Doubled Repetition Frequency in Regeneratively and Harmonically Mode-Locked Fiber Ring Laser

A regeneratively and harmonically mode-locked fiber ring laser that generated a pulse train with a doubled repetition frequency was experimentally demonstrated. We used a polarization-maintaining fiber laser and an electrical circuit that extracted a subharmonic clock to produce self-pulsing with a repetition frequency of 21.32 GHz while extracting the electrical clock at 10.66 GHz. The mode of operation depended on the bias point of the optical modulator. The doubled repetition frequency was observed when the modulator was biased in the vicinity of the voltage providing either the maximum or minimum transmission. Otherwise, the repetition frequency coincided with the modulation frequency. Detailed analyses of repetition frequencies suggested that the frequency-doubled pulse generation could be caused by rational harmonic mode-locking.