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Soviet Journal of Quantum Electronics

Phase conjugation under conditions of degenerate four-wave interaction involving simulated Raman scattering in a resonator

Anatolii A Afanas'ev, V V Drits, B. A. Samson

Published Dec 31, 1989• 0 citations
Publisher

Abstract

A system for phase conjugation of Stokes radiation by a four-wave interaction involving intracavity stimulated Raman scattering (STRS) in a Raman-active medium with a nonresonant nonlinearity. This method can be used in construction of high-power sources of highly directional Stokes radiation on the basis of round-trip STRS amplifiers coupled to a phase-conjugating element. It is shown that an STRS converter utilizing organic liquids and subjected to laser pumping of less than 100 MW/cm2 intensity inside a resonator can ensure a highly efficient phase conjugation with a peak reflection coefficient in excess of unity. Depending on the ratio of the Raman and nonresonant susceptibilities of a medium, there is an optimal value of the coefficient representing the degree of conversion of laser pump radiation into STRS radiation, which ensures the maximum phase conjugation efficiency.

DOI
10.1070/qe1989v019n12abeh010179

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A system for phase conjugation of Stokes radiation by a four-wave interaction involving intracavity stimulated Raman scattering (STRS) in a Raman-active medium with a nonresonant nonlinearity. This method can be used in construction of high-power sources of highly directional Stokes radiation on the basis of round-trip STRS amplifiers coupled to a phase-conjugating element. It is shown that an STRS converter utilizing organic liquids and subjected to laser pumping of less than 100 MW/cm2 intensity inside a resonator can ensure a highly efficient phase conjugation with a peak reflection coefficient in excess of unity. Depending on the ratio of the Raman and nonresonant susceptibilities of a medium, there is an optimal value of the coefficient representing the degree of conversion of laser pump radiation into STRS radiation, which ensures the maximum phase conjugation efficiency.
Part I of this thesis describes the theoretical and experimental aspects of a new area in coherent optics known as phase conjugate optics. By using nonlinear optical mixing techniques, the complex conjugate of an arbitrary incident monochromatic wave can be generated. In particular, the theory of the degenerate four-wave mixing technique is presented. Experimental observation of phase conjugate wave front generation in a liquid crystal material (MBBA) is described. The ability of the conjugate wave front to correct for phase aberration is demonstrated. The quantum mechanical origin of the nonlinear third order susceptibility responsible for the four-wave mixing process is illustrated in an atomic system. Because of a finite memory time of such an atomic system, the electromagnetic waves taking part in the mixing process need not occur simultaneously. The applications of the phase conjugation process to real-time information processing and the characteristics of a phase conjugate resonator are discussed in detail. The ability of an optical fiber to confine the propagation of electromagnetic waves over a long distance and within a small cross sectional area implies that large optical intensities can be maintained over a long " interaction length even at moderate incident optical power. Part II of this thesis describes some third order nonlinear optical phenomena that have been observed under this condition. Especially, phase conjugation by degenerate four-wave mixing in a fiber has been shown to require significantly less pumping power. The theoretical and experimental studies of four-wave mixing in a liquid-filled fiber are presented. Extensive experimental efforts have been devoted over the last few years to the study of stimulated Raman scattering and the oscillation of the Stokes radiation inside fibers. A detailed theoretical treatment will be given here to describe these effects. The analysis shows that pump depletion due to the nonlinear process is very important in describing the stimulated Raman scattering in low loss fibers and is the dominant saturation effect in a fiber Raman oscillator. Finally, the implications of nonlinear optical phenomena to fiber optic communication will be mentioned.
A wavefront reversal system is proposed for high-power optical radiation. This system is based on a resonant four-wave interaction without recourse to external pump waves. Stimulated scattering of the wave being reversed in a resonant medium placed inside a resonator creates two opposite pump waves and a parametric interaction between them generates reversed radiation. The efficiency of such self-reversal is comparable with the efficiency of conventional wavefront reversal as a result of a four-wave interaction in nonresonant media with a phase nonlinearity and it may reach 10 −3 .
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].
A model is developed for the description of stimulated Raman scattering. This model allows for changes in the level populations due to radiative transitions and it can be used also for an arbitrary detuning of the pump radiation from the center of a line of an atom in the active medium. A study is made of the conversion of wide-band laser radiation into a Stokes signal. It is shown that this conversion is quite efficient and the Stokes signal gain corresponds to coherent scattering. This result is related to the matching of the phases of the pump and Stokes signals. Numerical investigations carried out using this model under the experimental conditions described elsewhere give results in agreement with the experimental data.
As it is known, self-conjugation of the IR laser beam via Brillouin radiation stimulated scattering phenomenon is not realised because of some physical limits on radiation intensity which are the principal limitations, to the present time. That is why, one of the promising technique of phase conjugation of infrared radiation is a self-conjugation of a laser beam wavefront via four wave interaction in a nonlinear medium with feed-back loop (FWMF) [1]. Unlike conventional our wave mixing schemes, the FWMF self-conjugation is supposed to be realised without any additional beams pumping a phase conjugation mirror.