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Journal of the Optical Society of America B

Phase-locked phase conjugation using a Brillouin loop scheme to eliminate phase fluctuations

Kevin D. Ridley, A. M. Scott

Published May 1, 1996• 9 citations
Publisher

Abstract

We describe experiments investigating a technique that uses stimulated Brillouin scattering to phase conjugate a number of laser beams while preserving their relative phases. Phase locking of a number of beams focused into a stimulated Brillouin scattering cell is achieved by four-wave mixing with a pair of unfocused pump beams. The second pump is generated from the first by use of a stimulated Brillouin scattering loop geometry that minimizes phase fluctuations. We show that the reduction in phase fluctuations substantially improves the degree of phase locking. The requirements on four-wave mixing pump intensity are determined. The technique has the advantage of being insensitive to changes in alignment, with the potential to combine coherently a large number of laser amplifiers in a simple and compact geometry.

DOI
10.1364/josab.13.000900

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Four-wave mixing mediated by Brillouin scattering in the transient regime exhibits an instability which results in large phase conjugate reflectivities [A. M. Scott and K. D. Ridley, IEEE J Quantum Electron. QE-25, 438 (1989)]. In this paper we compare measurements of the threshold for the instability both as a function of the ratio of the second pump beam to the first pump beam and as a function of the phase mismatch. Phase conjugation in Brillouin enhanced four-wave mixing occurs through the interaction of a signal with two counterpropagating pump beams. In our experiments, the frequencies of all three input beams are different, and a small phase mismatch results. The magnitude of this mismatch was varied both by changing one of the input frequencies and the interaction length. The frequency of the second pump beam was changed by using stimulated Brillouin scattering in various mixtures. Thus we were able to vary the phase mismatch from four to thirteen resulting in a 50% change in the threshold intensity, in agreement with theoretical models. The ratio of the backward-to-forward pump beam intensities also affects the threshold, and measurements of this agree with the model.

Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
We describe experiments investigating a technique that uses stimulated Brillouin scattering to phase conjugate a number of laser beams while preserving their relative phases. Phase locking of a number of beams focused into a stimulated Brillouin scattering cell is achieved by four-wave mixing with a pair of unfocused pump beams. The second pump is generated from the first by use of a stimulated Brillouin scattering loop geometry that minimizes phase fluctuations. We show that the reduction in phase fluctuations substantially improves the degree of phase locking. The requirements on four-wave mixing pump intensity are determined. The technique has the advantage of being insensitive to changes in alignment, with the potential to combine coherently a large number of laser amplifiers in a simple and compact geometry.
Investigations of self-pumped phase conjugation using Brillouin-enhanced four-wave mixing in TiCl4 at 1 μm are presented. We have (1) observed both forward and backward Brillouin scattering into the optical cavity used for self-pumped phase conjugation during a single pump laser pulse and enhanced the forward-scattered component relative to the backward-scattered component by changing the angle of incidence of the pump beam, (2) demonstrated up to 80% coupling of pump radiation into cavity radiation, (3) examined quenching of the radiation in the optical cavity and eliminated both multiple Stokes orders from Brillouin scattering and Raman scattering as possible mechanisms, (4) observed mode beating in the cavity intensity profile coincident with the quenching, and (5) studied ring-resonator geometries, demonstrating both forward and backward scattering, unidirectional oscillation, and bidirectional oscillation leading to a triple-shifted phase conjugate.
The results of an experimental investigation of a new geometry for producing phase conjugation by Brillouin-enhanced four-wave mixing are presented. In this geometry, the four-wave mixing medium is carbon disulfide, and the backward-going pump wave is created from the transmitted forward-going pump wave by stimulated Brillouin scattering (SBS) in glycerol. The two pump waves are hence phase conjugates of each other, and the quality of the phase-conjugation process is not degraded even by the use of an aberrated pump wave. The probe wave is created by SBS in carbon disulfide, which has a Brillouin frequency half that of glycerol, and the conjugate wave is therefore generated at the same frequency as the probe. Since the pump and signal waves differ in frequency by the Brillouin frequency of the carbon disulfide four-wave mixing medium, high reflectivities (approximately 2000%) are obtained as a result of Brillouin resonance enhancement.
Brillouin enhanced four-wave mixing (BEFWM) is a judicious combination of SBS and FWM processes. In the traditional BEFWM experiments, two cells are used: one generates SBS, the other provides four-wave mixing. We demonstrated the generation of a phase conjugate signal by single cell BEFWM with focusing geometry. A single cell containing the optical nonlinear medium was used simultaneously for conventional SBS and FWM. Experiments were performed by using either CS2 or TiCl4 as the nonlinear medium; both have large Brillouin gain but differ greatly in their nonlinear indices of refraction. Thus the mechanism for four-wave mixing is shown to be Brillouin nonlinearity. In this experiment, the phase matching condition was satisfied and the frequency of the phase conjugate signal was not shifted from that of the master input beam. The operating regime of the experiment was far below the threshold of instability, which accounts for the low reflectivity that we observed. This single cell configuration is more compact and the optical alignment is easier than in the conventional BEFWM, in which two nonlinear cells are used.
We report on an investigation of Brillouin-enhanced four-wave mixing which provides phase conjugate reflectivities greater than 104. Theoretical modeling shows that the phase conjugate intensity grows with time until limited by pump depletion, and the main effect of varying the signal power is to increase the time delay before the conjugate beam reaches peak intensity. This is confirmed experimentally, and the maximum peak reflectivity is limited by the length of the laser pulses used in the experiment.
Four-wave mixing mediated by Brillouin scattering in the transient regime exhibits an instability which results in large phase conjugate reflectivities [A. M. Scott and K. D. Ridley, IEEE J Quantum Electron. QE-25, 438 (1989)]. In this paper we compare measurements of the threshold for the instability both as a function of the ratio of the second pump beam to the first pump beam and as a function of the phase mismatch. Phase conjugation in Brillouin enhanced four-wave mixing occurs through the interaction of a signal with two counterpropagating pump beams. In our experiments, the frequencies of all three input beams are different, and a small phase mismatch results. The magnitude of this mismatch was varied both by changing one of the input frequencies and the interaction length. The frequency of the second pump beam was changed by using stimulated Brillouin scattering in various mixtures. Thus we were able to vary the phase mismatch from four to thirteen resulting in a 50% change in the threshold intensity, in agreement with theoretical models. The ratio of the backward-to-forward pump beam intensities also affects the threshold, and measurements of this agree with the model.