Microwave pulse generation via employing an electric signal modulator to achieve time-domain mode locking in an optoelectronic oscillator.

An actively mode-locked optoelectronic oscillator (OEO) is proposed and demonstrated to generate chirp-free microwave pulse trains with variable repetition rates. Time-domain mode locking is realized by using an electric signal modulator to achieve amplitude modulation of the generated microwave signal in the OEO cavity. Through setting the externally applied electric signal frequency to be equal to the integral multiple of the free spectral range, a microwave pulse train with a low close-to-carrier phase noise is generated. In the experiment, microwave pulse trains with repetition rates of 179.94 and 360.00 kHz are generated through fundamental mode locking and 2nd-order harmonic mode locking, respectively. Under fundamental mode locking, the phase noise of the actively mode-locked OEO at 100 Hz frequency offset is measured to be 30 dB lower than that in a free-running OEO.