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Soviet Journal of Quantum Electronics

Amplification of a weak signal on reflection from an inverted medium

B. B. Boǐko, N. N. Uvarova

Published Jul 31, 1987• 0 citations
Publisher

Abstract

An experimental investigation was made of the influence of the reflection conditions and of the characteristics of the incident radiation on the amplification of light in the course of its reflection by an excited dye solution. A strong dependence of the reflection coefficient on the angle of incidence was observed and its maximum occurred near the critical angle of total reflection. The amplitude and width of the reflection peak depended on the parameters of the ambient media, as well as on the spectral composition and energy of the incident radiation.

DOI
10.1070/qe1987v017n07abeh009498

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An experimental investigation was made of the influence of the reflection conditions and of the characteristics of the incident radiation on the amplification of light in the course of its reflection by an excited dye solution. A strong dependence of the reflection coefficient on the angle of incidence was observed and its maximum occurred near the critical angle of total reflection. The amplitude and width of the reflection peak depended on the parameters of the ambient media, as well as on the spectral composition and energy of the incident radiation.
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An investigation is made of wavefront reversal of optical radiation in a nondegenerate four-wave interaction in a resonant medium. It is shown that for a pump intensity comparable with the saturation intensity of the absorption line of the resonant medium, the reversed wave spectrum has two side peaks positioned symmetrically relative to the central peak which corresponds to a degenerate interaction. These side peaks are attributable to regions of negative absorption (amplification) of weak radiation by the resonant medium situated in a high-power field of opposite pump waves. The dependences of the magnitudes and positions of these peaks on the pump intensity can be used to control the spectral and energy characteristics of the reflected radiation. The conditions for self-excitation of a reversing system based on resonant four-wave interaction are determined. In particular, the ranges of the pump intensity and the optical density of the resonant medium in which self-excitation of the reversing system may occur, are determined for a degenerate interaction.
We have measured as a function of incident angle, for a range of critical angles between 88° and 89° and of characteristic gain depths between 41 and 96 probe wavelengths, the single-pass reflectance of a He–Ne probe beam reflected from a transverse optically pumped solution of rhodamine B. In contrast to previously reported experiments, no anomalously high reflectances were observed. The theoretical formulas for the exponentially nonuniform-gain model reproduce well the observed reflectance curves including the locations, slopes, and heights of the amplification maximum in the vicinity of critical angle. Peak amplifications of 200–300% were predicted and observed for the given experimental conditions.
A theoretical analysis is made of the conditions which a resonantly excited medium must have to act as a wavefront-reversing mirror. In the absence of saturation by the incident signal, the absorption cross section at the frequency of this signal should be considerably less than at the reversed-wave frequency. In the case of strong saturation it is preferable to use substances with narrow spectral lines. If a wavefrontreversing mirror is used to compensate distortions of the front of a wave during amplification, the necessary contrast of the reversed wave is a function of the gain and of the width of its profile.
An orderly account is given of the processes of coherent propagation, amplification, and generation of pulses in polyatomic resonant media. The characteristic features of the propagation of solitons in resonant media of finite length are considered. Comparisons are made of the exponential and lethargic amplification of pulses in resonant media. It is shown that an allowance for the reflection of the field from the boundaries of an active medium gives rise to a qualitatively different dependence of the parameters of the pulses on the reflection coefficients under superradiance and lasing conditions, which makes it possible to introduce a criterion to distinguish lasing from superradiance. An analysis is made of the characteristics of collective superradiance, which is a new case of coherent generation of pulses. Experimental and theoretical investigations of rf superradiance emitted by systems of nuclear spins are reviewed.
A linear response of a resonant medium to a weak electromagnetic field is studied in the case when a medium is a gas of three-level atoms subjected to a strong saturating pulse. A noncoherent intense pulse and a coherent 2π pulse (of the kind encountered in the theory of self-induced transparency) are considered. The expressions obtained are used to study the problem of the spectrum of the reflection of light by a pulse moving in a medium. An analysis is made of the possibility of using the Doppler effect, which occurs when light is reflected by a moving pulse, in an investigation of the characteristics of pulse propagation, determination of its parameters, and study of the nonlinear characteristics of the medium itself. Possibilities of experimental observation of the predicted effects are discussed, particularly the possibility of conversion of an electromagnetic radiation spectrum.