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Pure and Applied Optics Journal of the European Optical Society Part A

Distortions of the probe light wavefront in a thermal lens pump-probe experiment

A Marcano O, Jimmy A. Castillo

Published May 1, 1994• 4 citations
Publisher

Abstract

Distortions of the probe beam wavefront due to scattering on the thermal lens, which is induced by a pump light field, are calculated using a non-linear geometrical model. It is shown that when the probe beam light spot is larger than the pump field one, the initial Gaussian distribution of intensities becomes ring shaped. When the probe beam diameter is smaller than the pump field one, the initial Gaussian distribution remains almost Gaussian. The predicted effects are confirmed by a calculation that takes into account the diffraction effects at the far field. Experimental evidence for the effect is shown for the case of light propagation through an ethanol solution of iodine.

DOI
10.1088/0963-9659/3/3/017

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Distortions of the probe beam wavefront due to scattering on the thermal lens, which is induced by a pump light field, are calculated using a non-linear geometrical model. It is shown that when the probe beam light spot is larger than the pump field one, the initial Gaussian distribution of intensities becomes ring shaped. When the probe beam diameter is smaller than the pump field one, the initial Gaussian distribution remains almost Gaussian. The predicted effects are confirmed by a calculation that takes into account the diffraction effects at the far field. Experimental evidence for the effect is shown for the case of light propagation through an ethanol solution of iodine.
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.
We analyze the deflection of a probe beam because of pump-probe interaction in a high-absorbing thermal medium. We extend the existing theory by accounting for translation of a finite-width probe because of deflection within the nonlinear sample. We also provide expressions for the number of resolvable angles of the probe for possible applications of the setup as a beam deflector and study conditions for the maximization of the deflection angle and the resolution. We present experimental results obtained with a solution of chlorophyll in ethanol as the thermal medium.
We show that in a gas laser the inhomogeneity in the distribution of amplifying atoms and the transverse Gaussian distribution of the saturating electromagnetic field give rise to different lens effects which are generally opposite to each other and influence the line shapes in different ways, shifting the Lamb dip either to the blue or to the red. For those laser phenomena which increase one or the other of the lens effects, the role of a diaphragm is of particular importance, since it determines the spatial distribution of the resonant field. We present here a new method for studying the resonant diffracted field in a laser. Also, we give a unified description of the various inhomogeneities. Doing away with the usual method of mean-field approximation, we numerically calculate, for a diaphragmed, monomode weak-gain gas laser, the intensity, variation of the beam diameter, and the radius of curvature as functions of the frequency.
A theoretical analysis is made of stimulated light scattering in a pump beam possessing both fine-structure inhomogeneity (and thus, "gray" divergence) and a continuously varying amplitude and phase envelope due to focusing of the beam with a lens into an unbounded medium. An equation is derived to describe the gain of the pump-reproducing field component, and a solution of this equation, excitable by spontaneous radiation noise, is found in the constant pump field approximation. In this case, the scattered wave is localized in a narrow (compared with the pump wave) near-axial zone and, unlike scattering in a waveguide, the reproduction of the pump wave is partial and dependent on the correlation between the "gray" and diffraction-limited divergence. The disagreement between this theoretical conclusion and the complete reproduction observed earlier experimentally indicates the importance of saturation effects in the scattering of focused beams.
Phase-modulation effects in self-diffraction with two pump beams incident on a nonlinear medium were studied theoretically and experimentally. Based on a two-level model and considering the influence of the phase modulation on the phase matching, expressions describing the first-order and second-order scattered intensities were derived. The influences of phase modulation on the pump-intensity dependence and the sample-concentration dependence of the scattered intensities were discussed via numerical calculations. Experiments were performed with tetrahydrofuran solution of 1,4-diethinyl benzene to demonstrate the theoretical results. Some interesting results predicted by the theory, such as the fact that the scattered intensities of the sample with lower concentration can be larger than that with higher concentration at suitable high intensity of the pump, have been observed. \textcopyright{} 1996 The American Physical Society.
An investigation is made of the wave problem of the stimulated scattering of light in a pump beam comprising a plane wave and a small (in terms of energy) admixture of fluctuating distortions. The existence of interference maxima and minima in the pump wave causes an appreciable (up to twofold) enhancement of the gain of the scattered wave, the form of which reproduces that of the pump field. This effect is associated with the previously observed reversal of the wavefront in the stimulated scattering of light.