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Optics Letters

Retromodulation/conjugation using a self-pumped atomic sodium phase-conjugate mirror

Celestino J. Gaeta, David M. Pepper

Published Jun 1, 1991• 5 citations
Publisher

Abstract

A self-pumped atomic sodium phase-conjugate mirror was modified to include an amplitude modulator, which thus formed a device capable of encoding temporal information onto the phase-conjugate return beam. Modulation rates of as much as 4 MHz with modulation depths in the range of 70-90% (both limited by the electronics and the modulator) have been obtained by using an intracavity acousto-optic modulator.

DOI
10.1364/ol.16.000802

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A self-pumped atomic sodium phase-conjugate mirror was modified to include an amplitude modulator, which thus formed a device capable of encoding temporal information onto the phase-conjugate return beam. Modulation rates of as much as 4 MHz with modulation depths in the range of 70-90% (both limited by the electronics and the modulator) have been obtained by using an intracavity acousto-optic modulator.
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 introduce a semi-self-pumped phase conjugate mirror (PCM) in which one pump is externally injected into a ring resonator while the other is self-induced as in a unidirectional ring resonator. One application is in one-way imaging through optical distortions using a phase conjugate window.1 The original realization of the phase conjugate window was similar to an externally pumped PCM and had the disadvantage of requiring a clean pump beam coherent with the distorted probe. This meant that the clean beam had to bypass the distortion, thus limiting the practical utility of the device. By replacing the externally pumped PCM portion of the device with a semi-self-pumped PCM, the clean beam can be self-induced as an oscillation beam. The second application is as a thresholding phase conjugate mirror which would be useful, for example, in some optical associative memories.2 In the semi-self-pumped phase conjugator, the presence of the externally injected pumping beam suppresses buildup of the self-induced pumping beam until the signal reaches a certain threshold intensity.
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.
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.