The efficiency of coupling an external radiation field into an integrated-optics waveguide mode using a distributed coupler such as a prism or a grating decreases with increasing power if one of the waveguide media is characterized by an intensity-dependent refractive index. The differences between the operating characteristics of such a nonlinear coupler and those of the familiar linear coupler are discussed. It is shown numerically that there exists an optimum detuning angle that varies with incident power for optimizing the coupling efficiency. This optimum efficiency, however, is still found to decrease with increasing power. Optimum values of incident-beam spot size and the ratio of spot size to characteristic linear coupling distance are also found to vary with guided-wave power. For coupling into lossy media, the coupling efficiency, which starts out much lower than for lossless media at low powers, becomes comparable with that for the lossless-media case at high powers. Ways of improving the coupling efficiency by using tapered films or chirped gratings are discussed, and calculations are given for specific cases.
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