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Soviet Journal of Quantum Electronics

Higher-order distributed feedback and generation of light

Pavel A Apanasevich, Anatolii A Afanas'ev, M. V. Korolkov

Published Jun 30, 1982• 2 citations
Publisher

Abstract

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

DOI
10.1070/qe1982v012n06abeh005195

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Sep 16, 2005•0 citations
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
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.
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].
Peculiarities of the implementation of a superradiant generation mode in a laser with a distributed feedback of the waves and an inhomogeneously broadened spectral line of the active medium are investigated. Based on a numerical solution of the semiclassical Maxwell-Bloch equations for the amplitudes of counterpropagating electromagnetic waves, polarization, and inversion of a two-level medium population, it is shown that a sequence of coherent high-power pulses is generated by the Bragg structure in a wide range of the laser parameters as a result of the selected mode and the cw pumping.
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.
This work presents theoretical and experimental studies of the energy efficiency of multiwave mixing in complex molecular media (dye solutions) exhibiting higher order nonlinearities. There is a great variety of such media, including the resonance ones revealing nonlinearities of higher orders due to the absorption saturation effect and transitions between different excited states of the molecules. However, the majority of previous studies of multiwave mixing have been performed in the resonant medium approximation disregarding the induced anisotropy effect. A new method for enhancement of multiwave mixing in solutions of laser dyes has been proposed and substantiated with the use of lasing as an additional light beam, whose absorption by the molecules in the excited states leads to the formation of thermal dynamic gratings resultant in the improved efficiency of the energy exchange. The theoretical models for the formation of nonlinear dynamic holograms adequately describing the control process over multiwave mixing by an independent light beam, the frequency of which is tuned into the induced absorption band from the molecules in the excited state, have been developed. Polarization multiwave mixing has been realized in Rhodamine-6G dye solution using nonlinearities up to the ninth order inclusive. It has been demonstrated that on orthogonal polarization of the hologram recording waves the diffraction efficiency is lowered by one-two orders, the diffracted wave polarization being dependent on the diffraction order (the polarization plane alignment is identical within the orders of the same evenness).