Simulation of a Simple Scheme to Generate Flat Frequency Comb Using Cascaded Single-Drive Mach-Zehnder Modulators

A scheme to generate flat optical frequency comb using two cascaded Mach-Zehnder modulators (MZMs) is proposed and demonstrated through simulation. By adjusting the amplitude of the modulation signal and the DC voltage supplied to the modulators, 7 comb lines with 1.15 dB flatness can be generated. An ideal 1×2 electrical splitter is used to split the RF (radio frequency) modulation signal ideally, so that both modulators are supplied with the same amplitude and frequency. In case using 90° hybrid coupler as a splitter, both modulators received different frequencies due to the 90-degree phase shift and the output was 3 and 5 comb lines with a corresponding flatness of 0.31 dB and 3 dB, respectively. Biasing was at the quadrature point to achieve linear modulation and avoid output optical distortion from the nonlinear region of MZM transfer function. The scheme is simple and cost-effective using only one signal generator, and it can be used mostly for analog optical links applications.