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Journal of the Optical Society of America B

Self-diffraction in population gratings

H. L. Fragnito, Silvania F. Pereira, Alvin E. Kiel

Published Aug 1, 1987• 25 citations
Publisher

Abstract

We present a nonperturbative theory of self-diffraction in thick, pseudostationary population gratings induced by the interference of two nearly collinear incident laser beams of arbitrary relative and absolute intensities. At large saturation parameters, the grating becomes nonsinusoidal, and high-order diffraction occurs efficiently. We obtain an exact expression for the Fourier coefficients of the grating that significantly simplifies the interpretation of the various coherent coupling mechanisms and permits efficient computer simulations. In the case of small depletion of the pump waves, we obtain simple analytic expressions for the scattered intensity into any order of self-diffraction. We present our results in a manner that should aid an experimenter in choosing the optical thickness of the unsaturated medium and the sum and ratio of the pump intensities to obtain optimum diffraction intensity in a given order.

DOI
10.1364/josab.4.001309

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We present a nonperturbative theory of self-diffraction in thick, pseudostationary population gratings induced by the interference of two nearly collinear incident laser beams of arbitrary relative and absolute intensities. At large saturation parameters, the grating becomes nonsinusoidal, and high-order diffraction occurs efficiently. We obtain an exact expression for the Fourier coefficients of the grating that significantly simplifies the interpretation of the various coherent coupling mechanisms and permits efficient computer simulations. In the case of small depletion of the pump waves, we obtain simple analytic expressions for the scattered intensity into any order of self-diffraction. We present our results in a manner that should aid an experimenter in choosing the optical thickness of the unsaturated medium and the sum and ratio of the pump intensities to obtain optimum diffraction intensity in a given order.
We investigate self-diffraction and multiwave scattering by a nonsinusoidal gain grating induced in a thick laser medium with pulses of duration much less than the population relaxation time. The energies of the transmitted and of the two first-order self-diffracted beams are calculated. Experimental measurements performed in a flash-lamp-pumped Nd:YAG amplifier at lambda = 1.064 microm show good agreement with the theoretical results. A total energy in the two first-order scattered waves of 1.1 mJ is obtained for a total incident energy of 14 mJ.
A theoretical analysis is made of the transformation of light waves by optically induced amplitude-phase gratings of different order in a saturable absorbing medium. Allowance is made for gratings appearing; due to the absorption saturation and inhomogeneous heating of the medium. It is shown that, under certain conditions, the higher-order gratings may play a considerable role in the diffraction process. The contributions made by multistage and direct mechanisms to the n-th order diffraction are calculated. Estimates are obtained of the conditions which must be satisfied by the intensity and duration of radiation pulses, as well as by the optical properties and thickness of the absorbing medium, for these contributions to be comparable.
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.
Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].
An investigation is made of a higher-order distributed feedback which appears in a medium characterized by nonharmonic modulation of the parameters due to different radiation diffraction processes. The averaging method is used to obtain reduced equations for the amplitudes of opposite waves and expressions for the gain (absorption coefficient) as well as for the coupling between the waves, subject to all the gratings induced in a medium. Competition between the contributions of various diffraction processes to distributed feedback of arbitrary order is studied. An analysis is made of the coupling coefficient of waves in a dye solution laser with a second-order distributed feedback due to gratings formed by the populations of levels participating in a resonance transition and also thermal gratings induced in the active medium because of saturation of spatially inhomogeneous pumping. A calculation is made of the parameters of the medium for which the competition between the various diffraction processes makes the greatest contribution to a higher-order distributed feedback