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Applied Physics Letters

New gain mechanism for wave amplification in photorefractive materials

Klaus H. Ringhofer, LASZLO SOLYMAR

Published Sep 19, 1988• 53 citations
Publisher

Abstract

It is shown that there is a gain mechanism in three- and four-wave forward interactions in which power is transferred inwards from the pump beams to the signal beams. The special case of amplifying a wave halfway between the pump waves is discussed.
DOI
10.1063/1.100058

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While the interaction of optical beams in photorefractive crystals can often be accurately described in terms of 1-D plane-wave approximation, 2-D models are sometimes more suitable, as suggested by recent work.1 Here we discuss some aspects of photorefractive interaction of finite size optical beams. We have solved the system of partial differential equations governing two- and four-wave mixing numerically by using iterative procedures. Simulations of beam amplification in two-beam coupling show strong beam deformation even without any spatial information imposed on them. Spatially nonuniform energy transfer is responsible for this effect. The pump beam loses energy during propagation and at the end of its passage through the beam intersection region (which was completely inside the crystal) it is strongly depleted and cannot contribute significantly to beam amplification. As the pump beam crosses through the probe, it amplifies most strongly that side of the probe that it meets first. By the time the pump has reached the far side of the probe, it is strongly depleted and less effective at probe amplification. In the case of phase conjugation through four-wave mixing, we found that the conjugate beam is strongly deformed only when the coupling constant is negative (corresponding to deamplification of the probe input beam). For positive coupling constants the output conjugate beam is a good replica of the input probe beam.
The properties of a self-pumped photorefractive phase conjugator as an amplifier of amplitude modulated signal beams are investigated. In principle the crystal acts as a self-adjusting beam combiner which allows homodyne detection of the weaksignalbeam superimposed on a stronger reference beam. In our experiments the mixing takes placevia a four wave interaction process in the crystal. In this paperwe investigate the salient characteristics ofthe self-pumped phase conjugate amplifier and explain details of the beam interaction inside the ciystal.
Photorefractively induced index-of-refraction phase gratings are shown to combine coherently the optical fields of a strong pump and a suitably amplitude- or phase-modulated signal beam in such a way that an apparent amplification of the modulating waveform appears as intensity modulation of the transmitted signal and pump beam intensities. The source of the signal gain is shown to be the square-law (intensity) detection of the coherently combined pump and modulated signal beams, just as in coherent optical communication systems in which a strong local-oscillator field is coherently added with a weak optical signal field by a beam splitter.