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Journal of Modern Optics

Picosecond Phase Conjugation in Sodium Vapour

János Hebling, P. Simon, Tassos A. Mikropoulos, A.C. Cefalas

Published Oct 1, 1987• 0 citations
Publisher

Abstract

Phase conjugation by degenerate four-wave mixing in sodium vapour was demonstrated using 80 ps transform-limited pulses. Values of the phase-conjugated reflectivity were measured as a function of the temperature of the vapour and the delay of the probe beam.
DOI
10.1080/09500348714551131

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Phase conjugated waves via degenerate four wave mixing with picosecond laser pulses having a full width at half maximum (FWHM) of 80 ps have been observed in alcali vapours. The dephasing time of the 2P3/2 state in sodium was found to be 7.4 ns. This is the lifetime of the transient volume grating of the atomic states which are involved in the phase conjugated experiment. The average time of the perturbation of the atomic wave functions due to a "hot" collision (from a frequency modulation of a Markov type) was found to be 5x10-14 sec.
We demonstrate experimentally that phase-conjugation (PC) by resonant degenerate four-wave mixing in Na vapour can reach large efficiencies in the c.w. regime (reflectivity up to 250%). With PC reflectivity exceeding unity, a c.w. phase-conjugate laser oscillation is obtained, for the first time on a resonant gas medium. We show that large Na densities are required, as well as large frequency detunings of the intense incident beams. The PC oscillation is characterized through threshold effect, pump depletion, and polarization characteristics.
The work reported here demonstrates for the first time degenerate four wave mixing (DFWM) phase conjugation in cesium (Cs) atomic vapor. The strong, spectrally narrow transition of cesium at 852 nm gives conjugate reflectivities up to 154% with low pump beam powers. Temperature tuning of the DFWM response over a 30 GHz bandwidth is also demonstrated. An improved experimental design to determine the DFWM angular response shows a field-of-view of 20 mrad at the half response points. Self-focusing effects are observed to be similar to those in sodium vapor with an onset of increased conjugate divergence for reflectivities greater than about 25%.
We have performed experimental studies designed to elucidate the parameters which limit the efficiency and fidelity of optical phase conjugate reflection of picosecond laser pulses in transparent Kerr media. The experimental data show that at low pulse energies, the reflectivity follows the expected square dependence on the pulse energies. However, it falls below the square at high energies even though, at these high energies, a phase conjugation efficiency of 300% was observed. We observed a rather strong dependence of the reflectivity on the ratio of pump to probe pulse energy. The quality of phase conjugation decreased at very high pump energies, but could under some conditions be improved by focusing the probe beam into the interaction region. Of the several mechanisms responsible for the decrease in efficiency and fidelity, we find that the predominant ones are pump depletion, small scale self-focusing, and dephasing owing to a difference in energy of the two pump pulses.