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Optics Letters

Phase conjugation through Brillouin-enhanced four-wave mixing over an extended atmospheric path

Paul S. Lebow, John R. Ackerman

Published Feb 15, 1989• 21 citations
Publisher

Abstract

We report what is to our knowledge the first demonstration of Brillouin-enhanced four-wave mixing over a long atmospheric path. Weak laser light reflected from a remote retroreflector was input to a high-gain conjugate mirror. The resulting amplified output returns to the retroreflector owing to its phase-conjugate nature. Despite atmospheric effects the phase-conjugate return energy measured at the retroreflector increased by a factor of 10(4) over the initial weak beam.

DOI
10.1364/ol.14.000236

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Annual Meeting Optical Society of America•Jan 1, 1986•0 citations
We report what is to our knowledge the first demonstration of Brillouin-enhanced four-wave mixing over a long atmospheric path. Weak laser light reflected from a remote retroreflector was input to a high-gain conjugate mirror. The resulting amplified output returns to the retroreflector owing to its phase-conjugate nature. Despite atmospheric effects the phase-conjugate return energy measured at the retroreflector increased by a factor of 10(4) over the initial weak beam.
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.
We report measurements of the gain and phase-conjugation fidelity of a nondegenerate four-wave Brillouin mirror (FWBM) based on high-pressure methane. In the experiments we used a Nd:YAG laser that produced single-mode pulses of 13-nsec duration. Depending on the energy of the pump we measured signal reflectivities between 10 and 10(3), constant over six orders of magnitude of signal energy. We also determined the conjugation fidelity of the FWBM by measuring the Strehl ratio of the conjugated return as a function of the energy of the input signal. We show that the Strehl ratio reduces to 0.5 for an input energy of 1.5 x 10(-13) J/mode of the FWBM. This noise-equivalent signal energy constitutes the limiting sensitivity of the FWBM. Our experiments show that a gas-filled FWBM could be used as a phase conjugator and high-repetition-rate optical image intensifier.
We have measured the conjugation fidelity produced by four-wave mixing in sodium vapor using weak probe beams that were aberrated by simulated atmospheric turbulence. Near-diffraction-limited (1.3 times the diffraction limit) return beams were obtained for both moderate (D/r(0) approximately 10) and strong (D/r(0) approximately 25) simulated turbulence, down to probe-beam pulse energies of 200 pJ. We believe this to be the first systematic study of conjugation fidelity as a function of probe energy.
It has previously been shown that Brillouin enhanced four-wave mixing can lead to large phase conjugate reflectivities for moderate laser powers.1,2 However, in the geometry considered by previous workers the pumps have equal frequency and hence the conjugate signal is shifted in frequency with respect to the probe. We have conducted an experimental and theoretical study of Brillouin-enhanced four-wave mixing for the different case in which the two pump waves differ in frequency by twice the Brillouin frequency of the four-wave mixing medium, and in which the probe wave frequency is halfway between those of the pump waves. In this case, the phase conjugate wave has the same frequency as the probe. Reflectivities > 100 % have been measured. Experimental results are compared to those of a theory that includes the effects of phase mismatch, pump depletion, and the finite response time of the medium.