Optical pulse interharmonic extraction and repetition rate division based on a microwave photonic phase detector.

Microwave photonic phase detectors (MPPDs) can extract ultrastable microwaves from a mode-locked laser (MLL), but their frequencies are often limited by the pulse repetition rate. Few works studied methods to break the frequency limitation. Here, a setup based on an MPPD and an optical switch is proposed to synchronize an RF signal from a voltage-controlled oscillator (VCO) to an interharmonic of an MLL and to realize the pulse repetition rate division. The optical switch is employed to realize pulse repetition rate division, and the MPPD is followed to detect the phase difference between the frequency-divided optical pulse and the microwave signal from the VCO, which is then fed back to the VCO via a proportional-integral (PI) controller. Both the optical switch and the MPPD are driven by the signal from the VCO. When the system reaches its steady state, the synchronization and repetition rate division are achieved simultaneously. An experiment is conducted to verify the feasibility. The 80½th, 80⅓rd, and 80⅔rd interharmonics are extracted, and pulse repetition rate division factors of two and three are realized. The phase noises at offset frequency of 10 kHz are improved by more than 20 dB.