Mode locking by phase-sensitive amplification
Mode locking is the mechanism by which lasers organize many optical modes into stable trains of ultrashort pulses. It underpins optical frequency combs, precision timing and spectroscopy, and provides the phase-coherent waveforms required for applications ranging from strong-field physics to optical frequency metrology. Conventional pulse-forming mechanisms select pulse amplitude without fixing its phase relative to an external optical reference, leaving pulse formation and optical-phase stabilization as separate functions. Phase-sensitive amplification offers a different possibility: the optical reference can participate directly in pulse formation. Here we demonstrate this principle in a continuously pumped oscillator. Kerr-induced phase evolution and cavity detuning turn pump-referenced parametric gain into a power-dependent discriminator that supports stable single-soliton operation. Roundtrip-resolved interferometry reveals that the soliton optical phase remains referenced to the master laser modulo $π$. These results establish phase-sensitive mode locking, in which pulse-amplitude selection and optical-phase referencing arise from the same coherent nonlinear feedback.