Performance prediction of an electroabsorption modulator at 1550 nm using GeSn/SiGeSn Quantum Well structure

The absorption spectra in strain-free Ge0.992Sn0.008/Si0.3Ge0.61Sn0.09 Quantum Well structure due to direct gap excitons formed in Ge0.992Sn0.008 wells reported by Chang and Chang are reproduced by an empirical expression. Two Gaussian distributions for heavy hole and light hole excitons and two exponential functions describing continuum transitions and The Sommerfeld factors are used for the fit. The expressions are then used to obtain the change in refractive index, Δn, with field (electrorefraction) using the Kramers–Kronig relation. With the calculated changes in absorption and refraction with bias, other performance parameters of an Electro-Absorption Modulator (EAM), like extinction ratio, chirp parameter, and a figure of merit (FoM) defined as the ratio of extinction coefficient and insertion loss, are evaluated. The FoM takes the largest value for 1 μm length of the EAM with 2 V bias applied to the p–i–n structure. The parameters are also calculated for different wavelengths as well as detuning lengths. For applications as short pulse generation the transmission T as a function of bias V should be linear. We have evaluated the best value of bias voltage that makes the sum of second and third derivatives of T with V equal to zero so that the variation of T with V becomes maximally linear for optimum operation as a short pulse generator.