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Optics Letters

Continuous-wave self-pumped optical phase conjugation in atomic sodium vapor

Celestino J. Gaeta, Jenny Lam, Richard C. Lind

Published Feb 15, 1989• 22 citations
Publisher

Abstract

Self-pumped optical phase conjugation has been demonstrated for the first time to our knowledge in a resonant atomic system. This process was implemented by using a single cw pump beam to excite a sodium-vapor oscillator. The counterpropagating optical fields inside this oscillator then combine with the pump beam in an internal fourwave-mixing interaction to yield the phase conjugate of the incident (pump) beam.

DOI
10.1364/ol.14.000245

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Europhysics Letters (EPL)•Nov 15, 1986•22 citations
Self-pumped optical phase conjugation has been demonstrated for the first time to our knowledge in a resonant atomic system. This process was implemented by using a single cw pump beam to excite a sodium-vapor oscillator. The counterpropagating optical fields inside this oscillator then combine with the pump beam in an internal fourwave-mixing interaction to yield the phase conjugate of the incident (pump) beam.
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.
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.
Recently, there has been increased interest in the use of resonant atomic systems for performing optical-wavefront conjugation. This interest has arisen because resonant systems are inherently fast, with response times in the nanosecond range. However, resonant systems tend to have lower efficiencies and greater optical power requirements than do photorefractive crystals, such as BaTiO3. In this paper, we explore the possibility of using a highly efficient Raman laser1 in a sodium-vapor cell to obtain efficient phase conjugation at greatly reduced optical power densities. In this demonstration we use a self-pumped (linear-cavity) conjugator for simplicity. With this setup, we have achieved phase- conjugate reflectivities of 2% with input powers of only 20 W/cm2. This is only 2% of the optical power density previously reported2 to obtain comparable reflectivity with a self-pumped scheme in sodium vapor.