Efficient wave mixing in photorefractive BSO/BGO crystals and applications to optical signal processing

Abstract

Photorefractive materials have been widely investigated for applications to optical signal processing; promising materials are sellenite type crystals Bi12SiO20 (BSO), Bi12GeO20 (BGO) in which fast response and high index modulation can be achieved in wave mixing experiments when the frequency of one beam is slightly shifted. This technique has resulted in large values of the exponential gain coefficient (Γ ≃ 8-12 cm-1) for optimized recording conditions (drift recording mode with an externally applied electric field E0). When the amplified incident or conjugate signal is fed back to the BSO, different types of oscillator are demonstrated in which the beam in the cavity is spontaneously frequency shifted. We review the conditions of efficient beam interaction with BSO-BGO crystals and demonstrate applications to image amplification, dynamic interconnection between fibers, ring and phase conjugate resonators. We also show that BSO-BGO crystals of different origins may exhibit significant differences in their photoconductive and photorefractive properties. This arises from noticeable differences in the migration lengths of the photocarriers and in the trapping center concentrations. We report high grating efficiencies (ηs ≃ 95%) when large electric fields (E0 ≃ 12 kV cm-1) are applied to low photoconductive crystals and nearly complete beam depletion in two-wave mixing with a moving grating (grating period Λ ≃ 20 μm).