The real-time information processing and manipulation of electromagnetic waves using nonlinear Qptical techniques has resulted in a myriad of new applications in diverse fields such as quantum electronics, image processing, optical computing, adaptive optics, and nonlinear spectroscopy. In this paper, we review and explore the field, provide a historical perspective, analyze several of the nonlinear interactions useful for the generation of phase-conjugate replicas, and conclude with a brief survey of potential applications and suitable nonlinear media.

Nonlinear Optical Phase Conjugation

Stimulated Brillouin scattering (SBS) is used for multiple-beam combining by overlapping the beams in the SBS interaction volume and for phase control by backseeding the interaction volume. Tripled Nd:YAG laser experiments and wave-optics analyses indicate the conditions of beam overlap, power, and focusing that are required to obtain effective beam combining and phase control.

Beam combination through stimulated Brillouin scattering

Seeded, transient, stimulated Brillouin scattering (SBS) is studied using a new noniterative, numerical algorithm to solve this two‐point boundary value problem. The coupled pump, Stokes, and phonon equations are solved numerically, including the effects of pump depletion and a finite phonon lifetime. The noniterative approach leads to efficient computation (<30 s of cpu time, VAX 8000 series computer for 500 by 5000 points in space and time, respectively). The numerical study predicts transient oscillations in the pump and the Stokes intensities. The frequency of these oscillations depends on the phonon lifetime and the depleted, exponential SBS gain. This work is the first known prediction of these oscillations.

Numerical study of transient stimulated Brillouin scattering

Phase conjugation in stimulated Brillouin scattering (SBS) from an optical waveguide is studied numerically, using a Cartesian two-dimensional propagation code (bounce) that includes pump depletion. The simulations show that light scattered back through a phase aberrator produces a far-field profile closely matching that of the incident beam; however, the near-field intensity exhibits large and rapid spatial inhomogeneities across the entire aberrator, even for conjugation fidelities H as high as 98%. This effect can be only partially removed by spatial-filtering techniques. The dependence of H on SBS gain, scattering length L, and the average intensity Ī and angular divergence θD of the pump beam is first studied in the low-reflectivity regime (i.e., ignoring pump depletion). For amplitude gains ≳e10, H decreases monotonically with the factor Ī/θD that is due to a small-scale intensity pulling effect. This result appears to be independent of whether θD arises entirely from the aberrator or from a combination of the aberrator plus curvature introduced by a lens. In all cases, pump depletion is found to enhance the fidelity by inhibiting the small-scale pulling effect. Under appropriate conditions, this can actually reverse the intensity dependence of H seen in the absence of pump depletion.

Numerical study of phase conjugation in stimulated Brillouin scattering from an optical waveguide

Phase conjugation of two beams by stimulated Brillouin scattering (SBS) was studied with a computer model and investigated experimentally. The relative phase of the return beams was found to vary randomly if the pump beams were well separated in the SBS cell but remained fixed and equal to the inverse of the pump relative phase if the beams overlapped. Backward Stokes injection was used to lower the threshold and increase the reflectivity of SBS.

Beam combination by stimulated Brillouin scattering

: Optical ray retracing in stimulated Brillouin backscatter from laser-produced plasmas is studied numerically, using a 2-D propagation code (BOUNCE). This code treats steady state behavior in the strong damping limit, and includes self focusing effects. Ray retracing phenomena are grouped into two limiting cases. In the 'whole beam' limit, the pump field at the lens has a broad spatial profile that can be focused to a long narrow waist within the plasma. This geometry selectively amplifies only those initial noise components that are propagating back along the axis, where the net gain is highest. The simulations show that such effects exhibit a pronounced threshold due to self focusing, and disappear when the focal width becomes comparable to the length of the gain medium. In the opposite limit, where the pump radiation at the lens is dominated by small scale transverse structure, the backscatter can reproduce this structure in detail (i.e., exhibit wavefront reversal) when the far field of the pump produces an interference pattern in the plasma.

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Numerical Study of Optical Ray Retracing in Laser-Plasma Backscatter.

Optical ray retracing in stimulated Brillouin backscatter from laser-produced plasmas is studied numerically, using a 2-D propagation code (bounce). This code treats steady-state behavior in the strong damping limit, and includes self-focusing effects. Ray retracing phenomena are grouped into two limiting cases. In the ''whole-beam'' limit, the pump field at the lens has a broad spatial profile that can be focused to a long narrow waist within the plasma. This geometry selectively amplifies only those initial noise components that are propagating back along the axis, where the net gain is highest. The simulations show that such effects exhibit a pronounced threshold due to self-focusing, and disappear when the focal width becomes comparable to the length of the gain medium. In the opposite limit, where the pump radiation at the lens is dominated by small-scale transverse structure, the backscatter can reproduce this structure in detail (i.e., exhibit wavefront reversal) when the far field of the pump produces an interference pattern in the plasma. The plasma then behaves as an active volume hologram as it amplifies the backward-propagating random noise fields.

K. A. Holder

Papers

Numerical Study of Optical Ray Retracing in Laser-Plasma Backscatter.

Numerical study of optical ray retracing in laser-plasma backscatter