Abstract

In recent years, self-pumped phase conjugation and self-induced photorefractive light oscillation have been reported, using different photorefractive materials. The main photorefractive materials used in these experiments are BaTiO₃ and Bi₁₂SiO₂₀. In ferroelectric BaTiO₃ the high electro-optic coefficients give rise to large reflectivities. In photoconductive and paraelectric Bi₁₂SiO₂₀ phase shifting techniques lead to enhanced self-diffracted intensities and phase conjugate reflectivities. KNbO₃ is such a material which in addition to its high photorefractive sensitivity³ and large beam coupling gain⁴, has been shown to become highly photoconductive if prepared properly, leading to optimized response speed⁵ down to 100 μs. In a degenerate four wave mixing configuration, maximum phase conjugate reflectivities up to 25% with an applied electric field have been observed several years ago⁶. In a previous paper⁷ and for the first time, the possibility of using KNbO₃ crystals for self-induced optical resonators has been reported. Both an unidirectional ring and a linear resonator configurations were realized and coherent light oscillations observed. Build up time constants of milliseconds were reported when an undoped reduced KNbO₃ crystal was employed. However the measured phase conjugate reflectivity for these configurations were very small. Later on self-pumped phase conjugation in KNbO₃ has been reported at 124°C⁸. In this paper we report on self-pumped phase conjugation experiments, performed with an iron doped KNbO₃ crystal at room temperature in both a linear and a passive ring resonator configuration. We also report on self-pumped phase conjugation in a configuration requiring no external optical elements. Results deriving from degenerate four-wave mixing are also presented.

© 1990 Optical Society of America