<title>Optical amplification by two-wave mixing in lithium niobate waveguides</title>

Abstract

ABSTRACT Photorefractive Fe:LiNbO3 planar waveguides have been fabricated by proton exchange method using Fe:LiNbO3 assubstrate and benzoic acid as exchange medium. The waveguide has a much larger refractive index increment than a Tidiffused one. Through end-face coupling, a pump beam and a signal beam were injected into the waveguide and the opticalamplification properties of the waveguide have been investigated experimentally. The results showed that a hundred foldamplification gain could be obtained in a wide range of grating periods and an amplification gain of 180 was achieved easily.This scheme has potential applications in optical signal processing at not very high incident intensity.Keywords: Two-wave mixing, optical amplification, lithium niobate, proton exchange, waveguide 1. Introduction Multi-wave coupling in nonlinear media has been extensively investigated for applications in wave-front amplification, laserbeam manipulation, parallel optical processing, phase-conjugation, and real-time holography. 1-3 A lot of photorefractivecrystals (such as LiNbO3, KNbO3, and SBN), have been studied for these purposes and have exhibited attractive capabilities.Meanwhile, much attention has also been paid to the photorefractive effect in waveguides of electro-optic and nonlinearoptical crystals for their potential applications in optical integrated circuits and interconnections. In comparison with bulkcrystals, waveguides permit a large interaction length, require low power sources, and enable easy integration. Hesselink, Ito,and Kamishilin et a! have demonstrated holographic storage in the fibers of SBN, Fe:LiNbO3 and Bi12TiO20, and obtainedhigh quality reconstructed images.4 Yu has studied the application of LiNbO3 fiber holograms to interconnection, switches,and massive memories.7'8 Optical amplification by two-wave mixing in planar waveguides also has been studied. Fischerachieved optical amplification in photorefractive BaTiO3 waveguide by lateral pumping of guide wave.9 Ito et a! performedoptical amplification in a periodically polarization-reversed BaTiO3 waveguide.'° It also has been shown by James et althat the response of two-wave mixing in ion-implanted BaTiO3 waveguide is two or three orders faster than that in bulkcrystal!2 In comparison with other crystals, iron doped LiNbO3 (Fe:LiNbO3) has higher photorefractive sensitivity and can