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Optics Letters

Phase conjugation in Cr^4+:YAG at 106 μm

A. Brignon, J.-P. Huignard

Published Aug 1, 1996• 7 citations
Publisher

Abstract

Phase conjugation in a Cr(4+):YAG saturable absorber by degenerate four-wave mixing with nanosecond pulses at lambda = 1.06 mu m is demonstrated for the first time to our knowledge. An experimental phase-conjugate energy ref lectivity of ~2% and an overall eff iciency of ~0.4% are achieved. Because of the presence of absorbing dipoles oriented along specif ic directions, it is shown that the phase-conjugated beam can be made orthogonally polarized relative to the signal beam or to both pump beams with almost no reduction of the energy reflectivity.

DOI
10.1364/ol.21.001126

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 11, 1996•2 citations
Phase conjugation in a Cr(4+):YAG saturable absorber by degenerate four-wave mixing with nanosecond pulses at lambda = 1.06 mu m is demonstrated for the first time to our knowledge. An experimental phase-conjugate energy ref lectivity of ~2% and an overall eff iciency of ~0.4% are achieved. Because of the presence of absorbing dipoles oriented along specif ic directions, it is shown that the phase-conjugated beam can be made orthogonally polarized relative to the signal beam or to both pump beams with almost no reduction of the energy reflectivity.
A model of degenerate four-wave mixing (DFWM) in Cr4+:YAG saturable absorbers is given in the transient regime. Because of the presence of absorbing dipoles oriented along specific directions, the DFWM process exhibits an anisotropic behavior. The effects of pump beam fluences, relative polarization of the beams, crystal orientation, absorbance, excited-state absorption, and nonresonant losses are analyzed. The theoretical results are supported by DFWM experiments in Cr4+:YAG performed at 1.06 µm with nanosecond pulses.
We describe a theory of four-wave mixing in the saturable absorber Cr4+:YAG{\mathrm{Cr}}^{4+}:\mathrm{Y}\mathrm{A}\mathrm{G}Cr4+:YAG and compare with experiments performed at 1.064 \ensuremath{\mu}m wavelength. We show that the strong polarization dependence of the saturable absorption along orthogonal crystallographic axes leads to unique polarization properties for wave mixing in Cr4+:YAG{\mathrm{Cr}}^{4+}:\mathrm{Y}\mathrm{A}\mathrm{G}Cr4+:YAG. Also there are a range of conditions that lead to vector phase conjugation of the polarization state of a signal beam. Optimum reflectivity for vector phase conjugation occurs when the pump beams have either orthogonal circular polarizations or parallel linear polarizations at 45\ifmmode^\circ\else\textdegree\fi{} to the crystallographic axes. The latter case is unique to this type of nonlinearity.
High-reflectivity four-wave mixing by gain saturation in a Nd:YAG laser amplifier is demonstrated. With a four-pass geometry, the phase-conjugate reflectivity of a Q-switched pulse (18-ns duration) is ∼2500, and for a long pulse (relaxation oscillation duration ∼100 μs) it is ∼30. Temporal and energy characteristics of the conjugate beam are presented. A phase-conjugate Nd:YAG TEM00 laser resonator is also produced in which the Nd:YAG rod is simultaneously the laser gain medium and the phase-conjugating device.
Degenerate four-wave mixing with orthogonally polarized pump beams at /spl lambda/=1.06 /spl mu/m in the saturable gain of a flash-lamp-pumped Nd:YAG amplifier is investigated, both experimentally and theoretically, using pulses of duration much less than the population relaxation time. We introduce a new set of coupled-wave equations in this short pulse regime that includes the effect of the four interacting waves and the laser gain experienced by all the beams. The dependence of the efficiency of the process on the small-intensity-gain-length product, beam ratios, and fluences is analyzed. A reflectivity of a weak probe beam of 25 percent and a transmission-gain-grating diffraction efficiency of 32 percent are achieved. Finally, the temporal shape of the conjugate signal is studied for various gain-saturation regimes.>