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Physics Letters A

Spectral intensity of thermal radiation in a medium for frequencies in the neighbourhood of an absorption line

M. Yamanoi

Published May 1, 1973• 0 citations
Publisher
DOI
10.1016/0375-9601(73)90942-0

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Abstract The problem of transfer of line radiation in sodium vapor under photoexcitation by laser radiation of a resonance line with wavelength λ = 589.6 nm is numerically solved. The influence of Doppler frequency shifts on photon emission coherent in the atomic reference system is taken into account in the formation of an emission line profile using the model of partial frequency redistribution. In the case of a small optical thickness of a medium τ0 ≤ 1, the Doppler frequency redistribution in the laboratory reference system yields higher intensity values in the emission line profile core in comparison with the model of complete frequency redistribution. For an optically dense medium, thermal motion of atoms leads to an increase of reabsorption in the emission line core, and intensity increases in the line wings as compared to the complete redistribution model. The dependence of the Holstein trapping factor on optical thickness is closer to the analytic dependence than the complete redistribution model. This is explained by the fact that the frequencies of emitted photons in the laboratory reference system fall into the spectral profile core, which increases the effects of resonance radiation trapping in a dense medium.
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 report a theoretical study on the possibility of amplification and generation of radiation by an inversionless two-level system in the ‘red’ wing of its spectral line under resonant diode pumping. The two-level system is used to simulate atoms of an active gas, while it is in the atmosphere of a high-pressure buffer gas. The effect results from the fact that the probability of stimulated emission in the ‘red’ wing of the spectral line exceeds the probability of absorption if the homogeneous broadening due to the interaction of particles with the buffer gas significantly exceeds the natural one (at high pressures of the buffer gas). It is found that the higher the buffer gas pressure and the higher the pump radiation intensity, the greater the inversionless amplification. It is shown that the gain in a two-level system in the ‘red’ wing of its spectral line can reach 0.011 cm −1 when the radiation intensity and the width of the radiation spectrum of pump diodes are 5 kW cm −2 and 4 cm −1 , respectively. The use of transverse diode pumping of an active medium placed in an optical cavity will make it possible to obtain lasing with frequency tuning and thereby solve the problem of converting incoherent broadband radiation into a coherent laser light in a gas of two-level active particles.