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Quantum Electronics

Selection of a phase-conjugate wave in an oscillator based on a four-wave interaction with feedback in an extended nonlinear medium

Alexander A. Betin, A V Kirsanov

Published Mar 31, 1994• 7 citations
Publisher

Abstract

A quasi-optical approximation without allowance for beam spreading over its envelope is used to show that selection of a conjugate wave occurs in a volume hologram whose dimensions are greater than the longitudinal correlation length of the fields forming the hologram. The condition under which the signal power exceeds the noise power is found. Phase conjugation of a speckle field with a divergence of 2 × 10 -2 by a four-wave interaction with feedback was achieved experimentally for neodymium-activated glass and the efficiency was 5%. The input energy threshold was 100 mJ and the maximum energy in the phase-conjugate wave was 10 mJ.

DOI
10.1070/qe1994v024n03abeh000058

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A quasi-optical approximation without allowance for beam spreading over its envelope is used to show that selection of a conjugate wave occurs in a volume hologram whose dimensions are greater than the longitudinal correlation length of the fields forming the hologram. The condition under which the signal power exceeds the noise power is found. Phase conjugation of a speckle field with a divergence of 2 × 10 -2 by a four-wave interaction with feedback was achieved experimentally for neodymium-activated glass and the efficiency was 5%. The input energy threshold was 100 mJ and the maximum energy in the phase-conjugate wave was 10 mJ.
A study is reported of a system, making use of four-wave mixing with feedback (FWMF), for high-quality phase conjugation of speckle-inhomogeneous radiation. The system includes a hologram formed on the basis of nonlinearity of gain saturation. A theoretical analysis is made of selective capabilities of a 'short' hologram, whose length is less than the longitudinal correlation lengths of the 'write' waves used to form it. The statistical-average values of the parameters of modes in an FWMF cavity are determined. The experimental results obtained for a neodymium glass system show that the fidelity of phase conjugation of speckle beams with a divergence (5 — 25) θ diffr is 80%. The maximum energy of the phase-conjugated component is 0.35 J and the maximum energy reflection coefficient of an FWMF mirror is 20.
One of the efficient methods of phase conjugation (PC) is the four-wave mixing with feedback (FWMF) (Fig. 1). To select the wave conjugate to the input radiation in a FWMF resonator, it has been suggested to use either an inhomogeneous phase plate or a long (larger than the length of the longitudinal inhomogeneity of the signal) nonlinear medium.1 The diffraction efficiency of a thick hologram and its noises for a plane reference wave are calculated in Ref. 2. This paper provides the calculation of the power of the conjugate and noise waves produced by the reflection from a long phase hologram (mirror) recorded by two speckled beams. The paper also deals with experimental realization of a PC FWMF speckled beam in a long medium.
The diffraction approximation is used to determine the structure of the radiation generated as a result of a four-wave interaction with feedback and the conditions for phase conjugation are identified. Phase conjugation can be expected in the case of a signal beam with a smooth distribution of the amplitude if the transverse size of this beam decreases as a result of passing through a feedback loop. In the case of a multimode signal beam it is additionally necessary to ensure that the resonator length or the length of the nonlinear medium exceeds the spreading length of the transverse inhomogeneity of the field.
An analysis is made of double phase conjugation in media with a local response. A threshold is found for the generation of a phase-conjugated wave by the double process in a medium with a local response and with additional amplification. An analysis is made of the further possibilities of generation of phase-conjugated radiation in the case of a four-wave interaction characterized by a feedback loop established when double phase conjugation takes place in a nonlinear medium. The threshold intensity of the input radiation ensures possible generation of a phase-conjugated wave as a result of a four-wave interaction characterized by a feedback loop and merging of counterpropagating beams in a nonlinear medium.
An analysis is made of Gabor dynamic volume phase holograms recorded by a signal wave which consists of a high-power homogeneous component and a relatively weak data-carrying field which determines the structure of the image formed with its aid. Reconstruction of such a reference-free hologram by a plane wave traveling opposite to the homogeneous component produces an inverted data field with a phase shift relative to the reconstructing wave. It is shown that when the maximum transfer of energy to the data-carrying component is attained, the phase shift becomes of the order of π and the conjugate image acquires a structure with an inverted contrast.