Pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser using optical feedback

Abstract

A novel method of pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser is proposed and experimentally demonstrated. This pulse-amplitude equalization method is based on two key points. One is that an optical pulse train introduced to a semiconductor optical amplifier (SOA) saturates the SOA gain quickly, and this can be used for the gain modulation of SOA. The gain modulation of SOA as a mode-locking process in a semiconductor ring laser can generate stable mode-locked pulse trains. The second point is that the SOA gain is equalized from pulse to pulse because the feedback optical pulse circulates inside the ring cavity. By combining the rational harmonic mode-locking using a LiNbO 3 electro-optic modulator with the gain modulation of the SOA, amplitude-equalized optical pulse trains with the multiplied repetition rates of ∼10.00 GHz (the third rational harmonic) and ∼16.72 GHz (the fifth rational harmonic) were successfully demonstrated using optical feedback with pulse intensity matching. This scheme can be used for upgrading existing rational harmonic mode-locked semiconductor ring lasers to a semiconductor ring laser with equalized amplitudes at high pulse repetition rates.