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Quantum Electronics and Laser Science Conference

Optical phase conjugation with a sodium Raman laser

Joanne DiFrancisco‐Donoghue, Jonathan S. Kane, Philip Hemmer

Published May 12, 1991• 0 citations

Abstract

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.

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Optics Letters•Feb 15, 1989•22 citations

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.

We report on the observation of retromodulation/conjugation using a self-pumped conjugator employing atomic sodium as the nonlinear medi- um. Previous observations of retromodulation/ conjugation employed a crystal of BaTiO3 as the nonlinear medium.1 Although the latter approach is simple in its configuration and high temporal data rates are possible, photorefractive materials possess a relatively limited response time to spatial changes of the incoming wavefront. In the present set of experiments, we demonstrated a retromodulator which potentially has several desirable features Including (1) high spatial bandwidth, (2) fast response time to wavefront changes (projected gigahertz bandwidth), (3) high temporal data rates possible (projected gigahertz bandwidth), (4) low threshold powers required (50 mW demonstated), and (5) rapid buildup time. These features indicate that such classes of retromodulators may be useful in low power level free space and guided-wave communication links as well as in novel optical resonators and gyroscopes.

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.

Self-oscillation of cavities closed by phase-conjugate mirrors has been observed with solids,1 liquids,2 and vapors.3 The case of vapors is particularly interesting because the nonlinearities are well known, and precise comparison between theory and experiments is possible. Furthermore, because of the huge nonlinearity close to a resonance transition, high reflectivities are obtained even with cw lasers.4 At last, the time response of the medium is expected to be of the order of the lifetime of the excited level.