Automatically starting, flat-top electro-optic frequency comb based on photonic sampling

An approach is proposed and demonstrated to realize automatically starting, flat-top electro-optic frequency comb (EOFC) generation utilizing cascaded electro-optic modulation based on a Mach-Zehnder modulator (MZM) and multiple phase modulators (PMs). The generated EOFC is used to achieve photonic sampling of two single-tone microwave signals with different frequencies in a feedback module. The optimal comb flatness of the EOFC is obtained by minimizing the power difference of the two down-converted signals in the first Nyquist zone after photonic sampling. By meticulously designing the frequencies of the two microwave signals for photonic sampling, this optimization process is achieved via low-frequency detection, avoiding optical spectrum monitoring in conventional methods. By using particle swarm optimization, the cumbersome multiple parameter optimization process is achieved, realizing automatically starting EOFC generation. Both simulation and experiment are carried out to verify the feasibility of the proposed method. In the experiment, a flat-top EOFC with 47 comb lines and a comb-tooth interval of 10.2 GHz within a spectral power variation range of 6 dB is automatically generated based on an MZM cascaded with two PMs. In addition, automatically starting flat-top dual EOFCs centered at 1541.8 nm and 1556.2 nm are generated by using this method.