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European Quantum Electronics Conference (EQEC)

Quantum spatial correlations of optical beams counterpropagating in Kerr media

Isabel Pérez–Arjona, A. Gatti

Published Jan 1, 2003• 0 citations
Publisher

Abstract

This study focuses on the case of a system consisting of a Kerr medium of thickness L, within which two beams, with forward (F) and backward (B) amplitudes, counterpropagate. A classical model for this system predicted the existence of a transverse modulational instability (MI) above a suitable threshold of the pump beam intensities. Instabilities of similar nature have been experimentally observed e.g. in sodium or rubidium vapors.

DOI
10.1109/eqec.2003.1314214

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This study focuses on the case of a system consisting of a Kerr medium of thickness L, within which two beams, with forward (F) and backward (B) amplitudes, counterpropagate. A classical model for this system predicted the existence of a transverse modulational instability (MI) above a suitable threshold of the pump beam intensities. Instabilities of similar nature have been experimentally observed e.g. in sodium or rubidium vapors.
We consider a vectorial Kerr-medium model including transverse spatial effects. We analyze cases in which, immediately above the threshold of the spatial instability, the homogeneous pump wave gives rise to two tilted waves corresponding to a stripe pattern in the near field. We analyze both the self-focusing and the self-defocusing case and we point out the existence of anti-correlations between the quantum fluctuations of the intensity of the pump and the sum of the intensities of the two tilted waves creating the transverse pattern in the near field. We also evaluate the efficiency of this scheme as a quantum nondemolition (QND) scheme which uses the tilted waves as a ``meter'' to measure the intensity fluctuations of the pump. Our results show the posibility of a QND measurement in the self-defocusing case. In this case, and for a linearly polarized pump, the output pump beam (uniform in the transverse plane) and the pattern have orthogonal polarization and could be easily separated experimentally.
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.
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.
Absolute instabilities of counterpropagating pump beams in a dispersive Kerr medium, placed inside a Fabry–Perot cavity, are analytically studied by use of the analysis and the results of part I [J. Opt. Soc. B14, 607 (1998)]. Our approach allows characterization of such a complicated nonlinear system in terms of a doubly resonant optical parametric oscillator. We consider the growth of modulation-instability sidebands associated with each pump beam when weak probe signals are injected through one of the mirrors of the Fabry–Perot cavity. The results are used to obtain the threshold condition for the onset of the absolute instability and the growth rate for the unstable sidebands in the above-threshold regime. As expected, the well-known Ikeda instability is recovered at low modulation frequencies. The effects of the group-velocity dispersion are found to become quite important at high modulation frequencies. Although the absolute instability dominates in the anomalous-dispersion regime, it exists even in the normal-dispersion regime of the nonlinear medium. Below the instability threshold, our analysis provides analytic expressions for the probe transmittivity and the reflectivity of the phase-conjugated signal that is generated through a four-wave-mixing process.