The basic limitations to light-controlled switching and logic gating when using nonlinear couplers fabricated from materials whose nonlinear response departs from ideal Kerr-like behavior are discussed. A critical value (≈10 dB) of total linear loss along a one-beat-length-long coupler exists: for greater absorptions the achievable power-dependent sharp switching at the output of a coupler is degraded greatly. Our analysis of spatial instabilities in the coupled-mode equations with linear-loss terms yields an analytical estimate of this critical absorption as well as of the switching efficiency. When paired with the exact results obtainable for a saturable nonlinearity without absorption, our description also may provide useful qualitative insight (in terms of trade-offs between switching power, efficiency, and throughput) when correct modeling of resonantly enhanced nonlinearities (e.g., a two-level absorber) requires numerical solutions of the coupled-mode equations.
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