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Soviet Journal of Quantum Electronics

Measurement of the gain of an inhomogeneous active medium

Naum K. Berger, Yu. N. Lukyanov

Published Apr 30, 1978• 0 citations
Publisher

Abstract

An analysis is made of the transmission of a Gaussian beam by an amplifier with a spatially inhomogeneous active medium. The results are used in estimating the precision of gain measurements by means of a probe beam. It is shown that even a slight inhomogeneity may distort the measured frequency dependences and spatial distributions of the gain.

DOI
10.1070/qe1978v008n04abeh010080

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Soviet Journal of Quantum Electronics•May 31, 1980•0 citations
An analysis is made of the transmission of a Gaussian beam by an amplifier with a spatially inhomogeneous active medium. The results are used in estimating the precision of gain measurements by means of a probe beam. It is shown that even a slight inhomogeneity may distort the measured frequency dependences and spatial distributions of the gain.
We have performed an experimental and theoretical analysis of how a probe beam is modified on propagation through an SBS gain medium for various input beam parameters and total gains through the SBS medium. For the case of an undepleted lowest-order Gaussian pump beam, we found that when the probe spot size is smaller than the pump spot size in the focal region, the input probe profile is essentially preserved during amplification. However, if the probe is larger than the pump, the beam is modified as it propagates through the medium and in the limit of high gain takes on the mode structure of the pump. To model these effects mathematically, we decompose the probe wave into Laguerre-Gaussian modes and solve numerically the equations describing the spatial evolution of the individual modes. The result of the nonlinear coupling is to transfer energy from the pump beam to the probe and to redistribute the energy among the probe modes. In some cases, the transmitted probe shows a pronounced ring structure.
An analysis is made of a one-dimensional problem involving superradiance of four-level active media. The rate equations for the populations and the transport equations for the superradiance fluxes are used to derive differential equations for the description of the behavior of the integrated population and the observed superradiance flux in the active medium excited by the pump flux. A detailed analysis is made of the superradiance decay. Analytic dependences are obtained to describe the decay of the normalized superradiance flux. The gain of the active medium at the onset of decay occurs as the parameter in these dependences and they are used as the basis of a technique for obtaining upper and lower estimates of this parameter.