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Optics and Spectroscopy

Effect of spatial-temporal coherence of medium excitation on the statistical characteristics of lasing

Yu. M. Golubev, V. P. Gryaznevich

Published Nov 1, 1983• 0 citations
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A Parametric Study of Mirrorless Lasing Dynamics

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There exists a lack of laser-based remote sensing methods that give signals with high intensity and directionality. According to previous research, mirrorless lasing can meet these requirements; however, neither the dependence on the gain medium geometry nor the light-matter interaction responsible for the emission has been investigated in detail. This thesis project examined the sideways lasing emission from a gain medium with a Fresnel number much smaller than one, generated by the overlap of two counterpropagating femtosecond laser pulses in xenon gas at atmospheric pressure. Both spatial and temporal measurements of the sideways lasing emission were made, with the latter being obtained using a streak camera. The emission was highly localised in space, suggesting it originated from the overlap. It also yielded a power dependence with a clear threshold and an interference pattern indicating coherence, both of which showed strong agreement with the predicted behaviour for superfluorescence (SF). The recorded temporal profile displayed a pulse with oscillations on the incline and the decline, appearing with a delay of about 70 ps relative to the excitation pulse and thus corresponding well to the theoretical predictions for SF emitted from a gain medium with a Fresnel number much smaller than one. It can therefore be concluded that the observed sideways lasing was generated by SF. Additionally, the dependence of the emission on the gain medium geometry was in good accordance with theory. These measurements, to the best of the author's knowledge, provide the first thorough investigation of mirrorless lasing at atmospheric pressure for this gain medium configuration, thus opening the way for both further fundamental research and the implementation of these techniques in applied laser diagnostics.

•Jan 1, 2026•0 citations
We study the dynamics of lasing from photonic paints excited by short, localized, optical pulses, using a time-dependent diffusion model for light propagating in the medium containing active atoms. The full time-dependent, nonadiabatic nonlinear response of the atomic system to the local optical field intensity is described using the Einstein rate equations for absorption and emission of light. Solving the time-dependent diffusion equation for the light intensity in the medium with nonlinear gain and loss, we derive detailed information on the spectral, spatial, and temporal properties of the emitted laser light. Our model recaptures the effects of scatterers to narrow the emission spectral linewidth and to narrow the emitted pulse duration, at a specific threshold pump intensity. Our model also describes how this threshold pump intensity decreases with scatterer density and excitation spot diameter, in excellent agreement with experimental results.
An analytic method of decoupling of physical factors is used to investigate the relationship between the temporal and spatial coherence of a randomly modulated light pulse representing a superposition of a signal and noise in a medium with a local nonlinearity. An analysis is made of the factors that influence the transformation of the spatial and temporal characteristics of a diffracted and spreading light pulse in a nonlinear medium. The relationships governing the mutual influence of the signal and noise, resulting in changes in the regular envelopes and statistical characteristics, are determined.
There exists a lack of laser-based remote sensing methods that give signals with high intensity and directionality. According to previous research, mirrorless lasing can meet these requirements; however, neither the dependence on the gain medium geometry nor the light-matter interaction responsible for the emission has been investigated in detail. This thesis project examined the sideways lasing emission from a gain medium with a Fresnel number much smaller than one, generated by the overlap of two counterpropagating femtosecond laser pulses in xenon gas at atmospheric pressure. Both spatial and temporal measurements of the sideways lasing emission were made, with the latter being obtained using a streak camera. The emission was highly localised in space, suggesting it originated from the overlap. It also yielded a power dependence with a clear threshold and an interference pattern indicating coherence, both of which showed strong agreement with the predicted behaviour for superfluorescence (SF). The recorded temporal profile displayed a pulse with oscillations on the incline and the decline, appearing with a delay of about 70 ps relative to the excitation pulse and thus corresponding well to the theoretical predictions for SF emitted from a gain medium with a Fresnel number much smaller than one. It can therefore be concluded that the observed sideways lasing was generated by SF. Additionally, the dependence of the emission on the gain medium geometry was in good accordance with theory. These measurements, to the best of the author's knowledge, provide the first thorough investigation of mirrorless lasing at atmospheric pressure for this gain medium configuration, thus opening the way for both further fundamental research and the implementation of these techniques in applied laser diagnostics.