Operation Strategy of InP Mach-Zehnder Modulators for Flat Optical Frequency Comb Generation

We investigate an InP-based semiconductor Mach-Zehnder modulator (MZM) for the purpose of generating a flat optical frequency comb (OFC). A nine-line OFC having flatness of <0.8 dB is experimentally obtained by using an InP MZM under an optimized operation condition. To determine the optimized drive voltages, we use a simple model that takes into account nonlinear changes in the refractive index (phase modulation) and optical absorption loss (amplitude modulation) induced in the InP MZM. We confirm that it can reproduce the measured spectra of OFCs generated by the InP MZM. Optimum drive voltages of the InP MZM for flat OFC generation are found by considering the amplitude differences of the linear and nonlinear phase modulations for each arm. It is found that nonlinear phase modulation with a moderate amplitude (0.1-0.2π) flattens the OFC. The OFC becomes less flat at larger amplitudes of the nonlinear phase modulation; however, this deterioration can be suppressed using the amplitude modulation induced by the dependence of the optical absorption loss on the applied voltage. A numerical analysis using the proposed model reveals feasible operation conditions for the InP MZM.