Observation of two distinct phase states in a self-phase-locked type II phase-matched optical parametric oscillator

We demonstrate self-phase locking in a type II phase-matched optical parametric oscillator by mutual injection locking. An intracavity quarter-wave plate provides polarization mixing between the orthogonally polarized signal and idler that induces signal–idler self-phase locking when their frequency difference is within the capture range. We observed two distinct phase states that differ in their oscillator thresholds and their signal–idler phase differences, in good agreement with theory.