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Numerical analysis of impact of DBRs' outermost layers on optical characteristics of a surface-normal electro-absorption modulator by the method of single expression

An optical model of a surface-normal electro-absorption (EA) modulator consisting of multiple-quantum wells (MQWs) embedded in an asymmetric Fabry-Perot (FP) cavity formed by high-reflective back DBR and moderate-reflective top DBR separated from MQWs at both sides by buffer layers is analysed numerically by the method of single expression (MSE). Reflectance of the modulator versus normalised thickness of the buffer layer is analysed for different alternation of quarter-wavelength layers of DBRs. Corresponding optical field patterns, distributions of power flow density and permittivity profiles along the modulator structures with different alternation of DBR layers are presented for low reflection state. The contrast of EA modulator with DBRs of high permittivity layers outermost depending on the operating wavelength is analysed for different values of imaginary part of a permittivity of quantum-well (QW) layers. In the result of modelling corresponding design rules for optimisation of surface-normal EA modulators are suggested.