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Annual Meeting Optical Society of America

Nonsaturable absorption due to stimulated emission of radiation

C. K. Hong, Howard Nathel, Roger W. Minich, J. C. Garrison, Burke Ritchie

Published Jan 1, 1989• 0 citations
Publisher

Abstract

We report an observation of nonsaturable absorption of light. When a laser medium in a small aspect ratio geometry is uniformly and transversely pumped, amplified spontaneous emission (ASE) builds up along the longitudinal axis. The higher ASE intensity at the ends of the cell repopulates the ground state through the stimulated emission process more rapidly than at the center of the cell. Therefore, the absorption of the pump light is enhanced at the ends and the transmitted pump beam shows a nonuniform profile with a peak at the center. When the occupation of the states other than the upper lasing state and the ground state can be ignored, the normalized profile of the transmitted pump beam becomes identical to that of the fluorescence depicting the upper state population distribution. This fact has been confirmed experimentally with a R6G dye cell pumped with various intensities of frequency doubled Nd:YAG laser (λ = 532 nm, 10-ns pulse duration). The percentage of the absorbed pump energy at the ends of the cell remains the same, virtually nonsaturable, whereas at the center it decreases as the pump intensity increases.

DOI
10.1364/oam.1989.tus8

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We report an observation of nonsaturable absorption of light. When a laser medium in a small aspect ratio geometry is uniformly and transversely pumped, amplified spontaneous emission (ASE) builds up along the longitudinal axis. The higher ASE intensity at the ends of the cell repopulates the ground state through the stimulated emission process more rapidly than at the center of the cell. Therefore, the absorption of the pump light is enhanced at the ends and the transmitted pump beam shows a nonuniform profile with a peak at the center. When the occupation of the states other than the upper lasing state and the ground state can be ignored, the normalized profile of the transmitted pump beam becomes identical to that of the fluorescence depicting the upper state population distribution. This fact has been confirmed experimentally with a R6G dye cell pumped with various intensities of frequency doubled Nd:YAG laser (λ = 532 nm, 10-ns pulse duration). The percentage of the absorbed pump energy at the ends of the cell remains the same, virtually nonsaturable, whereas at the center it decreases as the pump intensity increases.
A semiclassical dynamic theory of the nonlinear propagation of a few interacting intense light pulses is applied to study the nonlinear counterpropagation of amplified spontaneous emission (ASE) induced by three-photon absorption of short intense laser pulses in a chromophore solution. Several important results from the modeling are reached for the ASE process developing in the regime of strong saturation. Accounting for ASE in both forward and backward directions with respect to the pump pulse results in a smaller efficiency of nonlinear conversion for the forward ASE compared with the case in which forward emission is considered alone, something that results from the partial repump of the absorbed energy to the backward ASE component; the overall efficiency is nevertheless higher than for the forward emission considered alone. The efficiency of nonlinear conversion of the pump energy to the counterpropagating ASE pulses is strongly dependent on the concentration of active molecules so that a particular combination of concentration versus cell length optimizes the conversion coefficient. Under certain specified conditions, the ASE effect is found to be oscillatory; the origin of oscillations is dynamical competition between stimulated emission and off-resonant absorption. This result can be considered one of the possible explanations of the temporal fluctuations of the forward ASE pulse [Nature 415, 767 (2002)].
The stimulated emission threshold of a dye laser was found to depend on the method of formation of a longitudinal pulsed laser beam used as the pump. Excitation with a focused pump beam reduced the lasing threshold. The effect was strongest when the Q factor of the resonator cavity was low. This reduction in the lasing threshold by focusing the pump laser beam was attributed to an increase in the rate of spontaneous emission resulting from lasing of desqueezed states (enhancement of 'zero-field' fluctuations responsible for the appearance of spontaneous emission). An increase in the spontaneous emission rate reduced the buildup time of stimulated emission during the early stages and thus reduced the lasing threshold.
A calculation is made of the stimulated emission threshold of organic dye solutions as a function of the diffusion coefficient, intensity of induced singlet–singlet or triplet–triplet absorption at the stimulated emission wavelength, steepness of the leading edge of pump pulses, and other parameters. It is shown that, if the dipole moment of a fluorescence transition is perpendicular to the dipole moments of associated induced singlet–singlet or triplet–triplet absorption transitions, the laser threshold of high–viscosity solutions is considerably lower than that of low-viscosity systems. The maximum value of the ratio of the cross sections of the singlet-singlet absorption and stimulated emission, at which a given molecule ceases to emit stimulated radiation, increases from 1 to 3 when the rotational diffusion coefficient decreases from ∞ to 0.
The output of pulsed, homogeneously broadened amplified spontaneous emission (ASE) lasers operating under conditions of strong saturation is considered. An expression for the maximum power (energy) per unit volume that may be extracted from steady-state (pulsed) systems is derived and shown to be independent of the saturation intensity. Numerical calculations of the temporal evolution of the populations in the laser levels and the growth in the ASE fluence, peak intensity, and spectral width are presented for three types of laser system: quasi steady state, short pump pulses, and self-termination. In all cases the growth in the fluence of the ASE pulse is found to agree with an analytic solution derived for conditions of strong saturation. The calculated growth in intensity of the ASE pulse is compared with expressions derived by Pert [J. Opt. Soc. Am. B11, 1425 (1994)] for the steady-state case. As expected, good agreement is found for the quasi-steady-state system, but in the limit of strong saturation significant differences occur for the short pump pulse and self-terminating laser systems considered. For all cases considered, the variation of the spectral width of the ASE pulse is found to be in reasonable agreement with that predicted by Pert for steady-state conditions.
Paraxial Maxwell-Bloch theory is used to describe amplified spontaneous emission (ASE) in strongly inverted media. Counterpropagation of waves coupled through gain saturation and diffraction, and pumping by a laser beam which propagates transverse to the amplifier are included in the treatment. The reactive part of the medium polarization is calculated by population-rate equations in the limit of rapid dephasing of the atomic dipoles. The spontaneous emission noise source needed for the numerical simulation of Maxwell's equations is derived from microscopic theory by coarse-graining, thereby establishing the compatibility of the spherically symmetric microscopic source with paraxial propagation theory. The three coupled Maxwell equations for the counterpropagating ASE waves and the transversely propagating pump wave are solved numerically in time and two spatial dimensions. This simulation is used to calculate the longitudinal profile of the transverse fluorescence, which measures the saturation of the gain, the longitudinal profile of the transmitted pump wave, which measures the saturation of pump absorption, and the transverse coherence function of the radiation emitted from the ends of the amplifier.
The features of fluorescence emission in a dye-doped dense multiple scattered medium under pulsed laser pumping are considered in terms of confined excitation in small zones associated with laser speckles occurring in a pumped medium. The results of numerical modeling of the fluorescence emission kinetics are compared to the experimental data obtained using the rhodamine 6G-doped layers of the densely packed TiO2 (anatase) particles pumped at 532 nm by 10 ns laser pulses. The intensity of pump radiation during the action of laser pulses was varied from 1·105 W/cm2 to 5·107 W/cm2. In the recovery of the ratios of stimulated to a spontaneous emission, the spectra of the stimulated component were fitted using the spectral function derived by R. Dicke. In the framework of the considered concept, saturation of the ratio of the stimulated to a spontaneous emission and linear growth of an integrated fluorescence output with a practically unchangeable half-width of the emission spectra at high pump intensities are interpreted.