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We have demonstrated a loop phase-conjugate mirror that has produced up to 200 W phase-conjugate average output power at a 140% reflectivity for a CW input beam at a wavelength of 1.03 μm. A high conjugation fidelity was established by showing that more than 80% of the output beam power was contained within a 1.3–1.7 times diffraction limited beam after double passing a heavily aberrating amplifier chain.

We have demonstrated stable operation of a 2-kW Yb:YAG phase-conjugate master oscillator power amplifier (PC-MOPA) laser system with a loop phase-conjugate mirror (LPCM). This is the first demonstration of a continuous wave (CW)-input LPCM MOPA operating at a power greater than 1 kW with a nearly diffraction-limited output beam. The single-pass beam quality incident on the LPCM varied with the specific operating conditions, but it was typically ${sim}20$ times diffraction-limited (XDL). The measured beam quality with an MOPA output power of 1.65 kW was 1.3 XDL.

We describe an Er:SHARC fiber amplifier generating 1-nsec pulses having peak powers approaching 100 kW at a 30 kHz repetition rate. Pulse energies of 0.9 mJ were achieved with a longer pulse length of 15 nsec.

We have proposed, designed, manufactured and tested low loss dielectric micro-lenses for infrared (IR) radiation based on a dielectric metamaterial layer. This metamaterial layer was created by patterning a dielectric surface and etching to sub-micron depths. For a proof-of-concept lens demonstration, we have chosen a fine patterned array of nano-pillars with variable diameters. Gradient index (GRIN) properties were achieved by engineering the nano-pattern characteristics across the lens, so that the effective optical density of the dielectric metamaterial layer peaks around the lens center, and gradually drops at the lens periphery. A set of lens designs with reduced reflection and tailorable phase gradients have been developed and tested, demonstrating focal distances of a few hundred microns, beam area contraction ratio up to three, and insertion losses as low as 11%.

We modify the radiation transfer approach, accounting for refraction, and apply this model to the analysis of propagation of spatially incoherent beams in inertial nonlinear media. For double Gaussian beams, an oscillatory regime of nonlinear diffraction is revealed. An explicit analysis of the oscillations was carried out for logarithmic saturating nonlinearity. We show that ``big incoherent solitons,'' which possess an arbitrary transverse profile of intensity and exist for a wide class of nonlinear media, are steady state solutions of the radiation transfer equation.

Dielectric metamaterial layers are lossless, and exhibit relatively high transmission efficiencies (contrary to plasmonic metasurfaces). Subwavelength dielectric lenses have been developed - demonstrating beam area contraction ratio of three, and insertion losses of 11%.

A new class of optical fiber, the SHARC fiber, is analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined-gain dopants. The high-aspect-ratio (~30:1) rectangular core allows mode-area scaling well beyond 10,000 μm2, which is critical to high-pulse-energy or narrow-linewidth high-power fiber amplifiers. While SHARC fibers offer modally dependent edge loss at the wide "semi-guiding" edge of the waveguide, the inclusion of gain filtering adds further modal discrimination arising from the variation of the spatial overlap of the gain with the various modes. Both methods are geometric in form, such that the combination provides nearly unlimited scalability in mode area. Simulations show that for kW-class fiber amplifiers, only the fundamental mode experiences net gain (15 dB), resulting in outstanding beam quality. Further, misalignment of the seed beam due to offset, magnification, and tilt are shown to result in a small (few percent) efficiency penalty while maintaining kW-level output with 99% of the power in the fundamental mode for all cases.

SHARC fiber lasers offer very large rectangular core areas of 30,000 μm^2 or more, peak and average powers well beyond the state of the art, good beam quality, and a mechanical flexibility allowing compact coiling.

We demonstrated 5-mJ, 13-nsec pulses using a diode-pumped high-aspect ratio core (HARC) fiber with an active area of 30 x 580 μm2, in good agreement with our model, which predicts scalability to several 10s of mJ.

It is demonstrated that the self-starting generation for a phase-conjugate semilinear mirror is preceded by the silent zone, which is necessary for preparing a phase-conjugate generation structure in a multimode nonlinear resonator. The threshold for self-starting and the response delay are calculated. The experiment with a strontium barium niobate crystal demonstrates the reasonable quantitative agreement with the theory.

It is shown theoretically and experimentally that the appearance of lasing in a semilinear phase-conjugating mirror is preceded by a silent zone due to the formation of a mode with the conjugate wavefront in a multimode resonator.

We describe a hybrid narrowband fiber-slab laser system that produced 330 W CW output power at 1030 nm with a frequency bandwidth of 4 kHz and an average beam quality of 1.9 times diffraction-limited.

We have demonstrated stable operation of a 2-kW Yb:YAG phase-conjugate master oscillator, power amplifier (PC-MOPA) laser system with a loop phase-conjugate mirror (LPCM). This is the first demonstration of a CW-input LPCM MOPA operating at a power greater than 1 kW with a nearly diffraction-limited output beam. The single-pass beam quality incident on the LPCM varied with the specific operating conditions, but it was typically ~ 20 times diffraction-limited (XDL). The measured beam quality with a MOPA output power of 1.65 kW was 1.3 XDL.

For the first time, beam quality improvement of 2 μm laser using fiber based phase conjugation mirror has been demonstrated. Single frequency operation is necessary to lower threshold. The reflectivity of the PCM is ~50%.

For the first time, a 2-micron master-oscillator-power-amplifier laser using a fiber based phase conjugation mirror has been demonstrated. The beam quality improvement and 56% of the PCM reflectivity have been achieved.

Principal possibility of real-time low loss optical link between moving objects by means of aberrations cancelling phase conjugation (PC) technique is theoretically analyzed and experimentally verified using photorefractive SBN crystals. Angular tracking rate up to 0.5 degrees per second at 0.2 W light pumping power had been demonstrated in visible. Technical characteristics which are expected for real-time tracking via adaptive PC links are estimated in comparison with conventional electronics tracking systems. It is shown that a combined scheme, which incorporates an additional photorefractive double PC mirror for fine cancelling errors of the basic tracking system, seems to be the most promising one and would result in significant enhancement of a tracking accuracy. PC link between the Earth based multi- channel transmitter and the array of retroreflectors as a passive retranslator which is located at a geostationary orbit and distributes multiplexed signals of all the channels among many users at the Earth is proposed and discussed.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have produced 5 mJ, 13 ns pulses using a very large-mode-area rectangular fiber containing a high-aspect-ratio core with dimensions of ∼30 μm×580 μm. The rectangular fiber design retains a thin cladding dimension ∼0.5 mm, which is compatible with a compact coiled package. We have developed a theoretical model that achieves good agreement with the experimental data. This model indicates that an optimized fiber design can be scaled to pulse energies of several tens of millijoules. We also discuss a strategy for maintaining good beam quality in both transverse dimensions of this fiber.

The nonlinear problem of speckle-beam propagation in the photo-refractive crystals has been solved. The solutions corresponding to the beam trajectory self-bending have been obtained both in approximation of infinite beam diameter and for the case of finite one. The local tilting rate of the beam propagation direction proved to be proportional to the square of beam angular divergence. The influence of space-charge limit field on the trajectory curvature radius has been investigated as well.

A new class of optical fiber is presented that departs from the circular-core symmetry common to conventional fibers. By using a high-aspect-ratio (~30:1) rectangular core, the mode area can be significantly expanded well beyond 10,000 μm2. Moreover, by also specifying a very small refractive-index step at the narrow core edges, the core becomes "semi-guiding," i.e. it guides in the narrow dimension and is effectively un-guiding in the wide mm-scale dimension. The mode dependence of the resulting Fresnel leakage loss in the wide dimension strongly favors the fundamental mode, promoting single-mode operation. Since the modal loss ratios are independent of mode area, this core structure offers nearly unlimited scalability. The implications of using such a fiber in fiber laser and amplifier systems are also discussed.

We present, for the first time to our knowledge, an explicit experimental comparison of beam quality in conventional and confined-gain multimode fiber lasers. In the conventional fiber laser, beam quality degrades with increasing output power. In the confined-gain fiber laser, the beam quality is good and does not degrade with output power. Gain filtering of higher-order modes in 28microm diameter core fiber lasers is demonstrated with a beam quality of M(2)=1.3 at all pumping levels. Theoretical modeling is shown to agree well with experimentally observed trends.

Nondegenerate four-wave nonlinear mixing as a reflecting frequency filter is considered. Several simple methods to tune the central frequency, angle of reflection and selective properties of this filter are proposed.

A theoretical investigation is made of the influence of linear amplification as a result of stimulated Brillouin scattering in a multimode fiber waveguide. Calculations are made of the threshold of such scattering and of the efficiency of conversion of a pump wave into a Stokes wave under steady-state and transient conditions. The theoretical results are shown to agree well with experimental results obtained under small-gain conditions.

The variational principle is used to derive a system of equations describing the four-wave parametric oscillation in a medium with a reactive cubic nonlinearity. In the case of a transversely pumped configuration the integrals of the system can be used to obtain an analytic solution of the problem. The steady-state above-threshold amplitude of the signal waves is limited by their phase self-modulation and it is close to the amplitude of the reference waves.

Studies on the electrodynamic theory of the process of readout of holograms recorded in the interference of an object speckle field and a smooth reference wave are reviewed. These holograms are classified as thin, thick-layer, and volume holograms, and the process of reconstitution is examined from a unitary standpoint for all these types of holograms. Calculations are performed of the diffraction efficiency and noise level with account taken of various physical factors: saturation of the photoresponse, shift in frequency and angle during reconstruction, shrinkage of the hologram material, etc. Substantial advantages of volume holograms are demonstrated with respect to efficiency and quality of reconstruction in the next five years and in the prospects up to the year 2000.

Theoretical results concerning the wave front reversal (WFR) of spatially depolarized light in stimulated scattering are presented. The modes of scattering medium are found and the deterioration of reversal quality for the depolarized light is predicted. Experimental investigation of WFR in backward stimulated Brillouin scattering (SBS) in acetone was carried out when the light beam with λ = 1.06 μm was depolarized by the etched calcite plate. The share of scattered energy which has exactly reversed configuration, was measured. Experimental results quantitatively coincide with the theoretical conclusions.

A theoretical analysis is made of stimulated light scattering in a pump beam possessing both fine-structure inhomogeneity (and thus, "gray" divergence) and a continuously varying amplitude and phase envelope due to focusing of the beam with a lens into an unbounded medium. An equation is derived to describe the gain of the pump-reproducing field component, and a solution of this equation, excitable by spontaneous radiation noise, is found in the constant pump field approximation. In this case, the scattered wave is localized in a narrow (compared with the pump wave) near-axial zone and, unlike scattering in a waveguide, the reproduction of the pump wave is partial and dependent on the correlation between the "gray" and diffraction-limited divergence. The disagreement between this theoretical conclusion and the complete reproduction observed earlier experimentally indicates the importance of saturation effects in the scattering of focused beams.

A quantitative theory is developed for wavefront reversal in four-wave interaction in a field of coded reference waves, i.e., of waves having substantial fine-structure spatial inhomogeneity. It is found that by using conjugate coded waves, it is possible to reverse a weak signal with transfer of up to 50% of the reference wave energy to the reversed component and to eliminate the self-focusing instability of conventional four-wave wavefront reversal. Possible methods of producing conjugate coded waves are indicated.

An analysis is made of stimulated backscattering in the case when the pump (laser) radiation has a continuous frequency spectrum and a time-dependent (nonfactorizable) speckle structure. The gain increments μ(p) and the corresponding specklon solution for a Stokes wave are found in the case of an arbitrary ratio of the widths of the pump line Δωl, and of the stimulated scattering line Γ. These solutions correspond to partial wavefront reversal, i.e., to reversal of the spectral band of the pump radiation within an interval of width of the order of Γ/2 around a certain frequency p. A comparison of the specklon gain increments with the increments for uncorrelated waves makes it possible to determine the conditions for wavefront reversal.

A scheme of wave front reversal in four wave mixing (WFR-FWM) with angular tilting of the reversed wave is proposed, theoretically discussed and experimentally realized. It is based on detuning from counterpropagation of reference waves. Unlike the proper WFR-FWM, tilting on the angle γx reversal is effective only for the signal directions near the synchronism cone. when γx ⪡ 1 this cone is almost the plane which is perpendicular to the turning direction, and the angular width of effective reversal across the plane is δθx ∽ (kLγx)-1 where L is the depth of the nonlinear medium, k = 2π/λ. The experimental investigation of the interaction of λ = 1.06 μm radiation with the saturable dye solution for the tilting angle γx up to 10-2 rad was carried out. The measured angular selectivity of tilting reversal corresponds to the theoretical one. The energetic fraction of an ideally reversed field in the whole reflected wave was quantitatively measured.

The diffraction of light in the interior of a hologram recorded using a speckle field is considered. A quantitative calculation is made of the effects produced by a spatial resonance of a speckle field with inhomogeneities of a medium that reproduce the field intensity profile. It is shown that, contrary to the established view, the Kogelnik model of coupled plane waves gives quantitatively correct results for the diffraction efficiency even in the case of speckle-field holograms. Introduction of a speckle structure into recording beams shifts the spectral-angular selectivity curve, but does not change its profile in the case of phase holograms, whereas in the case of holograms with an amplitude component in the grating it alters additionally the maximum of this curve. Under normal conditions these effects are small and they become significant only when the holographic grating amplitude is less than or of the order of the total amplitude of the perturbations recorded in the hologram medium.

Wavefront reproduction or reversal, observed earlier experimentally and interpreted for stimulated Brillouin scattering, is considered now in the case of stimulated Raman scattering, i.e., for the scattering accompanied by a relatively large frequency shift. A condition is derived for the frequency shift which ensures that the reproduction effect occurs also in stimulated Raman scattering.

The principles of construction of lasers with wavefront reversing (WFR) mirrors are considered. Descriptions are given of methods for the determination of the transverse and longitudinal mode structures in such lasers. The advantages and shortcomings of various systems with WFR mirrors are reviewed. A comparison is made of the various systems with one another and with conventional mirrors. The future trends and the main lines of further research are forecast.

A classification of speckle-field holograms is proposed: these holograms are divided into thin-layer L

An analysis is made of the wavefront reversal (or reproduction) in pump beams with a highly developed transverse interference structure. When the difference between the pump and scattered field wavelengths is small, diffraction improves the reproduction process. On the other hand, pulling of the scattered field into the cores of diffraction filaments with high pump intensities (as in serpentine active microwaveguides) results in deterioration of the reproduction process but it increases the scattered-field increment. A calculation is made of the serpentine effect using a power-series expansion in terms of the intensity of a monochromatic pump beam with a randomly inhomogeneous transverse structure.

A review is given of theoretical and experimental studies of the wavefront reversal effect in stimulated scattering. Recent results obtained by the authors are reported.

Methods of the mode theory are applied to the following aspects of transient stimulated scattering: interpretation of the critical intensity, condition for interference suppression of stimulated scattering, and determination of the threshold for a broad-band-pumped stimulated scattering oscillator.

Theoretical and experimental investigations were made of the direction of energy transfer in the case of transient self-diffraction of waves incident on a nonlinear medium from opposite sides. It was found that, in spite of the different behavior of the intensities of the waves inside the nonlinear layer, the conditions governing the direction of energy transfer were the same for the concurrent (parallel) and opposed (antiparallel) geometries. The experimental results confirmed the theoretical predictions.

A study is made of the mechanism of the appearance, in the radiation reconstructed from a volume reflection hologram, of not only the original object field but also of a field traveling in the opposite direction (conjugate object wave). This effect owes its origin to the interaction of fields with a complex angular aperture in the interior of a hologram and may ensure a relatively high efficiency of conversion into the conjugate wave. The calculations are made within the framework of the mode theory.

An experimental investigation was made of the angular selectivity of stimulated Brillouin scattering holograms to variations in the direction of the reconstructing Stokes wave. The results were in good agreement with the theory.

A new method is proposed for the reversal of optical or acoustic wavefronts. It involves modulation of the reflecting properties of a surface at a frequency twice the frequency of the wave being reversed.

A theoretical analysis is made of wavefront reversal of depolarized light in the four-photon interaction. Methods for implementing full spatial-polarization reversal are suggested.

Theoretical and experimental investigations were made of the efficiency of self-defocusing of cw He–Ne laser radiation as a result of thermoelastic deformation of a specularly reflecting surface of a strongly absorbing material. The divergence of a focused single-mode beam of ~20 mW power after reflection from a deformed ebonite surface was thirty times higher than the diffraction limit.

A theoretical determination is reported of the dependences of the direction and efficiency of transient transfer of energy in a two-beam system in a medium with a cumulative nonlinear response on the ratio of the beam intensities, interaction geometry, beam structure, and behavior of the frequency-contrast characteristic of the medium. It is shown experimentally that energy transfer is affected by the difference between the angles of incidence of the interacting beams. Energy transfer in the anomalous direction (from the weak to the strong beam) is reported.

Wavefront reversal of He–Ne laser radiation was achieved by light-induced modulation of the shape of a reflecting surface during thermal expansion of a strongly absorbing viscous liquid. Theoretical and experimental investigations were made of the energy efficiency and dynamics of the process. Estimates were obtained of the feasibility of a thermal mechanism of recording dynamic surface holograms using pulsed lasers.

An analysis is made of the possible causes of the deterioration of the quality of wavefront reversal by stimulated Brillouin scattering in fibers: inaccurate focusing of the beam being reversed on the end of the light-guiding core, escape from the scattered line profile of angular components inclined strongly relative to the fiber axis, frequency shift due to the stimulated scattering, and spatial depolarization of the pump radiation. It is shown that in the case of short fibers for which the depolarization length is greater than one-thirtieth part of the fiber length, the main source of reduction in the fraction of the wavefront reversed radiation of a narrow-band laser is the escape from the line amplified by the stimulated Brillouin scattering of the strongly inclined angular components of the Stokes wave.

A theoretical analysis is made of the polarization properties of wavefront reversal under four-wave interaction conditions. A system for complete spatial-polarization wavefront reversal is proposed and implemented experimentally. This utilized propagation of the signal being reversed in the direction of the polarization unit vector of the reference waves.

A theoretical analysis is made of stimulated scattering in opposite pump beams. A condition for suppression of stimulated scattering by a weak opposite pump wave is determined. The threshold conditions for parametric generation of opposite Stokes and anti-Stokes components are found.

OPC in four-wave mixing (FWM) has been realized for various third-order optical nonlinearities. Irrespective of the type of third-order nonlinearity of a medium, a phenomenological study of the generation of a phase-conjugated wave in FWM is carried out in much the same way. In this chapter we will present a phenomenological study of this method which enables us to reveal its major features. The features of particular mechanisms of nonlinearity will be discussed in Chap. 7.

As has been mentioned in Chap. 1, the quality of OPC by SS is high when the pump field has a developed speckle structure in the interaction volume. Here we shall try to explain what a speckle structure is, under what conditions it is observed, and what properties the speckle fields possess. It is vary easy to see a speckle structure: a wall surface, a sheet of paper and, in general, almost any object illuminated by a laser beam looks to be covered with a large number of irregular bright speckles. Their positions change as the position of the observer, the light source, or of the object itself changes. Figure 3.1 presents a photograph of a speckle-field section.