An active mode-locking optoelectronic oscillator based on a dual-drive Mach-Zehnder modulator

An active mode-locking optoelectronic oscillator (OEO) based on a dual-drive Mach-Zehnder modulator (DDMZM) is proposed and experimentally demonstrated. The kernel of this scheme is that an electro-optic DDMZM driven by a lowfrequency sine signal whose period is equal to the OEO loop delay is employed to periodically regulate the loop gain for achieving mode locking. By adjusting the bias voltage of the DDMZM, the gain curve of the OEO cavity can be changed accordingly, resulting in the generation of different microwave pulse waveforms. Compared to the reported active modelocking OEO, the repetition frequency or the period of the microwave pulse train generated by the proposed scheme can be varied just by changing the bias voltage of the DDMZM, rather than the frequency of the driving low-frequency signal. An experiment is carried out to demonstrate the feasibility of this DDMZM-based active mode-locking OEO. In the experiment, under the condition of a 1.1-km cavity length and fixed driving frequency of 180 kHz, microwave pulse waveforms with repetition frequencies of 180 kHz and 90 kHz are generated by setting the bias voltage of the DDMZM to be different values. In addition, the autocorrelation result of the microwave pulse train with a repetition frequency of 90 kHz illustrates that there is a certain phase relationship between four pulses within a period, leading to the achievement of a phase-coded microwave pulse, which is beneficial for solving the problem of distance ambiguity in the radar system.