Abstract

A variety of phase conjugators have been recently demonstrated that require two incident beams which are usually mutually incoherent.^1-3 The two beams interact via shared self-organized holograms in a photorefractive crystal to generate the phase-conjugate replicas of each of the two beams. The main distinguishing feature of these various conjugators is the number of internal reflections of the beams at the crystal faces (either zero,¹, one,² or two³). We demonstrate a new conjugator which uses three internal reflections to generate the phase-conjugate beams. This conjugator uses two mutually incoherent beams incident on opposite a-faces of an xyz-cut crystal of cerium-doped Sr_0.75Ba_0.25Nb₂O₆ (SBN). Both incident beams have extraordinary polarization and propagate in directions ~66° off the c axis inside the crystal, with the beams fanning toward the negative c face of the SBN crystal. A similar configuration is used to make a mutually pumped phase conjugator in BaTiO₃ using one reflection,² except there the beam fanning is in the direction of the positive c face due to the opposite sign of the dominant charge carrier in that crystal. In the case of BaTiO₃, r₄₂ is the largest electrooptic coefficient, and the most efficient self-induced photorefractive gratings sharply bend (i.e., diffract) the light toward the +c face. In SBN, r₃₃ is the largest electrooptic coefficient, so that the preferred grating wave vectors are directed almost parallel to the c axis. Consequently, the angles of the self-diffacted beams differ between the two crystals, with the resulting angles being less steep in SBN, so that both incident beams are photorefractively channeled not to the c face but to opposite a faces where they are reflected again and continue to be channeled to a common point of reflection at the –c face. Cross readout of the photorefractive gratings along this complicated common beam path, which resembles a pair of frog legs, produces the phase-conjugate beams.

© 1988 Optical Society of America