Fig. 1. Structural diagram of the automatic phase-matching system for optical frequency combs generated by cascaded electro-optic modulation. LD, laser diode; MZM, Mach–Zehnder modulator; EVOA, electronically variable optical attenuator; PA, power amplifier; PM, phase modulator; PS, phase shifter; MCU, microcontroller unit; OSA, optical spectrum analyzer.

Fig. 2. Principle analysis and algorithm flow chart. (a) Changes in the OFC symmetric sideband optical power with the phase difference between MZM and PM. (b) Changes in the power difference between the two-order sidebands corresponding to the red line in (a). (c) Phase-matching algorithm of the MZM and PM flow chart.

Fig. 3. Principle analysis and algorithm flow chart. (a) Secondary PM optical carrier optical power difference (change in unit phase difference). (b) Three-level PM optical carrier optical power difference (change in unit phase difference). (c) Phase-matching algorithm flow chart of the MZM and multi-level PM.

Fig. 4. Results of multi-level PM phase-matching EOFC. (a) First-level PM output spectrum. (b) Second-level PM output spectrum. (c) Third-level PM output spectrum.

Fig. 5. Time to complete phase matching at different operating frequencies.

Fig. 6. Number of comb lines within 3, 5, and 10 dB flatness at different operating frequencies.

Fig. 7. System stability analysis chart.