We describe the use of photorefractive material in adaptive interferometers for the detection of small surface displacements generated by ultrasonic waves. Recent results on devices based on two-wave mixing and photo-emf detection will be presented. We will discuss techniques for tailoring materials properties for improved device performance.

Phase- and polarization conjugation has been realized by phase locking and phase conjugating two linearly polarized components of a depolarized beam by means of stimulated Brillouin scattering in nitrogen at 75 bar. Hereby, depolarization losses of an oscillator-amplifier laser system have been reduced from 36% without polarization conjugation to 7.1% with vector phase conjugation. An application in a novel oscillator double- amplifier laser system is presented where the amplifiers are operated in a parallel arrangement. An average of 81 percent of the total optical energy generated in the system has been transferred into the output beam with a beam quality of M² < 1.1 and excellent pointing stability.

Using stimulated Brillouin-scattering (SBS) in glass fibers as phase-conjugating process, the field of application of SBS phase-conjugators can be expanded, because fibers offer a decreased power threshold and a stable behavior even at high input powers. We have developed phase conjugators consisting of multimode fused silica fibers in standard or tapered geometry with core diameters between 25 and 400 μm. Such fiber phase conjugators were characterized for different lengths in respect of the phase conjugating parameters like threshold, reflectivity, and fidelity. Dependencies of all SBS parameters on the fiber length and on the coherence length of the input beams were determined. Such optimized fiber phase conjugators operate with more than 50% reflectivity, 93% fidelity, and less than 200 W threshold in the spectral range from IR down to the UV. A depolarization of SBS pulses could be reduced to less than 0.3% by proper configuration of the fiber. High power densities of more than 1 GW/cm² do not damage our fiber phase conjugators, because the absorption of the fiber materials used is very low. Furthermore, fiber phase conjugators are harmless to the environment and beat the performance of dangerous liquid or high pressure gas SBS- cells used in high power lasers up to now in all concerns. High economic benefits can be expected due to the popular fiber materials used from the glass fiber technology.

An analytical expression was obtained for the timing jitter of ultrashort solitons, taking into account the group velocity dispersion, the Raman effect and the third-order dispersion. This expression can also be useful to evaluate the timing jitter of a communication system using dispersion-decreasing fibers. It is shown that the higher- order effects increase the timing jitter beyond that of the Gordon-Haus value for solitons shorter than ~ 10 ps. Dispersion-decreasing fibers show a greater variation of the jitter control distance with the amplifier spacing, in comparison with the constant-dispersion fibers.

The method of computer simulations on nonlinear resonant magnetooptical effects developed for real multi-level atoms in the two laser fields of arbitrary intensity and external magnetic field is applied for the polarization effects of different types calculations and investigations of the dependence of the characteristics of these effects on magnetic field strength, intensities, polarization and detunings of laser fields for alkaline atoms. The essence of the method consists in simulations and analysis of the plots of dependence of quasi energies on parameters, which are obtained with the help of sorting subprogram, and selection of suitable algorithms for calculations of characteristics of nonlinear resonant magnetooptical effects. One photon and two photon resonant effects are investigated for wide range of magnetic field strength from Zeeman to Paschen Back effects. Some new features in the spectra of rotation of plane of polarization and circular dichroism of different types are predicted. The results show the agreement with known experiments. Such calculations of nonlinear resonant magnetooptical effects in the intense laser fields resonant to adjacent transitions and magnetic field show the opportunity of investigation the modifications of electronic structure due to intense radiation fields and strong external magnetic field in atomic gases and also may be used for the treatment of new methods of phase-polarization selection of modes of tunable lasers.

We presented wide range of laser-radiation optical limiters that are based on different physical principles, operating in 400-12000 nm spectral range with rate 100-1 ns and providing wide dynamic range of limiting required for eye and sensor protection. We propose and investigate a method for reduction of the optical limiting threshold due to the Bragg reflection in a system with distributed feedback. The limiting threshold with the Borman effect, accompanying by diffractional absorption suppress and stimulated reflection from the LC Bragg grating, is about (mu) J/cm² for nano seconds pulse. We investigate nonlinear optical limiters of laser radiation based on fullerene-contained materials, operating on the basis of reverse saturable absorption (RSA) as well as stimulated nonlinear reflection of laser radiation. Fullerenes C₆₀, C₇₀ and C₈₄ were studied with the second harmonic of Nd:YAG laser, 532 nm, single-mode radiation pulses of 30 ns duration and maximum energy 0.35 J. Definite thresholds of RSA limiting vary from 0.05 to 0.1 J/M²) depending on fullerene density and limiting scheme. Dynamic range of limiting sufficiently increases for multi-pass trains, the fluence decreases up to 10000 times. We elaborate 3000-12000 nm laser-radiation attenuators based on multi-layer interference containing vanadium-dioxide film. It is shown that these attenuators can operate due to both amplitude and diffraction effect. Attenuation is up to 1x10³- 1x10⁵ times for 1 MW/cm² beams, operating time being 30-100 ns. For sensors in cooled optical systems, radiation attenuators can be used with vanadium oxides having the phase transition at cryogen temperatures.

To the best of our knowledge, a novel self-phase-conjugation technique for laser radiation is proposed, which is based on the self-filtration of the phase-conjugate component in a nonlinear absorbing medium, bleached by a pump wave with the intricate spatial structure in the region of location of its speckles. Similar to the stimulated Brillouin scattering (SBS) case, the mechanism of the phase-conjugate wave discrimination against the noise background is based on the coincidence of the speckles in the pump and conjugate waves throughout the bulk of the medium. To compensate for a drop in the signal power due to total absorption, the amplification in conventional laser gain cells is used. The feasibility of the phase conjugation with reflectivities from unity to tens of percent is demonstrated by analytical estimates and numerical calculations. The technique can find application in the middle and far IR spectral regions, where the implementation of phase conjugation via SBS is either hindered or impossible at all.

The laser can be focused on a distant target with stimulated Brillouin-scattering (SBS) effect in the atmosphere. To a moving target, the target can be tracked with an isoplanatic angel between the phase conjugate light and the beacon light. We present a method to produce an angle between the light in SBS.

We show a compact multi-pass amplifier, based on a single dual-rod laser-head whihc produces ultra-high gain. A double-pass produced a maximum small signal gain of 4x10⁸. Another pass was permitted by including a specially designed Brillouin phase conjugate mirror (PCM). This enabled a total gain of 7.7x10¹⁰, which raised an input signal of 10pJ to 770mJ output signal. To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. The amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply. We show that this system can be utilized for producing high-energy long temporally-smooth narrow linewidth pulses, as well as high power controllable, temporally-modulated pulses.

The design of a BBO OPO cavity based on cylindrical focusing of the pump beam in the insensitive plane of the non-linear crystal is presented and characterized. A variety of beam sizes in the sensitive plane of the BBO crystal are investigated, and in all cases this cavity design is found to significantly lower the operational threshold pulse energy of 355 nm pumped type I BBO OPOs. With optimal beam focussing parameters, the measured threshold intensity of 45 MW/cm² is similar to that of conventional OPOs, but the threshold pulse energy of 0.4 mJ is some twenty times lower than that found in circularly symmetric pump beams. Pump pulse energies in this range can now be routinely achieved with diode-pumped Q-switched lasers, and the combination of these sources with cylindrically-focussed OPO cavities should result in a new class of all-solid-state high repetition rate and high average power non-linear light sources that can be tuned over the entire visible wavelength region.

We report the first successful demonstration of optical parametric oscillation in periodically poled RbTiOAsO₄ (PPRTA). We have achieved >400mW of signal output power with 3.0W of 1.064μm pump at 20kHz. We have likewise demonstrated significant temperature tuning of the PPRTA crystals, covering a signal wavelength of 1.65-1.69μm over 25-75°C.

By integrating a free running cavity and a narrow bandwidth cavity, it is possible to create a compound OPO design which possesses the most desirable properties of each. In this example, we have combined a 355 nm-pumped idler-resonator free running type I BBO OPO cavity with a grating-narrowed signal-re-injection arm using only standard, low-cost optics. The output from such an implementation retains the high efficiency and low divergence of the free-running OPO, but its bandwidth is dictated by that of the high resolution cavity.

A review of nonlinear optical techniques to be used in tunable far IR lidar, radar and spectrometer system is presented. As an example, a tunable IR remote sensing technology demonstrator system operating at 8-12 μm is described. It includes an OPO and OPA pumped by Nd:YAG laser as well as Raman emitter and receiver pumped by the OPA output. Relatively small variations of an OPA wavelength (1.85-2 μm)provide a broad operative range at far IR (8-12 μm). The unique feature of OPO is pumping by two phase conjugated beams. A pulse from the pump laser is transmitted through the OPO. A portion of the transmitted pulse is reflected in a phase conjugate cell, and the remaining pulse energy is semi- compressed in stimulated Brillouin scattering (SBS) cells. These phase conjugate (PC) cells are filled with highly purified and stable SiCl4 and GeCl4 (R=80%). This process reduces the OPO threshold as well as forms short and narrow linewidth pulses. Being amplified up to 140 mJ in a 2 ns pulse in the OPA, these pulses are directed to a hydrogen Raman emitter. Due to auxiliary pumps at 1064 nm and 1907 nm with total energy approximately 30 mJ providing phonon excitation, the SRS conversion of an OPA output in the Stokes wave starts from a relatively high phonon seed so a quantum efficiency of more than 50 percent at far IR is achieved. The high sensitive detector of far IR weak signals (~1000 photons)is based on a Raman-induced four wave mixing of these signals with a delayed OPA output. A subsequent readout of the Raman grating is performed using a 532 nm pulse. The Raman-shifted 683 nm pulse is proportional to that of a weak IR signal.

A theoretical model to describe localization of high-power electromagnetic field and abrupt local increasing of its amplitude in transparent nonabsorbing dielectric is presented. It is based on formation of unstable field structure in dielectric microstructures. Possibility for field instability to arise is estimated making use of model of diffraction of high-amplitude plane wave on sphere and cylinder which radii are about radiation wavelength. Field instability is shown to be threshold-like phenomenon. There are estimated threshold amplitude of incident wave and field amplification in the microsphere and cylinder. Possible laser-induced processes in transparent dielectric are discussed.

We propose a modified scheme of a four-wave-mixing phase- conjugate mirror with the feedback comprising a nonreciprocal Brillouin amplifier placed inside the feedback loop. A simulation of the loop scheme showed the unidirectional oscillation of a phase-conjugate wave to be feasible in the optimum mode of operation of the loop-type PC mirror when the energy of a probe wave at the SBS amplifier input is more than an order of magnitude lower than the SBS threshold. Our experiments demonstrated that the proposed scheme of the loop-type PC mirror ensured pulse-to-pulse stable unidirectional oscillation of a phase- conjugated wave. The characteristics of the output radiation are studied and the high phase conjugation fidelity is demonstrated. The problems arising in scaling up the PC mirrors of such a type are considered.

Nonlinear propagation of plane high-power wave through both single dielectric layer and multilayer coating is investigated with computer modeling. Laser intensity is supposed to be high enough to induce considerable variations of refraction index. Laser-field evolution is investigated for cases of coatings with step and gradient sine-shaped variations of refraction index for both high-reflection and anti-reflection coatings. Formation of multiple field disruptions has been observed and shown to begin under certain combination of field parameters. The disruptions can result in formation of positive feedback resulting in super local blow-up instability of field amplitude developing within less than one tenth of laser period. Influence of coatings parameters upon blow-up field instability is investigated. Results of modeling are compared with theoretical calculations and discussed from the viewpoint of laser-induced damage of optical coatings.

There are presented results of computer modeling of laser- field evolution in nodular defects in multilayer optical coatings. It is investigated dynamics of laser-wave propagation through both linear and nonlinear nodules. Formation of field maxima is studied. There are presented theoretical estimates of field amplification for resonant case. The presented results are discussed from the viewpoint of laser-induced damage of optical coatings.

We describe a number of experiments devoted to compensation for aberrations in the primary mirror of a telescope using a volume dynamic holography technique, where the writing and reading of the hologram are carried out independently. Three sets of experiments demonstrating holographic aberration correction have been performed. In all cases transmitting holograms were formed in photorefractive crystals such as BSO and SBN. In experiments with a small diameter primary mirror the image is formed by a multicolor point-like object. The system includes a unit which compensates for the hologram dispersion. The aberration correction of a six- segmented primary mirror of 150 mm in diameter is demonstrated in conditions where there is a pronounced difference in the wavelengths of the recording beams and that of the source observed (Δλ=0.12 μm). The experiments on observation an extended object of continuous spectrum (Δλ=0.35 μm) with a single primary mirror of 150 diameter have been also made. All these experiments indicate an efficient compensation of distortions of the primary mirror by the hologram.

We discuss in this paper, highly efficient techniques for CW non-linear frequency conversion using novel pump laser sources and high efficiency laser gain materials We also address the limitations of non-linear conversion processes and evaluate the performance of conventional non-linear optical materials that have recently been modified for non- critical phase matching (NCPM) applications, such as NCPM- KTP. Moreover, we outline the significant improvements in performance of new non-linear materials for frequency conversion of fundamental wavelength laser sources, utilizing recent advances in application tailored materials with high non-linear coefficients such as PPLN. In particular, the issues related to efficient intracavity higher harmonic generation, including intracavity doubling and intracavity tripling as well as intracavity and extracavity optical parametric oscillator development are described.

The development of practical high power fiber lasers has created a resurgence of interest in Raman amplifiers for telecommunications systems. In this paper, we review the applications of cladding-pumped fiber lasers and cascaded Raman lasers to distributed and discrete, analog and digital Raman amplifiers at both 1.3 and 1.5 micrometers and to remotely pumped amplifiers in repeaterless communication systems.