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

D. Grandclément, G. Grynberg, M. Pinard

Published Apr 26, 1987• 0 citations
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Abstract

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.

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Observation of phase-sensitive noise in sodium vapor

Mari W. Maeda, Prem Kumar, Jeffrey H. Shapiro

Squeezed states of light exhibit phase-sensitive quantum noise measurable via optical homodyne detection. They have been predicted to be generated via degenerate four-wave mixing, resonance fluorescence, and nonlinear propagation.1 We report observing phase-sensitive noise via optical homodyning on a cw dye laser beam which has propagated through sodium vapor near the D lines. A p-polarized dye laser beam of ~5-W/cm2 intensity was passed through sodium vapor near its D lines. Homodyne measurements were made on both the s- and the p-polarized components of the transmitted beam using the dual-detector subtraction scheme.2 A spectrum analyzer monitored the noise in a 6–15-MHz band. In the absence of sodium, the noise in this band was vacuum-state quantum noise limited and was insensitive to the relative local-oscillator beam/transmitted laser beam phase θ. In the presence of sodium, the noise was sensitive to θ, with dependence of the form As,p + Bs,p cos2(θ − ϕs,p), where ϕs = 0 for s polarization and ϕp = π/2 for p polarization. For one set of data we measured As = 2 dB, Bs = 3 dB, Ap = 3 dB, and Bp = 2.5 dB above the noise levels in the absence of sodium. The s-poiarization data were collected with the local-oscillator power PLO far in excess of the s component of the transmitted pump power PTP; for p polarization, PLO ~ PTP prevailed. The measurement uncertainty in the spectrum analyzer data is ±0.5 dB. Possible physical mechanisms to explain these results are discussed.

Annual Meeting Optical Society of America•Jan 1, 1985•0 citations
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.
We describe our efforts to observe squeezing, and related nonclassical effects, in three forward four-wave mixing experiments using sodium vapor as a nonlinear medium. In a single-beam experiment with homodyne detection, we found optically phase-sensitive noise, which was radio-frequency phase insensitive, in a cw dye-laser beam that had propagated through sodium vapor. The minima of the phase-sensitive noise were always at or above the coherent-state value. In our forward four-wave mixing experiment with probe–conjugate-beam direct detection and correlation, we found that the sodium-vapor interaction produced positive noise correlation between the probe and conjugate beams. The correlation, however, did not exceed the excess noises on these beams, so that a nonclassical behavior was not demonstrated. Finally, our forward four-wave mixing experiment with probe–conjugate-beam combination and homodyne detection did demonstrate the generation of squeezed-state light in our setup. Optically phase-sensitive noise with a minimum falling 4% below the coherent-state level was observed.
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.
Squeezed states of light exhibit phase-sensitive quantum noise measurable via optical homodyne detection. They have been predicted to be generated via degenerate four-wave mixing, resonance fluorescence, and nonlinear propagation.1 We report observing phase-sensitive noise via optical homodyning on a cw dye laser beam which has propagated through sodium vapor near the D lines. A p-polarized dye laser beam of ~5-W/cm2 intensity was passed through sodium vapor near its D lines. Homodyne measurements were made on both the s- and the p-polarized components of the transmitted beam using the dual-detector subtraction scheme.2 A spectrum analyzer monitored the noise in a 6–15-MHz band. In the absence of sodium, the noise in this band was vacuum-state quantum noise limited and was insensitive to the relative local-oscillator beam/transmitted laser beam phase θ. In the presence of sodium, the noise was sensitive to θ, with dependence of the form As,p + Bs,p cos2(θ − ϕs,p), where ϕs = 0 for s polarization and ϕp = π/2 for p polarization. For one set of data we measured As = 2 dB, Bs = 3 dB, Ap = 3 dB, and Bp = 2.5 dB above the noise levels in the absence of sodium. The s-poiarization data were collected with the local-oscillator power PLO far in excess of the s component of the transmitted pump power PTP; for p polarization, PLO ~ PTP prevailed. The measurement uncertainty in the spectrum analyzer data is ±0.5 dB. Possible physical mechanisms to explain these results are discussed.