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Optics and Spectroscopy

Controlling the intensity and fluctuations of light upon its selective reflection

Ya. A. Fofanov

Published Sep 1, 2005• 2 citations
Publisher

Abstract

Selective reflection of a semiconductor laser beam by a resonant gas is studied. The experiments were performed under conditions where the laser frequency is determined by the laser cavity mode structure. In this case, characteristic steps—intensity quasi-levels—arise on the reflectivity resonances. It is shown that the height of the quasi-levels depends on the degree of saturation of the resonance reflection by the laser field. Low-noise reflectivity regimes with different sensitivities to external signals are found on these quasi-levels.

DOI
10.1134/1.2055943

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Physical Review A•Nov 1, 1990•30 citations
Selective reflection of a semiconductor laser beam by a resonant gas is studied. The experiments were performed under conditions where the laser frequency is determined by the laser cavity mode structure. In this case, characteristic steps—intensity quasi-levels—arise on the reflectivity resonances. It is shown that the height of the quasi-levels depends on the degree of saturation of the resonance reflection by the laser field. Low-noise reflectivity regimes with different sensitivities to external signals are found on these quasi-levels.
The transformation of intensity fluctuations accompanying the nonlinear resonance reflection of light from a glass-resonant gas interface is discussed. The illumination of the interface at angles close to the critical angle for total internal reflection is found to have certain advantages over orthogonal incidence. At the maximum of the strongest resonance, the reflected light intensity amounts to more than 50% of the incident light. In these conditions, a squeezing of the amplitude (photon) fluctuations is observed in the radiation generated by a semiconductor laser-nonlinear selective reflector system. A simple semiclassical theory adequately describes some important characteristics of this system: the shape of the reflection resonances, the strength of the saturation fields, and so on.
This work is devoted to study of the strong selective reflection of obliquely incident laser beam (inclined geometry) from the interface "glass-rubidium vapour". It is shown that the contrast of the strongest refection resonances exceeds 500% at the moderate heating of reflecting cells. The new nonlinear properties of selective reflection resonances have been investigated - asymmetry of saturation of resonant total internal reflection (TIR) and nonlinear structures that appear under conditions close to resonant TIR. The observed structures can not be associated with well-known sub-Doppler resonances in selective reflection. The nonlinear structures described here do not have saturation properties in usual sense. They exist only in the inclined geometry and in a narrow range of saturating values of the surface power density at the interface. The transformation of the fluctuations of reflected light was also investigated at the optical saturation of selective reflection resonances. For this purpose the intensity of incident light was slightly modulated. The reduction of the corresponding intensity modulation was registered in the reflected light. This reduction of the intensity fluctuations ("classical squeezing") and the relatively high selective reflection coefficient offer good possibilities for the squeezing of quantum fluctuations in the reflected light. In the process of appropriate studies it was found that the reduction (squeezing) of photons (quantum) fluctuations in the reflected light beam is about of 5% in compare with the coherent light.
Nonlinear selective reflection of obliquely incident laser beam (inclined geometry) has been investigated. The new structures were detected on the tops of strong reflection resonances under conditions of optical saturation of the D2-lines in the natural mixture of rubidium isotopes. The detected nonlinear structures have not saturation properties in usual sense and exist only in the close area of saturating power density values. This phenomenon can be useful for the amplification the nonlinear characteristics of the optical interface in order to reduce the fluctuations of selective reflected light.
Resonance reflection of light from the interface between a dielectric and an alkali vapor is studied. Two cases are considered in which a laser light beam is polarized either parallel or perpendicular to the plane of incidence. Upon scanning of the frequency of the incident laser radiation, in the first case, the reflection conditions vary from approximately the conditions of Brewster reflection to the conditions characteristics of total internal reflection. In the second case, the Brewster reflection conditions are not realized, and the variations in the intensity of the reflected light observed upon frequency scanning are smaller. The rotation of the plane of polarization and the laser frequency tuning make it possible to rapidly switch between these reflection conditions.
The dipole–dipole broadening of the spectrum of the selective reflection of intense resonance light from the interface between a transparent dielectric and a gas of the natural mixture of Rb isotopes has been studied experimentally. The case of a high gas density where the Doppler broadening can be neglected has been investigated. It has been shown that dipole–dipole broadening is reduced with increasing the number density of excited atoms. When the laser beam intensity is much higher than the saturation intensity of a resonance transition, a significant broadening due to the very high laser beam intensity has not been observed in the reflection spectrum from the transparent dielectric/gas interface. The observed intensity dependence of the spectral width has been explained by the quenching collisions of the excited atoms with the interface.
Intense light incident on a dense-gas--dielectric interface, from the dielectric side, is reflected because of the combined effect of the discontinuities in the index of refraction at the interface and at the transition region in the gas where the degree of atomic saturation changes abruptly. This latter discontinuity gives rise to a self-reflected wave. At low intensities this self-reflected wave is weak, the problem is linear, and the net reflected wave intensity taken as a function of frequency has a simple shape that should allow measurement of the hitherto undetected self-induced frequency shift in a collision-broadened gas at an absorption resonance. Calculations show that in the weak intensity regime, r'', the imaginary part of the reflection coefficient, is quasi-Lorentzian and shifted by an amount comparable to its width. At high light intensity r'' broadens, shifts, and distorts. The distortion, which nonuniformly modulates the Lorentzian shape, comes from an interference of the forward and self-reflected waves. In the wings of the resonance line the self-reflected wave can be neglected, and the shape of r'' although not distorted is shifted from where it would be if there were no local field correction.