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Optical Engineering

Optical Beam Phase Conjugation By Stimulated Backscattering

R. W. Hellwarth

Published Apr 1, 1982• 21 citations
Publisher

Abstract

We review the history and nature of stimulated scattering of optical beams and of the large phase-conjugate component often observed in stimulated backscattering, whether it be of Raman, Brillouin, Rayleigh, or other origin. We review the theory of the magnitude of the phase-conjugate component in stimulated backscattering of waves in a multimode optical guide (the usual experimental configuration) or in free focus. Wide applicability of stimulated backscattering for optical phase conjugation is found.

DOI
10.1117/12.7972891

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We review the history and nature of stimulated scattering of optical beams and of the large phase-conjugate component often observed in stimulated backscattering, whether it be of Raman, Brillouin, Rayleigh, or other origin. We review the theory of the magnitude of the phase-conjugate component in stimulated backscattering of waves in a multimode optical guide (the usual experimental configuration) or in free focus. Wide applicability of stimulated backscattering for optical phase conjugation is found.
Various aspects of optical phase conjugation are discussed: the properties of the conjugate wave, its potential applications, the basic conjugation methods, and a brief history of the question. The theory for phase conjugation in stimulated scattering is set forth in detail. The basic experimental results on this conjugation method are also discussed in detail. Phase conjugation occurs because that configuration of the back-scattered field which has the conjugate wavefront is amplified to the greatest extent (at a doubled gain) in the intense speckle-inhomogeneous conjugate wave in a medium in which stimulated scattering occurs. Because of the large overall amplification in stimulated scattering, all the other, uncorrelated, configurations of the spontaneously scattered nucleating waves are amplified by a factor of 107 less and are discriminated against. The intervals of values of the various parameters in which the conjugate configuration (the specklon) exists are discussed theoretically, as is the effect of nonlinear selection and saturation on phase conjugation in stimulated scattering. There is a review of experimental results on the first observation of the effect, on the measurement of the angular structure of the uncorrelated waves and of the extent to which they are discriminated against, on the phase fluctuations of the conjugate wave, on the conjugation of subthreshold and depolarized radiation, and on phase conjugation in stimulated scattering in focused beams and for other scattered-wave amplification mechanisms.
We consider the backward optical wave stimulated by a multimode, monochromatic, incident optical wave in a waveguide filled with a transparent nonlinear medium, when the incident wave is negligibly perturbed by the nonlinear processes. We derive the conditions on guide length, area, mode number, and Stokes shift in order that a given high percentage of the power in the backscattered field be the “phase conjugate” of the incident field, i.e., be proportional to its complex conjugate in the entrance plane of the waveguide.
Optical phase conjugation by stimulated Brillouin scattering, excited by an inhomogeneous light field with a self-focusing strong component, has been investigated. The conjugation fraction \ensuremath{\eta} depends on the intensity ratio \ensuremath{\sigma} of the strong component to the total pump light, attaining a maximum at a certain value of \ensuremath{\sigma}. Theoretical analysis shows that a four-wave-mixing process is involved in its physical mechanism. By taking into account the effect of self-focusing, a fair agreement with experiments is obtained.
Optical phase conjugation is concerned with the use of nonlinear optical effects to precisely reverse the direction of propagation of each plane wave in an arbitrary beam of light. This causes the return beam to exactly retrace the path of the incident beam, leading to many applications of automatic pointing and tracking (even in the presence of an inhomogeneous medium). The main goals of this course are familiarization with how each of the many nonlinear optical effects gives rise to a phase conjugate signal, and understanding and appreciation of the many potential applications for optical phase conjugation. Conjugation techniques involving Kerr-like degenerate four-wave mixing, resonant degenerate four-wave mixing, stimulated Brillouin scattering, photorefractive effects, surface phenomena, and others will be carefully reviewed. Applications of particular importance to this course will include distortion correction, pointing, tracking, laser-resonator design, optical information processing, etc. Throughout the course, key references will be identified for those wishing to pursue any topic more vigorously.
The demonstration of optical phase conjugation in photorefractive materials has been accomplished by numerous methods, including i) externally-pumped four-wave mixing,1ii) self-pumped (passive) phase conjugation,24iii) stimulated backscattering,5,6and iv) mutually-pumped phase conjugation.7-10Stimulated backscattering and self-pumped phase conjugators have only a single incident beam. On the other hand, externally-pumped four-wave mixing has two auxiliary pumping beams in addition to the incident probe beam. Different still is the cross-conjugation of the two incident beams in mutually pumped phase conjugators. To use these conjugators in a given application, the compatibility between the processes generating the phase-conjugate wave(s) and the application must be considered. The distinguishing features of the different conjugators translate to advantages or disadvantages for certain applications.
This paper reviews the main experimental and theoretical developments in stimulated Raman scattering of light, which has been actively investigated since 1962. The effect cannot be discussed properly without paying attention to related nonlinear effects which include stimulated Brillouin and Rayleigh scattering, self-focusing of light, and modulation of the refractive index. The physical processes are emphasized and no encyclopedic survey of all the data and the research literature is attempted, but a brief historical account is added in the final section.