Comb Line Multiplication in an InP Integrated Photonic Circuit Based on Cascaded Modulators

We report the broadband characterization of an indium phosphide monolithically integrated optical frequency comb generator based on cascaded modulators. The device consists of a dual-drive Mach-Zehnder modulator (MZM), two cascaded phase modulators, and is also equipped with an integrated laser source, resulting in an extremely compact device, tunable both in wavelength and comb line frequency. Comb generation is shown in the range 4 to 8 GHz, and comb line multiplication is demonstrated, for the first time in an integrated device, exploiting the dual-drive configuration of the MZM. In particular, by feeding the MZM upper and lower arms with two synchronous sinusoidal RF signals, where one signal frequency is multiple of the others. At the repetition rate of 1 GHz, this technique allows to obtain up to 55 comb lines within a 3 dB power variation. A characterization of the comb phase noise when line multiplication is performed is also reported.