Theory of unidirectional photorefractive ring oscillators

Amplification owing to holographic two-wave mixing in photorefractive crystals can be utilized to achieve unidirectional ring oscillation. Unlike for the conventional gain medium (e.g., He–Ne), the gain bandwidth of photorefractive two-wave coupling is very narrow (a few hertz for BaTiO3). Despite this fact, the ring resonator can still oscillate over a large range of cavity detuning. A theory is presented that describes how the oscillating mode attains the round-trip phase condition.