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Soviet Journal of Quantum Electronics

Asymmetry of the amplification of light in optically dense active media

S. G. Voloshenyuk, L. V. Vovk, E I Zabello, Eugene A. Tikhonov

Published May 31, 1989• 2 citations
Publisher

Abstract

An asymmetry effect in the amplification and efficiency of an optically dense amplifier is predicted theoretically and confirmed experimentally for opposite and concurrent collinear signal and pump paths. The effect is due to a difference in the losses as a result of the two-stage absorption of pump radiation in a system of luminescing centers described by a model of three energy levels. Calculations are made of various characteristics of this type of amplifier using the quasisteady-state approximation.

DOI
10.1070/qe1989v019n05abeh008020

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An asymmetry effect in the amplification and efficiency of an optically dense amplifier is predicted theoretically and confirmed experimentally for opposite and concurrent collinear signal and pump paths. The effect is due to a difference in the losses as a result of the two-stage absorption of pump radiation in a system of luminescing centers described by a model of three energy levels. Calculations are made of various characteristics of this type of amplifier using the quasisteady-state approximation.
A theoretical study is made of the influence of changes in the relaxation of a three-level system in a resonance field on the dispersion, absorption, and difference between the populations of levels activated by noncoherent pumping. This change in the relaxation is due to a tendency of a thermal reservoir to create an additional difference between the populations of quasienergy states that appear under the influence of the resonance field. It is shown that as a consequence there is a considerable change in the nature of the interaction with the radiation field in laser crystals with three-level active centers when the pump power is close to the threshold. The lasing conditions are found subject to such changes. It is shown that the threshold pump power can be reduced in this case to a value at which the difference between the populations of the active levels vanishes and lasing appears because of an inversion of the populations in a system of quasienergy levels. Under transient conditions the lasing frequency near the threshold should then depend on the pump
We here present the theory of lasing without inversion for systems with a split upper laser level. We show that the effect of amplification without inversion may be realized for at least two different situations. In the first situation, the effect is due to the asymmetry of radiative characteristics of quasidegenerate transitions. In the second situation, the effect can be provided without any asymmetry and is achievable for the definite phase of the transfer of coherence between the quasidegenerate levels (quasiparametric amplification). We also show that for laser amplification without inversion, the rate (frequency) of mixing of the quasidegenerate levels should be significantly higher than the linewidths of these levels. This mixing of quasidegenerate levels can be realized by low-frequency electromagnetic radiation or by interaction with an auxiliary third level. In addition, we derive the conditions for providing laser action for such systems rigorously, and we give some specific examples for the realization of this effect in gaseous and solid-state media. One of the most promising configurations may be realized in solid-state media when two narrow quasidegenerate levels are embedded in quasifree states (low-density state conduction band). We also discuss the use of a parametric mechanism for the creation of spatial gratings and for the construction of new types of electro-optical devices.
Results of numerical simulation of coaxial propagation of an intense pump beam and a weak probe beam in a three-level medium with different types of energy diagrams in conditions of inversionless amplification are presented. The effect of the beam self-action and diffraction on the spatial and frequency characteristics of the probe wave were studied. These effects, in combination with linear absorption and refraction, give rise to complex spatial profiles of gain along the beam propagation path. The annular structure of the amplified beam and a small asymmetry of its frequency characteristics, resulting from the nontrivial gain profile over the beam cross section, were found and studied.
The amplification of intense light pulses in resonance active media with an optical Stark effect and coherence effects is examined theoretically. In the course of the amplification, the pulse may go through a stage of an incoherent Stark quasisoliton. The envelope of this quasisoliton has an approximately constant, highly asymmetric shape. The energy of the quasisoliton is approximately constant. Upon further amplification, the Stark quasisoliton "breaks up" into a train of shorter pulses. Over a sufficiently long amplification distance, a true soliton may form in the active medium. In general, this true soliton is not the same as a standard π pulse in a medium without a Stark effect.
The two-level approximation is used to make a theoretical analysis of four-wave mixing in the active medium of a laser whose radiation wavefront is to be reversed. Explicit allowance is made for changes in the amplitudes of the pump waves as a result of their amplification during propagation in the active medium. Both the completely degenerate case, when the frequencies of all four waves are the same, and the case of a small detuning of the signal wave frequency relative to the pump waves are considered. For the completely degenerate case a system of reduced equations allowing for saturation of the inversion was solved by computer using an iterative method. It was found that, under optimum conditions in the saturation regime, the power of the wave reflected from a four-wave mixer may be higher than that of the signal wave.