Photorefractive resonators

Abstract

The strong nonlinear optical coupling in photorefractive crystals, such as BaTiO3, readily permits the construction of efficient phase-conjugate reflectors and self-pumped optical resonators. These latter devices1 exhibit two peculiar properties. First, the oscillating beam differs in frequency2 from the pumping beam by a small amount (Δω/ω ~ 10−15) and, second, the self-oscillation apparently occurs at any optical cavity length. Careful experiments reveal that, when the self-oscillation is confined to a single spatial mode, the frequency difference between the oscillation and pumping beams is directly proportional to the amount of cavity length detuning away from an integer number of optical wavelengths (mλ). That is, when the resonator cavity length is slightly less than mλ, the self-oscillation frequency is upshifted with respect to the pump frequency; when the length is slightly greater than mλ, the oscillation frequency is downshifted; and when the length exactly equals mλ, there is no frequency difference. This observation is explained by an appreciable optical phase shift that accompanies nondegenerate two-wave mixing. In a photorefractive unidirectional ring resonator, the theoretical threshold conditions for oscillation are uniquely determined by the two-wave mixing gain (γL), the response time of the photorefractive crystal, and the ring cavity losses. Theoretical predictions agree with the experimentally observed oscillation threshold behavior.