A resonator design strategy for high-power end-pumped solid- state lasers is described, which reduces the beam distortion due to thermally-induced lens aberrations. The approach is based on a resonator design satisfying two criteria: firstly, the laser mode radius must be appreciably smaller than the pump beam radius, and secondly, the laser mode size must decrease with decreasing power (increasing focal length) of the thermal lens. Experimental results are presented for diode-bar-pumped Nd:YAG lasers confirming the validity of this approach. A simple technique, employing a single quarter-wave plate, for reducing the depolarization loss due to thermally-induced birefringence is also described. Using this approach, the depolarization loss for a diode-bar-pumped Nd:YAG laser operating at 946 nm has been reduced from approximately 1.7% to <0.001%, resulting in 2.9 W of linearly-polarized output for 14.3 W of incident pump power.

High power laser diodes and especially high power laser diode modules made enormous progress in the last few years. Different aspects of high power laser diodes are treated starting from general description of high power laser diodes and their mounting techniques, characterizing the electro- optical behavior of single laser bars and finally presenting beamshaping optics for the collimation of large modules. The later technique allows for symmetrical focal spots in the kilowatt range with a beam quality of about 170 mm*mrad. Different aspects of current applications of high power laser diodes are presented.

In this paper we present recent results obtained by our group on diode-pumped bulk erbium-ytterbium lasers. Modeling of the active material is first presented to outline design criteria for the optimization of laser cavities. Different operating regimes and particular properties of these novel laser devices are subsequently analyzed and discussed, including single-frequency operation, frequency tuning, intensity and frequency stabilization, mode-locking and frequency-modulation operation. Due to the high performance in terms of emission characteristics, these lasers are attractive for a large number of applications and, in particular, for optical communications in 1530 - 1570 nm wavelength region.

A major problem using laser diodes for longitudinal pumping of solid state lasers is the poor beam quality and the non symmetric beam profile of the diodes as the astigmatic beam emitted from the array of diodes exhibits a limited focusability in the direction of the slow axis of the diodes. To overcome this problem an optic which turns the astigmatic beam of a stacked diode array into a radially symmetric (stigmatic) beam has been designed. This symmetric beam is then focused into an axially water cooled disk laser to serve as a longitudinal pump source. Up to now different laser crystals have been investigated as laser source. Using Yb:YAG an output power of 60 W in qcw operation has been realized whereas Nd:YVO₄ delivers a qcw power of 113 W. The optical to optical efficiencies are 18% and 32.3% respectively.

We report on the influence of the different specifications of a single emitter laser diode (type A) and a laser diode (LD) of the partially phase locked type (type B), on some characteristics of our diode pumped solid state lasers (DPSSL). We find that the use of the type B LD is preferable to that of type A with respect to smaller M² of the DPSSL-beam, superior noise behavior, and smaller full width at half maximum of the LD emission.

Orthogonally polarized modes have been operated in a Nd:YAG laser with weakly anisotropic cavity. Different intensity dependences of the orthogonal modes on the orientation of the pump polarization plane are discovered. The power spectrum of each polarization mode reveals three relaxation peaks in a good agreement with predictions of the laser model modified to include polarization effects.

The problem of swift steering of laser beams is important for many applications, (projection, marking, engraving, etc.). An approach of intracavity laser beams spatial control was proposed and developed by several teams [1 -9, 14] . Laser beam movement is provided, with thisapproach, by the positioning of the "ruling diaphragm" across the surface of one of mirrors of conjugate laser cavity. The "ruling diaphragm" can be done as a Spatial Light Modulator (SLM). Such modulators are based on various physical effects and can use various modulating media: electrooptical crystals [2, 10], thin semiconductor layers [1 1], ferro-electrical ceramics [12-14], or liquid crystals [6,8]. The electrically-addressed intracavity Spatial Light Modulators (SLMs) were experimentally shown to provide computer-controlled beam addressing to any pixel insidelaserFieldOfView (FOV) [12, 13,14] In the presented article the following aspects are treated of such laser development: (a) design and properties of iniracavity LC SLMs with random addressing to arbitrary pixel in 2D field, and (b) some peculiarities ofDPSSL with intracavity beam scanning, in terms of SLM operation inside the cavity, and thermal lens influence on the laser FOV

We have studied the Yb³⁺ (reversible reaction) Tm³⁺ nonradiative energy transfer processes responsible for population of the ¹G₄ thulium level under Yb³⁺ or Tm³⁺ selective laser excitation of YLF:Tm³⁺,Yb³⁺ crystal. The microparameters and the rates of the energy transfer via cross-relaxation schemes are determined. It is concluded that the process of populating the ¹G₄ thulium level is greatly affected by up-conversion processes not only from the ³H₄ but also from the ³F₄ level, proceeding within the static decay model.

We have studied both theoretically and experimentally the energy transfer processes in YbLiF₄:Ho³⁺ (0.4%) and YbLiF₄:Ho³⁺ (0.4%),Tm³⁺ (10%) crystals and in the series of crystals KYb(WO₄)₂:Tm³⁺,Ho³⁺ (0.4%) with the thulium concentration 5; 10; and 20%. The population kinetics of the ⁵I₇ holmium level was studied under the 1.047 micrometers Q-switched YLF:Nd³⁺ laser pumping. The efficiency of energy transfer processes in Ho³⁺ doped crystals, codoped with Yb³⁺ and Tm³⁺, was demonstrated. With an YbLiF4:Ho³⁺ (0.4%) laser rod we have obtained lasing at 2.06 micrometers with the total efficiency of 4.8% and the slope efficiency of 11%. In the KYb(WO₄)₂:Tm³⁺ (10%),Ho³⁺ (0.4%) crystal, lasing was obtained for the first time at 2 micrometers . The pumping threshold being approximately 150 mJ, total efficiency 6% and slope efficiency 9.7% was achieved.

Spectro-luminescent characteristics were determined for LaSc₃(BO₃)₄-crystals, activated by ions of the ytterbium and the erbium. Quantum efficiency of nonradiative transfer from the ions Yb³⁺ to the Er³⁺ is closed to 80% for concentration of ions [Yb] equals 10²¹ cm^-3, [Er] equals 5 (DOT) 10¹⁹ cm^-3. Quantum efficiency of energy transmission from the Cr³⁺ to the Er³⁺ is about 90%. Having achieved population inversion in the Er-ions the efficiency of energy transmission decay to 70%. We have fulfilled comparison of generation properties for lasers with active elements made of LSB-crystal, that contents [Cr³⁺] equals 0.5 at.%, [Yb³⁺] equals 10 at.%, [Er³⁺] equals 0.5 at.%, and phosphate glass that contents [Cr³⁺] equals 0.3 wt.%, [Yb³⁺] equals 20 wt.%, [Er³⁺] equals 0.2 wt.%.

The absorption and fluorescence spectra of Er-doped phosphate and silicate laser glasses were investigated in temperature range of 20 - 300 degree(s)C. The heating resulted in 5 - 10% decrease of oscillator strengths of spectral bands. On the base of temperature variations of spectral profiles we proposed an energy level model, which allowed to explain the thermal variation of spectrum of Er-laser at 1.54 micrometers .

The goal of the project is the construction of a wideband gravitational gradiometer based on a laser 100-m Fabry- Perot-Michelson interferometer on free masses in deep rock surroundings. The device is aimed at registering bursts of gravitational radiation with an amplitude of the dimensionless metric variations as small as 10^-21 in the frequency range 10² - 10³ Hz. The auxiliary purpose is to measure low-frequency variations of the Earth gravity gradients bearing information on the internal dynamics of our planet. The deep under-rock position of the installation inside tunnels of Baksan Neutrino Observatory provides a two-order of magnitude decrease in the geophysical noise level which allows the combination of parallel astrophysical and geophysical studies. For geophysics, the long-based interferometer with hanging mirrors presents a new type of geophysical instrument--an angular gravity gradiometer at low frequencies 10^-4 - 10^-5 Hz with a resolution of 10^-8 - 10^-10 rad, which is enough to register weak geodynamical perturbations due to oscillations of the Earth interiors.

The wide-band laser interferometer with a measurable armlength of 75 m has been installed in an underground tunnel (the North Caucasus), in one of the most active in geodynamic respect region of Russia. The optical scheme of the interferometer is the two passes (N equals 2) Michelson- type interferometer with unequal arms working in regime of space separated beams which is realized by use of the corner reflectors. The mirrors, the beam-splitter and the optical passes are placed in vacuum (5 X 10^-5 mbar). The long-term monitoring strains of the Earth is provided in two frequency bands: below 0.1 Hz and in the bandwidth of 0.5 Hz around frequency of 1.62 kHz (the monitoring of quadrature components). The sensitivity in the high frequency band is about 10^-15 Hz^-1/2. In the regime of long- term monitoring the Earth strains the interferometer is operating from the beginning of 1993. The observation data, thanks to the unique capabilities of the laser interferometer with respect to both the frequency and the dynamic ranges, are used for studying some geophysical and astrophysical problems.

The results of the study of dynamic peculiarities of deformational oscillations of Earth crust with periods in the range from 10 to 300 sec by He-Ne laser dephormograph placed in underground gallery are presented. The mean square amplitude of deformational noise observed during one month is about 0.01 micrometers. It has constant components and variable one with period 24 hours. A mechanism of the deformational noise generation and a practical implementation of the laser apparatus and observed phenomenon for geodynamic and seismic monitoring are discussed.

Optical noise is inherent in fringes obtained with the electronic speckle pattern interferometer (ESPI) system. When used in the subtraction mode the optical noise is readily decreased and high contrast fringes may be seen. However, if fast dynamic events are studied a pulsed laser may be used in order to capture particular moments of such an event. A fast repetition rate, 60 pulses per second, Nd:YAG diode seeded laser is introduced in the ESPI system. This laser has twin cavities allowing the firing of two pulses with a minimum time separation, between them, of less than 10 ns. Thus two pulses can be fired within a single TV frame (TV frame rate is 30 Hz) of the ESPI system, interrogating at two different moments in time the fast dynamic event under observation. This process creates a speckle pattern for each pulse, and since two pulses are fired within a single TV frame an addition fringe pattern is obtained, assuming the event has produced a physical measurable change. This addition fringe pattern has an amount of optical noise that is approximately twice that of a subtraction fringe pattern. Results are presented where the addition fringe pattern contrast may be enhanced by using real time electronic filtering.

The influence of the wave phase and amplitude distortions on the local angular distribution and on the spatial amplitude distribution of the modes of laser resonator was analyzed by the methods of the waveguide theory. The theoretical evaluations are in a good agreement with the experimental data.

In this article we describe an efficient method of the compensation of thermally induced birefringence in high power Nd:YAG rod lasers. Losses at a resonator internal polarizer due to depolarization in the rods are reduced, and a polarized cw laser with an output power of 300 W is realized. Resonators with a small stability range and therefore beam quality near the diffraction limit are introduced. To improve the variability and stability of such resonators an adaptive high reflecting mirror is used as resonator mirror. The maximum average output power of the resonator with adaptive mirror is 145 W at a beam quality of M² equalsV 2.

The main purpose of our investigations consists in creation the solid state laser with power approximately 500 W and high beam quality. The beam quality, as well known, is described with parameter M², that characterizes the focusing properties of laser radiation. Notice that the high power technological lasers with lamp pumping have the typical value M² from 20 to 100. It could be decreased by means of use the special technical methods of compensation of birefringence and aberrations in the induced thermal lenses of active elements.

The results of investigation and development method for multi-cascade pulsed solid-state lasers optical schemes are represented. The analysis of processes for lasers with active and passive Q-switch was made. It is shown the variation of output laser parameters versus the inter- cascade coupling.

A high reproducibility of the feedback stabilized prelase dynamics in the actively mode locked and Q-switched laser is shown experimentally. The reliable generation of 65 - 70 ps pulses with 1 mJ energy at the relative root-mean-square deviation 0.6% is achieved due to the feedback-assisted timing to prelase signal of all cavity Q-control events.

Experimental results have been given on laser-damage resistance of the liquid crystal modulator with longitudinal operating electrical field. It was shown that laser-damage resistance of the modulator is limited by the ITO transparent electrodes and equals 2.5 - 2.9 J/cm² at 1.06 nm, (tau) equals 15 ns. To improve this parameter we proposed an liquid crystal structure controlled by a transverse electric field in which the ITO electrodes are removed out of the zone intense laser radiation. The main characteristics of this mode liquid crystal modulator are discussed.

New device, enabling more than tenfold shortening and smooth variation of a laser pulse duration using repeated nonlinear total internal reflection of the laser beam from a border of transparent and absorbing media, is proposed. The shape and duration of the reflected and transmitted parts of the incident pulse are calculated in dependence on parameters of neighboring media and conditions of the experiment.

The results of a comparison analysis of the laser radiation harmonic generation in biaxial crystals for all types of interaction are represented. The calculation of the phase- matching angles, bandwidths (angular, spectral, temperature), `field' of effective nonlinearities, phase- matching curves and etc. have been carried out.

To obtain wide-band antireflection coatings for the visible and the middle IR regions of the spectrum transparent optical films with different refractive indexes and small absorption coefficients are required. Promising BaF₂, CaF₂, SrF₂ films with optical thickness more than 3 - 5 micrometers are not transparent in the visible region. Alloyed BaF₂ films, which were transparent in the all mentioned diapason, were obtained and investigated. Spectra selective coatings which based on alloyed BaF₂ were synthesized, made and researched for the visible and the middle IR range. The synthesis was realized on the base of the method of equivalent layers.

We present the results of experimental and theoretical study of opportunities for improving of spatial homogeneity in the near field of Nd:YAG-laser radiation at (lambda) 532 nm by means nonlinear optical limiting of laser radiation, that take place in fullerene-containing medium (with C₆₀). A scope for the smoothing out a profile of generation impulse was studied. The experiments were performed with fullerene- containing composites C₆₀-solution. The mechanism of optical limiting is discussed.