Simple and accurate analytic expressions are provided for the maximum reflectivity and tolerances of an impedance-matched asymmetric Fabry-Perot used as a high-contrast spatial light modulator when electroabsorptive quantum wells provide loss modulation. When the device geometry is optimized, these expressions depend only on material properties. The maximum reflectivity depends only on the fractional absorption change and is independent of the front-mirror reflectivity. The most important tolerance is on the flatness of crystal growth; the fractional-length tolerance is proportional to the absorption coefficient. These formulas agree with experimentally reported results from multiple-quantum-well modulators and previous numerical analyses; they are useful for quickly predicting optimized performance of possible new materials. The normally on and normally off geometries are compared. The effect of finite back-mirror reflectivity is clarified. Deviations from impedance match enable increased reflectance difference at the expense of contrast ratio, an approach which is evaluated as a function of material parameters.
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