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Europhysics Letters (EPL)

Continuous-Wave Self-Oscillation Using Pair Production of Photons in Four-Wave Mixing in Sodium

M. Pinard, D. Grandclément, G. Grynberg

Published Nov 15, 1986• 22 citations
Publisher

Abstract

An oscillation is observed when a sodium cell interacting with two counterpropagating pump beams is placed inside a Fabry-Perot cavity. We have shown that, in our experimental conditions, this self-oscillation comes from a pair production of photons in degenerate four-wave mixing. We have found that self-oscillation occurs with a single passage gain equal to 1% even when the transmission losses through the mirrors are 6%.

DOI
10.1209/0295-5075/2/10/003

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Study of cw self-oscillation using pair production of photons in four-wave mixing in sodium

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Optics Letters•Jul 15, 1991•7 citations
An oscillation is observed when a sodium cell interacting with two counterpropagating pump beams is placed inside a Fabry-Perot cavity. We have shown that, in our experimental conditions, this self-oscillation comes from a pair production of photons in degenerate four-wave mixing. We have found that self-oscillation occurs with a single passage gain equal to 1% even when the transmission losses through the mirrors are 6%.
There is presently a growing interest in nearly degenerate four-wave mixing, both for application in optical phase conjugation and the generation of squeezed states. In particular, the self-oscillation of a system interacting with one or two pump beams has been investigated in great detail in the case of photorefractive materials, but very few experiments have been done in vapors in the cw regime. The results that we obtained using sodium are presented here. They are strikingly different from the results obtained with photorefractive materials. For example, the oscillating beam and pump beam frequencies coincide with a precision better than 10−2 Hz. Another interesting aspect of this oscillating beam is that it exhibits a phase sensitive noise: just above threshold, the noise on one quadrature of the field can be 10−2 times smaller than the noise on the other quadrature.
Collisional damping can induce a coupling between two nonresonant light beams propagating in an atomic vapor. This coupling is sufficiently important to induce a gain process for a weak probe beam interacting with an intense pump beam. We present here the first observation of a cw oscillation generated by this process using a cell which contains sodium and helium, the pump beam being slightly detuned from the D2{\mathrm{D}}_{2}D2​ resonance line. We also show how the self-oscillation due to four-wave mixing is modified by the presence of a buffer gas.
We have used a two-wave mixing gain process in sodium vapor to construct a self-pumped phase conjugator. This was done by forming a resonator around a quartz sodium cell equipped with Brewster angle windows. A pump beam is then directed into the sodium cell at a small angle to the longitudinal axis of the resonator. At sufficiently high cw pump intensities, the ac-Stark effect pro- vides sufficient gain to achieve oscillation. Although this gain is unidirectional it occurs on the wings of the sodium resonance so that the backward traveling optical field in the resonator does not experience loss due to sodium absorption. The counterpropagating optical fields in the resonator interact with the pump in an internal four- wave mixing process to produce a fourth optical field (phase conjugate to the pump field). Our experiment was conducted at the sodium D2 resonance. The pump laser was a 699 autoscan single frequency ring dye laser providing pump intensities up to several hundred watts per square centimeter.