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Annual Meeting Optical Society of America

Polarization instabilities in an isotropic nonlinear Kerr medium

Alexander L. Gaeta, Robert W. Boyd

Published Jan 1, 1987• 0 citations
Publisher

Abstract

The steady-state polarizations of fields counterpropagating in nonlinear Kerr media can exhibit complicated behavior.1 We examine the temporal behavior for the case of an isotropic nonlinear Kerr medium and show that the polarizations are temporally unstable above a certain threshold intensity, even when the input polarizations are linear and parallel to each other. This polarization configuration is known to be spatially stable and is often chosen for the two strong pump waves in phase conjugation by degenerate four-wave mixing. We find that the threshold intensity for the polarization instability is approximately the intensity at which the phase conjugate reflectivity is equal to unity. Depending on the ratio of the two input intensities, this instability just above threshold can manifest itself as an abrupt rotation of the output polarizations to a new steady-state value or as a periodic oscillation of the output polarizations. At higher intensities the polarization can evolve chaotically in time, and depending on the ratio of the transit time through the medium to the response time of the medium the output intensity can fluctuate chaotically as well.

DOI
10.1364/oam.1987.mi4

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Journal of the Optical Society of America B•Nov 1, 1986•11 citations
The steady-state polarizations of fields counterpropagating in nonlinear Kerr media can exhibit complicated behavior.1 We examine the temporal behavior for the case of an isotropic nonlinear Kerr medium and show that the polarizations are temporally unstable above a certain threshold intensity, even when the input polarizations are linear and parallel to each other. This polarization configuration is known to be spatially stable and is often chosen for the two strong pump waves in phase conjugation by degenerate four-wave mixing. We find that the threshold intensity for the polarization instability is approximately the intensity at which the phase conjugate reflectivity is equal to unity. Depending on the ratio of the two input intensities, this instability just above threshold can manifest itself as an abrupt rotation of the output polarizations to a new steady-state value or as a periodic oscillation of the output polarizations. At higher intensities the polarization can evolve chaotically in time, and depending on the ratio of the transit time through the medium to the response time of the medium the output intensity can fluctuate chaotically as well.
Instabilities in the interaction of counterpropagating laser beams in a nonlinear medium can have important consequences for nonlinear optical processes such as optical bistability and phase conjugation. We have performed a theoretical study of the polarization characteristics of two counterpropagating waves in an isotropic Kerr medium in order to determine the stability of the multivalued solutions found by previous workers.1 Our method involves numerical integration of the coupled-amplitude equations describing the spatial and temporal evolution of the counterpropagating vector waves.2 Our results show that the multiply stable solutions for the output polarizations of the two vector fields show hysteresis, and that the order in which the two fields are turned on determines which solution is reached. For even higher input intensities we find that the output polarizations fluctuate in time.
The phase-conjugate reflectivity produced by degenerate four-wave mixing in a Kerr medium in the Bragg scattering regime is shown to be unstable when the intensity of the external pump beams is greater than a threshold value that can be predicted analytically. Below this threshold, a single branch of the reflectivity curve is stable, eliminating any possibility for bistability. A numerical simulation indicates that the unstable regime is characterized by fast oscillations with a period related to the transit time in the nonlinear medium. A finite relaxation time of the nonlinearity eliminates the high-frequency oscillations but can increase the amplitude of the low-frequency components. The insertion of a phase-conjugate mirror into an optical cavity leads to an instability whose threshold can be evaluated analytically. The temporal evolution of the field intensity in the cavity is shown to change from a stable regime to periodic oscillations and then to quasi-periodic oscillations as the pump intensity is increased.
We have observed that the polarizations and intensities of counterpropagating laser beams in sodium vapor are temporally unstable. The output from a stabilized dye laser tuned near to the 3S1/2 → 3P1/2 atomic transition was split to form the two counterpropagating beams. These beams were polarized with the same linear polarization and were weakly focused to a spot size of ~750 μm in sodium vapor having a density of ~3 × 1013 cm−3. We monitored the light generated in the polarization orthogonal to that of the pump beams. For a detuning of 1.01 GHz to the high-frequency side of the 3S1/2(F = 2) → 3P1/2(F = 2) transition and a power of 160 mW in one of the beams, we found that the threshold for the instability occurred at a power of 15 mW in the other beam. Slightly above threshold, the instability was found to be periodic in nature. As the intensity in the counterpropagating beam was increased far above the threshold value, wild fluctuations in the polarizations and intensities of the transmitted beams were observed. We have found that these fluctuations are chaotic and that the system evolves on a strange attractor whose dimension increases with increasing laser power. These observations are in qualitative agreement with the theoretical predictions of Gaeta et al.,1 which predict that the polarizations of counterpropagating beams in a Kerr medium are unstable to the growth of temporal fluctuations.
We consider degenerate four-wave mixing in a Kerr medium with a standard noncollinear geometry and the polarization directions of all the beams parallel to one another. The two pump beams, the probe beam, and the phase-conjugate beam are denoted by the subscripts 1,2,4 and 3, respectively. The nonlinear medium is placed between z = 0 and z = 1; the values of the intensities of the beams at z = 0 and 1 are denoted by Ii0 andIi1, where i = 1,2,3,4. Beams 2 and 4 are incident at z = 1; and beam 1 is incident at z = 0. We have determined the dependence of the phase-conjugate reflectivity R = I31/I41 and the pump transmission T = I11/I10 on the following parameters: the probe ratio, q=I41/(I10 + I21); the pump ratio, r = I21/I 1 0; the absorption coefficient–path length product, al; and the coupling constant of the medium, q. In the absence of nonlinear refraction, pump depletion, and linear absorption in the medium, R = tan2q. When all of these effects are taken into account, the maximum value of R is given by Rmax = exp (-2α1)/(q(1 + r)). The minimum value that T can reach is proportional to q for q < 1. Even for small values of q, multistable solutions for R and T are obtained when q is greater than a threshold value, which we denote by qth. We have determined the dependence of qth on the parameters q and r.
The exact theory of degenerate four-wave mixing in a Kerr-like medium in the Bragg scattering regime is used to study the reflectivity of phase-conjugate mirrors pumped by intense laser beams. The analysis takes into account pump depletion, nonlinear phase modulation and the relaxation time of the nonlinearity. The steady-state response exhibits multivalued reflectivity, hysteresis and potential bistability; however, the transient response shows that at most one value of the phase-conjugate reflectivity is stable. Periodic or chaotic pulsations are predicted beyond threshold values of pump intensity. The onset of a natural oscillation in the Kerr-like medium is also predicted in the absence of a continuous probe beam; the resulting field amplitude is stable and weakly sensitive upon the presence of external noise.
This paper presents an exact solution to the slowly varying envelope equations for the probe and signal fluxes in degenerate four-wave mixing in Kerr media. The analysis is performed for a noncollinear geometry, and all beams have the same linear polarization in the nonlinear medium. It is assumed that the angle separating the directions of the counterpropagating pumps and probe and signal beams is large enough so that cross-coupled waves do not contribute to the envelope equations describing the spatial evolution of the pump, probe, and signal beams. Multiple values of the transmitted probe are obtained for equal pump fluxes above a threshold determined by the nonlinear phase shift in the medium. Significant pump depletion occurs in this regime, but it can also occur without the appearance of multiple solutions. Since exact optical phase conjugation occurs only in the limit where the pumps are undepleted in Kerr media, this result implies that there are fundamental upper limits on the pump intensity that can be used to increase signal reflectivities in optical phase conjugation.