Microwave pulse triggered at phase extrema via phase modulation in an actively mode-locked optoelectronic oscillator

We propose and experimentally demonstrate a brand-new, to the best of our knowledge, active mode-locking scheme for optoelectronic oscillators (OEOs) based solely on phase modulation. By incorporating an electrical phase shifter into the OEO loop as the mode-locking element, periodic phase modulation is imposed on the intracavity signal. Microwave pulses are naturally triggered at the phase extrema, where the instantaneous frequency shift vanishes. Experimentally, sinusoidal, square, and triangular modulation waveforms all successfully produce two microwave pulses per period. Notably, the duty cycle of the square wave enables independent control over the two pulse widths within the same period, while the symmetry of the triangular wave allows flexible adjustment of their relative temporal spacing. These findings establish phase modulation as a simple yet effective active mode-locking mechanism for OEOs, significantly enriching the toolbox for reconfigurable microwave pulse generation.