge, the designed SiN waveguidetaper and crossing have the smallest footprints.

Skeleton extraction and inpainting from poorbroken ESPI fringe with M-net convolutional neuralnetwork

Chenxiu Liu, Chen Tang, Min Xu, Fugui Hao, and Zhenkun Lei

DOI: 10.1364/AO.391501 Received 25 Feb 2020; Accepted 07 May 2020; Posted 08 May 2020  View: PDF

Abstract: Extracting skeletons from fringe patterns is the key to the fringe skeleton method, which is used to extract phaseterms in electronic speckle pattern interferometry (ESPI). Because of massive inherent speckle noise, extractingskeletons from poor broken ESPI fringe patterns is challenging. In this paper, we propose a method based on amodified M-net convolutional neural network for skeleton extraction from poor broken ESPI fringe patterns. In ourmethod, we pose the problem as a segmentation task. The M-net does excellent segmentation and we modify itsloss function to suit our task. The broken ESPI fringe patterns and corresponding complete skeleton images areused to train the modified M-net. The trained network can extract and inpaint the skeletons simultaneously. Weevaluate the performance of the network on two groups of computer-simulated and two groups of experimentallyobtained ESPI fringe patterns. Two related recent methods, the gradient vector fields based on variational imagedecomposition (VID-GVF) and the U-net based method, are compared with our method. The results demonstratethat our method can obtain accurate, complete, and smooth skeletons in all cases, even where fringes are broken. Itoutperforms the two compared methods quantitatively and qualitatively.

Improving Carrier Transport in AlGaN Deep UltravioletLight-Emitting Diodes Using a Strip-in-a-BarrierStructure

Ravi Teja Velpula, Barsha Jain, Thang Bui, FATEMEH MOHAMMADI SHAKIBA, Jeffrey Jude, Moses Tumuna, Hoang-Duy Nguyen, Trupti Ranjan Lenka, and Hieu Nguyen

DOI: 10.1364/AO.394149 Received 31 Mar 2020; Accepted 07 May 2020; Posted 08 May 2020  View: PDF

Abstract: This paper reports on the illustration of electron blocking layer (EBL) free AlGaN light-emitting diodes (LEDs)operating in the deep ultraviolet (DUV) wavelength at ~270 nm. In this work, we demonstrated that the integrationof an optimized thin undoped AlGaN strip layer in the middle of the last quantum barrier (LQB) could generateenough conduction band barrier height for the effectively reduced electron overflow into the p-GaN region.Moreover, the hole injection into the multi-quantum well active region is significantly increased due to a large holeaccumulation at the interface of the AlGaN strip and the LQB. As a result, the internal quantum efficiency andoutput power of the proposed LED structure has enhanced tremendously compared to conventional p-type EBLbased LED structure.

Q-Switched YDFL Generation by MAX Phase SaturableAbsorber

Ahmad Razif Muhammad, A.A.A. Jafry, Arni Munira Markom, A. H. A. Rosol, Sulaiman Wadi Harun, and Preecha Yupapin

DOI: 10.1364/AO.393719 Received 26 Mar 2020; Accepted 07 May 2020; Posted 07 May 2020  View: PDF

Abstract: A stable Q-switching pulse train in the Ytterbium-doped fiber laser (YDFL) cavity was generated by using MAX-PVAthin film, which composes of layered ternary metal carbide (Ti3AlC2) as a passive saturable absorber (SA) andincorporates it in the laser cavity. MAX-PVA SA was fabricated using drop a solution-casting method. A self-startedpulse train with a maximum repetition rate of 92 kHz and a decreasing pulse width (from 3.5 µs to 1.92 µs) withincreasing pump power (from 214 mW to 260mW) was produced. The laser’s stability was highlighted with a highsignal-to-noise ratio (SNR) of 4552 dB was obtained at the highest repetition rate of 2792 kHz. Henceforth, thisfinding suggests that MAX-SA could be used as an alternative to other SAs in Q-switched fiber laser.

Identification of microplastics in a large water volumeby integrated holography and Raman spectroscopy

Tomoko Takahashi, Zonghua Liu, Thangavel Thevar, Nicholas Burns, Sumeet Mahajan, Dhugal Lindsay, John Watson, and Blair Thornton

DOI: 10.1364/AO.393643 Received 25 Mar 2020; Accepted 07 May 2020; Posted 07 May 2020  View: PDF

Abstract: A non-contact method for identification of sparselydistributed plastic pellets is proposed by integrating holography and Raman spectroscopy in thisstudy. Polystyrene (PS) and poly(methyl methacrylate)(PMMA) resin pellets with a size of 3 mm located ina 20 cm water channel were illuminated using a collimated continuous wave laser beam with a diameter of4 mm and wavelength of 785 nm. The same laser beamwas used to take a holographic image and Raman spectrum of a pellet to identify the shape, size and composition of material. Using the compact system, the morphological and chemical analysis of pellets in a largevolume of water was performed. The reported novelmethod demonstrates the potential for compact noncontact continuous in situ monitoring of microplasticsin water without collection and separation.

Camera calibration based on the commonpole-polar properties between two coplanarcircles with various positions

Sixin Liang and Yue Zhao

DOI: 10.1364/AO.388109 Received 15 Jan 2020; Accepted 07 May 2020; Posted 08 May 2020  View: PDF

Abstract: Computer vision camera calibration is widely performed using parallel circles.Various cases of two coplanar circles are algebraically explained, proving the common pole islocated at the line at infinity for all relative positions, and the corresponding polar passesthrough the centers of the two circles. The two common poles of the two coplanar circles arethe points at infinity when concentric; one common pole of the two coplanar circles is a pointat infinity when non-concentric. Accordingly, the vanishing line can be obtained by using thecommon pole-polar properties of two groups of two coplanar circles, and the camerasintrinsic parameters are solved according to the constraints between the image of the circularpoints and the imaged absolute conic. The camera calibration can be solved using only threeimages of two coplanar circles. Simulation and experiments verify the proposed algorithmsare effective.

Plastic optical fiber sensor for temperature-independent high-sensitivity detection of humidity

Nianbing Zhong, Xin Xin, Huimin Liu, XIAOYI YU, HAIXING CHANG, Bing Tang, DENGJIE ZHONG, M Zhao, Hongmin Zhang, and JIE ZHAO

DOI: 10.1364/AO.391090 Received 20 Feb 2020; Accepted 06 May 2020; Posted 08 May 2020  View: PDF

Abstract: A simple U-shaped plastic optical fiber evanescent-wave sensor was fabricated for a temperature-independent highly sensitive detection of humidity. The sensing region of the sensor was subjected to five cycles of heating–cooling to improve the temperature independence. The effects of the polyimide (PI) coating thickness, number of graphene oxide (GO) coating layers, and alternate PI–GO coating sequence were investigated to optimize the sensitivity. The fabricated sensor exhibited a high temperature independence and very good sensitivity of 0.17 × 10−2 (% relative humidity)−1 in the temperature range of 10 to 70 °C.

Contrast analysis in two-beam laser interferencelithography

David Miller, Adam Jones, and Robert McLeod

DOI: 10.1364/AO.393741 Received 26 Mar 2020; Accepted 06 May 2020; Posted 07 May 2020  View: PDF

Abstract: Interference lithography enables large area, sub-µm, periodic patterning without photomasks or projection lithography tools. We show that the fundamental design variable of interference lithography, theoptical contrast, enforces coupled constraints on source coherence, beam pointing stability, field size, polarization state, and intensity balance between beams. The analysis enables selection and alignmenttolerance of components to meet a particular design requirement. In particular, the analysis reveals thatgrating beam splitters are significantly less sensitive to beam pointing and polarization misalignmentthan plate beam splitters.

Imaging Through a Homogeneous CircularCylinder: The Role of Virtual Caustics, RainbowGlare Points, and Image Fragmentation

James Lock

DOI: 10.1364/AO.390330 Received 13 Feb 2020; Accepted 06 May 2020; Posted 06 May 2020  View: PDF

Abstract: Small air bubbles on the rear inside surface of a water-filled cylinder, near its edges, appear horizontallyelongated, joined in pairs, and take on color. Similarly, if an extended object is sufficiently close to the waterfilled cylinder, three images of the object are seen when looking through the cylinder. The center image joinsonto the left or right image as the observer moves his or her head back and forth in front of the cylinder. The firstobservation is explained in terms of glare points of light, and the real and virtual parts of the external caustic ofthe light transmitted through the water-filled cylinder. The second observation is explained as an example ofBerry’s caustic touching theorem which describes the topological method of fragmentation of an object’s imageinto multiple images. For the situation studied here, an imaginary cylindrical aberration caustic of the waterfilled cylinder decomposes object space into a three-ray region sandwiched between two one-ray regions. As anextended object crosses the caustic boundary from one of the one-ray regions into the three-ray region, animage-pair creation event occurs, which is followed by an image-pair disconnection event producing the threeimages. Similarly, when the extended object crosses the caustic boundary from the three-ray region into one ofthe one-ray regions, an image-pair merging event occurs, which is followed by an image-pair annihilation eventproducing the one remaining image.

Variable Precision Depth Encodingfor 3D Range Geometry Compression

Matthew Finley, Jacob Nishimura, and Tyler Bell

DOI: 10.1364/AO.389913 Received 05 Feb 2020; Accepted 06 May 2020; Posted 07 May 2020  View: PDF

Abstract: State-of-the-art 3D range geometry compression algorithms that utilize principles ofphase-shifting perform encoding with a fixed frequency; therefore, it is not possible to encodeindividual points within a scene at various degrees of precision. This paper presents a novelmethod for accurately encoding 3D range geometry within the color channels of a 2D RGBimage that allows the encoding frequency—and therefore the encoding precision—to be uniquelydetermined for each coordinate. The proposed method can thus be used to balance betweenencoding precision and file size by encoding geometry along a statistical distribution. Forexample, a normal distribution allows for more precise encoding where the density of data ishigh and less precise encoding where the density of data is low. Alternative distributions may befollowed to produce encodings optimized for specific applications. In general, the nature of theproposed encoding method enables the precision to be freely controlled at each point or centeredaround identified features of interest, ideally enabling this method to be used within a wide rangeof applications.

JCW: Zernike monomials in wide field of view optical designs

Tim Johnson and Jose Sasian

DOI: 10.1364/AO.392305 Received 11 Mar 2020; Accepted 05 May 2020; Posted 05 May 2020  View: PDF

Abstract: Zernike polynomials are universal in optical modelling and testing of wavefronts, however their polynomialbehavior can cause a misinterpretation of individual aberrations. Wavefront profiles described by Zernikepolynomials contain multiple terms with different orders of its pupil radius (ρ). Zernike polynomials are a sum ofhigh and low orders of ρ to minimize the RMS wavefront error and to preserve orthogonality. Since the low orderpolynomials are still contained in the net Zernike sum, there is redundancy in individual monomials. Monomialaberrations, also known as Seidel or Primary aberrations, are useful in studying an optical design’s complexity,alignment, and field behavior. Zernike polynomial aberrations reported by optical design software, are notindicative of individual (monomial) aberrations in wide field of view designs since the low order polynomial arecontaminated by higher order terms. An aberration node is the field location where an individual (monomial)aberration is zero. In this paper, a matrix method is shown to calculate the individual monomial aberrations giventhe set of Zernike polynomials. Monomial aberrations plotted as a function of field angle (H) indicate the field order(Hn) and the location of true aberration nodes. Contrarily, Zernike polynomial vs field (ZvF) plots can indicate falseaberration nodes, due to the polynomial mixing of high and low order terms. Accurate knowledge of the monomialaberration nodes, converted from Zernike polynomials, provides the link between a raytrace model or lab wavefrontmeasurement and Nodal Aberration Theory (NAT). This method is applied to two different optical designs: 1) 120°circular FOV fish-eye lens and 2) 120° x 4° rectangular FOV, off-axis, freeform 4-mirror design.

Phase retrieval and adaptive optics correctionfor systems with diffractive surfaces

Emily Finan and Thomas Milster

DOI: 10.1364/AO.392643 Received 18 Mar 2020; Accepted 05 May 2020; Posted 05 May 2020  View: PDF

Abstract: Adaptive Optics (AO) is a powerful technique for correcting for extrinsicaberrations, such as those caused by atmospheric turbulence or biological sample thicknessvariations, by using measured phase information and a wavefront correcting element. Toextend AO techniques to systems with diffractive surfaces, considerations need to be madefor additional components of the measured phase that are due to diffraction from the objectand are not a part of the extrinsic aberration. For example, light reflected from a diffractivesurface of an optical storage disk contains an additional phase due to the diffracted ordersfrom the grating-like structure of the data tracks. In this work, a modified Gerchbergalgorithm is presented as a viable method of phase retrieval to detect the total aberration, andcorrection for extrinsic aberrations is shown for light reflected from a grating. Anexperimental microscope system demonstrates successful AO correction, verifying simulationresults.

Efficient manipulation of probe pulse forachieving optical storage and switch in triplecoupled quantum dots

Nuo Ba, Jin-you Fei, Dong-fei Li, Xin Zhong, Dan Wang, Haihua Wang, Qian-Qian Bao, and Lei Wang

DOI: 10.1364/AO.392732 Received 16 Mar 2020; Accepted 05 May 2020; Posted 05 May 2020  View: PDF

Abstract: We investigated the steady optical properties of the triple quantum dots which obtaintunneling induced effect by using the external electric field. Our numerical results shown that theelectromagnetically induced transparency can be achieved via the tunneling induced effect. Inaddition, we examined the propagation dynamics of a probe field in this system and found that theprobe field group velocities and its absorption were related to the tunneling coupling intensities.So, we could control the probe field to obtain ultraslow group velocities and tunable opticalswitch. Finally, this scheme was possible to store and release the probe field by modulating thetunneling coupling sequences.

Design and analysis of a hybrid optical systemcontaining multilayer diffractive optical element withimproved diffraction efficiency

Shan Mao, Jianlin Zhao, and Meixuan Li

DOI: 10.1364/AO.392025 Received 04 Mar 2020; Accepted 05 May 2020; Posted 28 May 2020  View: PDF

Abstract: We present a compressive image quality evaluation for a hybrid optical system containing a multilayerdiffractive optical element (MLDOE). We take effects from both incident angle and waveband into considerationto improve diffraction efficiency of MLDOE. It ensures an accurate image quality evaluation of hybrid opticalsystem from the principle. Additionally, a refractive-diffractive hybrid optical system is taken as an example foroptimal design of surveillance lenses containing the MLDOE. The results show that this design enables adiffraction efficiency-improved MLDOE, and further ensures an accurate image quality of modulation transferfunction (MTF) and better optical system structure of this hybrid surveillance lenses. It is of great significancefor the quantitative and optimal design of hybrid imaging optical systems.

Dual-Beam Potassium Voigt Filter for Atomic LineImaging

Michael Kudenov, Brett Pantalone, and Ruonan Yang

DOI: 10.1364/AO.393649 Received 25 Mar 2020; Accepted 04 May 2020; Posted 05 May 2020  View: PDF

Abstract: Spectrally narrowband imaging in remote sensing applications can beadvantageous for detecting atomic emission features. This is especially useful in detectingspecific constituents within rocket plumes, which are challenging to discern from naturallyoccurring sunglints. In this paper, we demonstrate a dual-beam technique, implemented witha Wollaston prism, for calibrating a Voigt magneto-optical filter for a linear polarizer’s finiteextinction ratio, as well as optical misalignment between the linear polarizers’ transmissionaxes. Such a strategy would be key towards expanding the filter’s field of view whilemaintaining its classification capabilities. Validation of the potassium Voigt filter isdemonstrated using the simulation tool ElecSus in combination with a potassium hollowcathode lamp. RMS error, between the filter’s temperature response and that of thesimulation, was approximately 2%. We then demonstrate the detection of a potassium modelrocket motor outdoors alongside a sunglint. Results indicate a 20-fold increase in contrastwhen using our dual-beam calibration strategy.

Near-Zone Transmission Caustic of a Hanging WaterDrop

James Lock

DOI: 10.1364/AO.390328 Received 13 Feb 2020; Accepted 03 May 2020; Posted 04 May 2020  View: PDF

Abstract: A water drop hanging from a house siding board after a rain shower is near-normally illuminated by sunlighteither shortly after sunrise or before sunset. A focusing caustic consisting of a bright V-shape or U-shape with asmall bright elliptical-shape immediately above it is frequently seen on the next lower siding board. Inaddition, there are two broad regions of illumination immediately above the caustic, fanning out to the upperleft and upper right. This complicated pattern, composed of a bright V-shape or U-shape, and the bottom half ofthe small bright elliptical-shape immediately above it, is caused by the hyperbolic umbilic diffraction causticnear the condition of maximum focus. This can be observed because, by a stroke of good fortune, the distancebetween the lower edge of a siding board and the flat portion of the next siding board beneath it is nearly equalto the paraxial focal distance of the caustic. Blocking off the light incident on the top, bottom, left side, andright side of the drop was used to determine the portion of the drop responsible for different parts of thecaustic. The results were found to match the predictions for the hyperbolic umbilic caustic.

Digital all-sky polarization imaging of the totalsolar eclipse on 21 August 2017 in Rexburg,Idaho, USA

Joseph Shaw, Laura Eshelman, Martin Tauc, Taiga Hashimoto, kendra gillis, William weiss, Bryan Stanley, Preston Hooser, and Glenn Shaw

DOI: 10.1364/AO.391736 Received 28 Feb 2020; Accepted 03 May 2020; Posted 04 May 2020  View: PDF

Abstract: All-sky polarization images were measured from sunrise to sunset and during acloud-free totality on 21 August 2017 in Rexburg, Idaho using two digital three-camera allsky polarimeters and a time-sequential liquid-crystal-based all-sky polarimeter. Twenty-fivepolarimetric images were recorded during totality, revealing a highly dynamic evolution ofthe distribution of skylight polarization, with the degree of linear polarization becomingnearly zenith-symmetric by the end of totality. The surrounding environment wascharacterized with an infrared cloud imager that confirmed the complete absence of cloudsduring totality, an AERONET solar radiometer that measured aerosol properties, a portableweather station, and an ASD spectrometer with MODIS satellite data that measured surfacereflectance at and near the observation site. These observations confirm that previouslyobserved totality patterns are general and not unique to those specific eclipses. The hightemporal image resolution revealed a transition of a neutral point from the zenith in totality tothe normal Babinet point just above the sun after 3rd contact, providing the first indication thatthe transition between totality and normal daytime polarization patterns occur over of a timeperiod of approximately 13 s.

Analyzing colors and spectra of naturalrainbows with hyperspectral imaging

Raymond Lee

DOI: 10.1364/AO.387757 Received 08 Jan 2020; Accepted 01 May 2020; Posted 01 May 2020  View: PDF

Abstract: Few colorimetric analyses of natural rainbows (i.e., bows seen in rain showers)have been published, and these are limited either to approximate techniques (colorimetricallycalibrated RGB cameras) or to rainbow proxies (bows seen in sunlit water-drop sprays).Furthermore, no research papers provide angularly detailed spectra of natural rainbows in thevisible and near-IR. Thus some uncertainty exists about whether the published spectra andcolors differ perceptibly from those in natural rainbows. However, battery-powered imagingspectrometers now make possible direct field measurements of the observed chromaticitiesand spectra in such bows. These data (1) show consistent spectral and colorimetric patternsalong rainbow radii, and (2) let one subtract additively mixed background light to reveal theintrinsic colors and spectra produced by rainbow scattering in nature.

Below the horizon – the physics of extreme visual ranges

Michael Vollmer

DOI: 10.1364/AO.390654 Received 14 Feb 2020; Accepted 01 May 2020; Posted 01 May 2020  View: PDF

Abstract: Visual ranges of up to 440 km have recently been documented by photographs of earth based observers. An old report from 1948 even claimed a record visual range from a plane of more than 530 km and a similar recent observation from 2017 was documented by a photo. Such extreme visual ranges can in principle be explained by the interplay of refraction and light scattering. However, they require very special conditions of the atmosphere, and cleverly chosen locations of object and observer as well as observation time.

Dislocated spots and triple splittings ofnatural rainbows generated by large dropdistortions, oscillations and tilts

Alexander Haussmann

DOI: 10.1364/AO.391405 Received 28 Feb 2020; Accepted 01 May 2020; Posted 01 May 2020  View: PDF

Abstract: For an accurate modeling of natural rainbows, it is necessary to take into accountthe flattened shape of falling raindrops. Larger drops do also oscillate, and their axes exhibittilt angles with respect to the vertical. In this paper, I will discuss two rare rainbowphenomena which are influenced by these effects: Bright spots belonging to various rainboworders, but appearing at remarkable angular distances from their traditional locations; as wellas triple-split primary rainbows. While the former have not been observed in nature so far, thelatter have been documented in a few photographs. This paper presents simulations based onnatural drop size distributions using both a geometric optical model, as well as numericallycalculated Möbius shifts applied to Debye series data.

Zernike Coefficients from Wavefront Curvature Data

Virendra Mahajan and Eva Acosta Plaza

DOI: 10.1364/AO.391958 Received 03 Mar 2020; Accepted 30 Apr 2020; Posted 30 Apr 2020  View: PDF

Abstract: The concept of curvature sensing is reviewed, and a comprehensive derivation of the curvaturepolynomials is given whose inner products with the wavefront curvature data yield the Zernike aberrationcoefficients of an aberrated circular wavefront. The data consist of the Laplacian of the wavefront across itsinterior and its outward normal slope at its circular boundary. However, we show that the radial part of thecurvature polynomials and their slopes at the boundary of the wavefront have a value of zero, except when theangular frequency of the corresponding Zernike polynomial is equal to its radial degree. As a result, the effectof noise on the corresponding Zernike coefficients is lower because the noisy data at the boundary of thewavefront is not used to determine their values. The use of the curvature polynomials to determine the Zernikecoefficients is demonstrated with simulated noisy curvature data of an aberration function consisting of tenZernike coefficients, namely, defocus, and primary, secondary, and tertiary astigmatism, coma, and sphericalaberrations.

Generalized shift-rotation absolute measurementmethod for optical surface shapes with polygonalapertures based on migration recognition by Radontransform

Dong Yang, Zhongming Yang, Jiantai Dou, and Zhaojun Liu

DOI: 10.1364/AO.392851 Received 17 Mar 2020; Accepted 29 Apr 2020; Posted 29 Apr 2020  View: PDF

Abstract: A generalized shift-rotation absolute measurement method for optical surface shapes with polygonal aperturesbased on migration recognition by Radon transform is proposed. The rotation angles and translation distances ofthe test surface, measured three times, are calculated through migration recognition. The absolute shape of the testsurface with the polygonal aperture is fitted by orthogonal Zernike polynomials. Compared to the existing absolutemeasurement method for polygonal apertures, our method ensures test surface measurement accuracy withouthigh-precision attitude control and repeated adjustments. The measurement is simple and coherent, whichreduces the measurement time and improves the efficiency.

High Precision THz-TDS via Self ReferencedTransmission Echo Method

Jon Gorecki, Nick Klokkou, Lewis Piper, Sakellaris Mailis, Nikitas Papasimakis, and Vasilis Apostolopoulos

DOI: 10.1364/AO.391103 Received 20 Feb 2020; Accepted 28 Apr 2020; Posted 28 Apr 2020  View: PDF

Abstract: Terahertz time-domain spectroscopy is a powerful characterisation technique whichallows for the frequency dependent complex refractive index of a sample to be determined. This isachieved by comparing the time-domain of a pulse transmitted through air to a pulse transmittedthrough a material sample, however the requirement for an independent reference scan canintroduce errors due to laser fluctuations, mechanical drift, and atmospheric absorption. In thispaper we present a method for determining complex refractive index without an air reference, inwhich the first pulse transmitted through the sample is compared against the ’echo’ pulse wherethe internal reflections delay the transmission of the echo pulse. We present a benchmarkingexperiment in which the echo reference method is compared to the traditional air method, andshow the echo method is able to reduce variation in real refractive index.

Correction of Non Common Path Aberrationsin Pyramid WFS to recover the optimalmagnitude gain using a deformable lens

Martino Quintavalla, Maria Bergomi, Demetrio Magrin, Stefano Bonora, and Roberto Ragazzoni

DOI: 10.1364/AO.393499 Received 24 Mar 2020; Accepted 28 Apr 2020; Posted 05 May 2020  View: PDF

Abstract: Adaptive Optics (AO) correction based on Pyramid Wavefront Sensors (P-WFS)has been successfully implemented in several instruments for astronomical observation due tothe P-WFS advantages in terms of sensitivity with respect to other wavefront sensors, such asthe Shack-Hartmann. The correction of Non Common Path Aberrations (NCPA) between thesensing and the scientific arm, commonly performed introducing offsets to the Zernikecoefficients of the measured wavefront in the AO closed loop, reduces the sensitivity of PWFS causing a loss in sky coverage and scientific throughput. We propose a technique toexploit the full capabilities of P-WFS compensating the NCPA up to the 4th order on the WFSchannel by means of a Multi-actuator Adaptive Lens (MAL). We show the preliminary resultsobtained in a dedicated laboratory test-bench.

Trap stiffness modification of an optically trappedmicrosphere through directed motion ofnanoparticles

Shruthi Iyengar, Praveen P, Sharath Ananthamurthy, and Sarbari Bhattacharya

DOI: 10.1364/AO.389500 Received 31 Jan 2020; Accepted 26 Apr 2020; Posted 29 Apr 2020  View: PDF

Abstract: We report an enhancement in the corner frequency of an optically trapped non-magnetic microsphere in the planeperpendicular to the laser propagation direction on addition of ferrofluid to the suspension medium. We conjecture that adirected motion of the nanoparticles towards the trap in this plane is responsible for the augmentation. Changes in thecorner frequency in the presence of external magnetic field gradients lend credence to this conjecture. Corner frequencyaugmentation is also observed when Zinc Oxide nanoparticles are used. Here, however, no further changes are seen in thepresence of magnetic field gradients.

A Surface Plasmon Resonance TemperatureSensor Based on Photonic Crystal Fiber filledwith silver nanowires

Tonglei Cheng, Xu Li, Shuguang Li, Xin Yan, Xuenan Zhang, and FANG Wang

DOI: 10.1364/AO.391226 Received 24 Feb 2020; Accepted 25 Apr 2020; Posted 28 Apr 2020  View: PDF

Abstract: A surface plasmon resonance (SPR) temperature sensor based on a photonic crystalfiber (PCF) filled with silver nanowires is proposed in this paper. We inject ethanol solutionfilled with silver nanowires into the grapefruit PCF to realize temperature sensing. Thesensitivity of the sensor can reach -433 pm/ by numerical simulation, and the experimentalresult is -160 pm/. Simulations and experiments show that the wavelength of the resonancepeak will blue shift with the invalidity of silver nanowires, and the resonance effect of thesensor will weaken. It can provide reference for the realization and application of other SPRsensors based on PCF.

Experimental evaluation of the self-shadow and itscorrection for on-water measurements of water-leavingradiance

Lin Hua, Zhongping Lee, Gong Lin, and Xiaolong Yu

DOI: 10.1364/AO.391633 Received 28 Feb 2020; Accepted 25 Apr 2020; Posted 05 May 2020  View: PDF

Abstract: Accurate determination of the water-leaving radiance (Lw) is key to correctlyinterpret in-water optical properties and to validate the atmospheric correction schemesin ocean color studies. Among the various approaches adopted to measure Lw in thefield, the skylight-blocked approach (SBA) is the only scheme that can potentiallymeasure Lw directly. However, the apparatus associated with an SBA system willintroduce self-shading effects to the measured Lw, which is required to be corrected foran accurate Lw determination. In this study, we experimentally evaluate several factorsthat could contribute to the self-shading effects of the SBA-measured Lw, includingsolar zenith angle (~18° - 64°), water’s optical properties, and cone size (radius of 22mm and 45 mm). For waters with the total absorption coefficient at 440 nm as high as~6.0 m-1, the normalized root-mean-square difference (NRMSD) between theSBA-measured Lw after shade correction and the “true” Lw is generally between ~5%and ~10% for wavelengths in the range of 400 - 750 nm. These results suggest that SBAcan obtain highly accurate and precise Lw in nearly all natural aquatic environments.

Use of embedded and patterned dichroicsurfaces with reflective optical power to enablemultiple optical paths in a micro-objective

David Vega, Travis Sawyer, Nancy pham, and Jennifer Barton

DOI: 10.1364/AO.391654 Received 27 Feb 2020; Accepted 14 Apr 2020; Posted 15 Apr 2020  View: PDF

Abstract: We demonstrate the use of patterned dichroic surfaces with reflective optical powerto create multiple optical paths in a single lens system. The application of these surfaces enablesa micro-endoscope to accommodate multiple imaging technologies with only one optical system,making the packaging more compact and reliable. The optical paths are spectrally separatedusing different wavelengths for each path. The dichroic surfaces are designed such that the visiblewavelengths transmit through the surfaces optically unaffected, but the near-infrared wavelengthsare reflected in a telescope-like configuration with the curved dichroic surfaces providing reflectiveoptical power. We demonstrate wide-field visible monochromatic imaging, and microscopicnear-infrared imaging using the same set of lenses. The on-axis measured resolution of thewide-field imaging configuration is approximately 14 µm, and the measured resolution of themicroscopic imaging configuration is approximately 2 µm. Wide-field white-light imaging ofan object is also demonstrated for a qualitative perspective on the imaging capabilities. Otherconfigurations and applications in fields such as optical metrology are discussed to expand on theversatility of the demonstrated optical system.

Improving the quality of full-color holographicthree-dimensional displays using depthrelated multiple wavefront recording planeswith uniform active areas

Yan-Ling Piao, Munkh-Uchral Erdenebat, Yu Zhao, Ki-Chul Kwon, Mei-Lan Piao, Hoonjong Kang, and Nam Kim

DOI: 10.1364/AO.387377 Received 09 Jan 2020; Accepted 08 Apr 2020; Posted 07 May 2020  View: PDF

Abstract: In this paper, a depth-related uniform multiple-wavefront recording plane (UMWRP) method is proposed for enhancing the image quality of point cloud-based holograms.Conventional multiple WRP methods, based on full-color computer-generated holograms(CGHs), experience a color uniformity problem caused by intensity distributions. To solvethis problem, the proposed method generates depth-related WRPs to enhance coloruniformity, thereby accelerating hologram generation using a uniform active area. The aim isto calculate depth-related WRPs with designed active area sizes that then propagate to thehologram. Compared with conventional multiple WRP methods, reconstructed images havesignificantly improved quality, as confirmed by numerical simulations and opticalexperiments.

Phase-shift laser range finder technique based on optical carrier phase modulation

Hongyu He, Jianfeng Sun, Zhiyong Lu, Mengmeng Xu, Chenzhe Lao, RONGLEI HAN, XINYU CAI, and YUEXIN LI

DOI: 10.1364/AO.387196 Received 07 Jan 2020; Accepted 19 Mar 2020; Posted 08 May 2020  View: PDF

Abstract: A coherent laser range finder based on optical phase modulation and phase shift measurement is presented. In the proposed laser range finder, emitted laser is modulated by an electro-optic phase modulator(PM) using 20MHz sine signal and the received laser is mixed with local oscillator using a 90° optical hybrid. Compared with traditional laser phase shift range finder, the proposed laser range finder can measure the velocity and range at high precision simultaneously. Algorithm to calculate the range and velocity is deduced. Our preliminary experiments on moving target indicate that, when the measurement rate is 100kHz, the root mean square error (RMSE) of range and velocity respectively are 9.3×10-4m and 4.735×10-4m/s.

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