Photorefractive Oscillators

We review recent developments, both theoretical and experimental, involving the nonlinear interactions of light in photorefractive crystals. When pumped by light beams in various configurations, certain crystals, such as barium titanate (BaTiO3), respond with the self-build up of gratings in the crystal and formation of new light beams. The focus of this paper is on these devices, known as photorefractive oscillators, the mutual light crystal interactions which govern their operation, and all optical applications which are based on these oscillators. The role of optical phases is shown to be prominent in the operation of various photorefractive oscillators. We have developed a theory which accounts for a spontaneous self-frequency detuning of the oscillating beams. This detuning, on the order of 1–10 Hz in BaTiO3, is due to a grating motion in the crystal, which itself is inherently dependent upon the optical phases and amplitudes of the interacting beams in the oscillators and a uniform electric field across the crystal. Experimental evidence points to the presence of an internal electric field in the BaTiO3 crystal which affects the detuning. New experimental results of the detuning dependence on nonreciprocal phases are presented, via the Faraday effect and amplitude aperturing on the oscillating beam. We discuss applications for this new type of dynamic interferometry. The double phase conjugate mirror (DPCM), a novel oscillator which we have demonstrated, couples mutually incoherent beams in a bidirectional, dynamic holographic link. This new capability significantly relaxes operating requirements, and offers many potential applications in adaptive optics, laser coupling, optical image processing and computing, interferometry, and optical communications. We present new experimental measurements of the transmissivity and reflectivity in the DPCM. Phase-conjugate reflectivity amplification is observed, as predicted by theory. With our invention of the double-color pumped oscillator (DCPO), the DPCM-type configuration is extended to nondegenerate wavelengths. In the DCPO, the two pumping beams are of different colors; nevertheless, a photorefractive oscillation builds up in the crystal. We review the various applications for the DPCM and DCPO, such as beam steering with automatic Bragg matching, image color conversion, and dynamic optical communication interconnects with wavefront matching.