Proposed Scheme for Ultra-Flat Optical Frequency Comb Generation Based on Dual-Drive Mach-Zehnder Modulators and Bidirectional Recirculating Frequency Shifting in Single Loop

Recirculating frequency shifting has attracted much attention for its advantages in the generation of the flexible and high-quality optical frequency comb. A new scheme of ultra-flat optical frequency comb generation system based on single-loop bidirectional recirculating frequency shift is proposed and studied in this paper. The generation system employs two pairs of dual-drive Mach-Zehnder modulators and several polarization devices. Compared with the method of single-loop unidirectional recirculation frequency shift, under the same cycles, the number of comb lines generated by the proposed method is doubled, and the generated optical frequency combs have less noise accumulation and better flatness. The theoretical model is established, and the proposed scheme is verified by software simulation. A 111-line optical frequency comb with the spacing of 12.5 GHz, the flatness of 0.76 dB, and the optical signal-to-noise ratio of 27.39 dB was obtained by adopting the proposed scheme.