Generation of Flat Optical Frequency Comb Using Cascaded PMs With Combined Harmonics

Flat optical frequency comb (OFC) can be generated by using single phase modulator (PM), where combined harmonics are applied to drive the PM. However, the number of comb lines and flatness of OFC are limited under this scheme. In this work, we use cascaded PMs with combined harmonics to generate flat OFC. The optimized phases and modulation indices in terms of amplitudes for each harmonic are obtained by solving optimization problem to minimize the fluctuation of OFC with a certain number of comb lines. Both simulation and experiment are carried out to evaluate the feasibility of the proposed scheme. Regarding the case of combined fundamental tone, third harmonic, and fifth harmonic, a 15-line OFC is generated with the flatness of 0.29 dB and 0.65 dB observed in simulation and experiment, respectively. Other feasible solutions to flat OFC with combined harmonics are also investigated.