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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Conjugate wave selection in four-wave mixing with feedback in a long nonlinear medium

Alexander A. Betin, A. V. Kirsanov

Published Jan 1, 1994• 0 citations
Publisher

Abstract

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.

DOI
10.1364/cleo_europe.1994.cwf61

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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.
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.
As it is known, self-conjugation of the IR laser beam via Brillouin radiation stimulated scattering phenomenon is not realised because of some physical limits on radiation intensity which are the principal limitations, to the present time. That is why, one of the promising technique of phase conjugation of infrared radiation is a self-conjugation of a laser beam wavefront via four wave interaction in a nonlinear medium with feed-back loop (FWMF) [1]. Unlike conventional our wave mixing schemes, the FWMF self-conjugation is supposed to be realised without any additional beams pumping a phase conjugation mirror.
An experimental investigation was made of the influence of the state of optically polished surfaces of various materials on the scattering coefficient which governs the power of the intrinsic noise of phase-conjugate mirrors based on a degenerate four-wave interaction. The angular dependence of this coefficient was also investigated. The noise power of a phase-conjugate mirror made of InAs, operating at the optimal pump wave intensities 1— 2 MW cm -2 , may be reduced to ~2 × 10 -7 W per one spatial mode of the radiation to be phase-conjugated. This was achieved by increasing the angle between the axes of the signal and the pump beams up to ~1 rad without a significant reduction of the reflection efficiency, which can be ~500%.
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.