Characterization of Self-Focusing and Self-Defocusing of Light in Sodium Vapor

Abstract

Self-focusing of light in sodium vapor was first observed on a cw basis in 1974. Recently at Northwestern University, efforts to develop a quantum optical communications network employing squeezed states of light have required quantitative characterization of the self-action effects. It has been determined that self-focusing and self-defocusing change the spatial structure of the output beams of the forward four-wave mixer used in the experiments, thus worsening the homodyne-detection efficiency by creating a mismatch between the squeezed output beam and the local-oscillator beam. Consequently, the need to characterize the self-action effects in sodium vapor has arisen. By characterizing the self-action effects as a function of the sodium cell temperature, input beam intensity, and the dye laser frequency, it will be possible to modify the localoscillator wavefront to compensate for the spatial mismatch, and thus improve the homodyne-detection efficiency. This paper reports the results of an experiment carried out in the Fall and Winter Quarters of the 1987/1988 school year as an Honors Project in Electrical Engineering. The theories of self-focusing of optical beams and Gaussian beam propagation are developed early in the paper in order to lay the groundwork for the presentation and interpretation of the experimental results. A general description of the laboratory setup is given, and the experimental procedure is described in detail. Finally, the paper concludes with a presentation and interpretation of the experimental findings.