Abstract
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.