The history, modern state, and perspectives of high power multibeam CO₂ laser development in Russia are described.

In this paper we present the first report on experiments with a compact, high-power discharge, tube laser with slow pumping (diffusion cooling) of the working gas in which the power has been increased by close, parallel packing of the tubes within a common resonator. In contrast with the traditional lasers of this kind, in which the laser beam passes through all the tubes in succession, the present case has the radiation produced in a set of parallel beams.

We report the development of a multibeam laser, excited by an ac discharge in the frequency range approximately 10 to 70 kHz. The results are given of an investigation of the active medium and of the dependence of the output characteristics of such lasers on the discharge tube diameter, pressure in the gas mixture, and frequency of the pumping current.

A report is given of the construction of a high-power (P approximately 1 kW) multibeam cw CO₂ laser, with diffusive cooling of the gas mixture and excited by a 10 kHz ac discharge. The design of the laser is described and experimental measurements of its basic characteristics presented. These included measurements of the laser divergence and an investigation of the influence of the pressure, composition, and circulation velocity of the gas mixture on the laser output power. The experimental data obtained and a comparison of the parameters with those of other known lasers are used to discuss its advantages and prospects for use in technological applications are discussed.

A discussion is given of the fundamental principles underlying the construction of high-power multibeam gas-discharge lasers with diffusive cooling. An analysis is made of the constructional features of the main systems of the multibeam CO₂ laser Iglan-3. The experimental dependences are given of the laser output power on the discharge current, the working gas pressure, and the composition and circulation velocity of the laser mixture. Laser output powers of 4.6 and 3.5 kW respectively were obtained on using exit mirrors of ZnSe and KCl. The gain and saturation intensity of the active medium obtained by different methods were analyzed. The laser resonator efficiency was estimated for different exit mirrors. The electro-optic laser efficiency was found to be 14% and the energy efficiency 10%. Experimental data on the laser radiation beam divergence and on the intensity distribution in different cross sections of a focused beam are presented.

A multibeam technological cw CO₂ laser (Iglan-10) with an output power of 10 kW was constructed. Some of its characteristics are reported and potential applications are discussed.

Some results of recent development of HP industrial ac-excited multichannel (MTL) CO₂- lasers are presented. The 2 - 4 kW MTL models are described; some results of surface-heat treatment applications of these lasers are described. A nev 6 kW MTL model for laser welding applications is presented.

Presently, technological multibeam laser systems intended for heat strengthening, surface alloying, melting-up, etc. are extensively used in industry. These modular systems are based on the multibeam, diffusion-cooled, slow-axial-flow lasers. The systems can operate at output power levels from 1 to 10kW. The high output-power level unusual for diffusion-cooled lasers was achieved due to placing the array of parallel small-diameter gas-discharge tubes in a common cavity. In a first approximation, output power extracted from laser per a length unit of the discharge-tube does not depend on the tube diameter and is equal to approximately 50 W/m. Owing to that, by decreasing the tube diameter and increasing the number of tubes, we have managed to create a high-power laser keeping all advantages peculiar to diffusion-cooled tube lasers 13 These advantages include simplicity and reliability of design, long lifetime, output-power stability, no need for gas-pumping equipment, and a low noise level. These features are of particular importance for industrial lasers designed for a long-term continuous operation in a technological cycle.

Experimental data are presented on the dissociation of CO₂ molecules in the active medium of a multibeam gas-discharge laser as a function of the circulation velocity of the active mixture and of the discharge current. An analysis is given of the physical processes responsible for degradation of the active mixture, and its results are in agreement with data from other studies on both the value of the dissociation rate constant and the dependence of the degree of dissociation on the discharge current. A closed-cycle laser operating procedure is proposed in which the active mixture is regenerated in a high-temperature reactor at atmospheric pressure. Tests of this system showed that the regenerator provides for stable laser operation in the closed-cycle regime and enables the gas flow rate to be reduced by almost a factor of 30. Record values of the specific flow rate for high-power industrial gas discharge CO₂ lasers, of the order of 6 liters kW^-1 h^-1, are reported.

The use of gas regeneration systems in powerful diffusion-cooled multibeam CO₂-lasers, that feature high consumption of working gases, makes it possible to decrease running costs and an environmental impact. In this paper, we consider problems related to the use of heat regenerators. Impurity content in a gas- vacuum loop of the laser system and an active gas degradation degree have been evaluated. Concentrations of nitrogen oxides and CO were measured when running the multibeam laser system. Measurements of gain coefficient have been made.

A determination was made of the dependencies of the small-signal gain, saturation power, and gas temperature on the discharge tube diameter, working pressure, and discharge current in a wave-guide CO₂ laser excited by a longitudinal alternating-current (5 - 100 kHz) capacitance discharge. The results established that the scaling laws were applicable and a comparison was made of the efficiency of this excitation method with other methods.

An experimental investigation was made of the operating regimes of a pulse-periodic waveguide CO₂ laser employing diffusive gas cooling and excited by a longitudinal 10 kHz ac capacitative discharge. A specific average radiation output power of 12 W/m was obtained at repetition frequencies of 1 kHz and feasibility of control of the shape of the lasing pulse was demonstrated. The results of these experiments can be used to construct multibeam pulse-periodic CO₂ lasers for heat processing.

A study was made of the influence of misalignment of the plane mirrors of a waveguide CO₂ laser on the output power and mode structure of the radiation. It was found that misalignment of the resonator only weakly affected the output power, but exerted a strong influence on the beam divergence and maximum radiation intensity in the far-field zone. A qualitative agreement was obtained between the experimental results and the proposed theoretical model.

A gain coefficient of multibeam, multimodule CO₂-lasers can be determined by the method of measuring integral power of laser emission from all the channels for the different numbers of modules in operation. The gain coefficient depends on the reflection and transmission coefficients of resonator mirrors, a resonator length, and the number of modules. However, other research has failed to take into account the effect of spectral composition and waveguide components in the flux on the gain coefficient. The purpose of the present paper is to determine the active medium gain coefficient from spectral characteristics of laser emission.

Interest has recently turned to multibeam CO waveguide lasers1'2 as simple, compact sources of intense infrared light. A multibeam laser is an array of parallel discharge tubes (elements) between common plane mirrors. When the elements of the multibeam laser generate light independently, the intensities of all the beams are summed in the far zone (in the focusing plane), and the size of the focus is determined by the divergence of the beam from an individual element and the overall aperture of the multibeam laser. This circumstance imposes an upper limit on the light intensity at the focus. Another consequence of the independent operation of the elements of a random distribution of the light spectrum and polarization over the output aperture. This random distribution in turn has the consequence that it is not possible to realize the full potential of "focusators,t' which are optical devices of a new type for producing a given intensity distribution in a focus3 There is accordingly considerable interest in operating the various elements coherently, so that the multibeam laser operates as a single spatially coherent source. In this case the far- zone light intensity distribution is the result of an interference of all the beams. According to estimates in Ref. 4, operating all the elements of the multibeam laser of Ref. 2 coherently wifi lead to a maximum light intensity in the far zone about 50 times as high as during independent operation of the elements. It has also been shown that a coherent array of waveguide CO lasers can be used for L1DAR4 to produce short light pulses with a high repetition frequency.4'6 Phase locking of the elements of a multibeam laser will therefore substantially extend the possible range of applications and, in particular, permit an increase in the light intensity at the focus.

It is predicted theoretically and verified experimentally that an effective phase-locking of a two-dimensional array of lasers can be achieved by making use of the reproduction of the wave front of a periodic structure of coherent radiators.

An experimental investigation was made of the emission characteristics of a coherently operating array of waveguide CO₂ lasers. The lasers were phase locked by self- reproduction of periodic light fields. Three supermodes of the array were found to exist, an investigation was made of the radiation power distribution over the aperture as a function of the active medium pumping level, and the efficiency in the coherent lasing regime was determined.

An experimental investigation was made of a new method for phase locking of a periodic laser array. The advantages of the method include an afocal position of a spatial filter and the ease of variation of the coefficient representing the coupling between the lasers in the array, which makes it possible to select the optimal value of this coefficient to ensure the highest total radiation power.

An investigation was made of phase locking of radiators in an industrial MTL-2 multibeam laser by a spatial filter located at the focus of an intracavity telescope. Depending on the filter parameters, it was possible to vary the degree of coupling of such radiators and, consequently, the quality and power of the output radiation.

Some methods of multichannel CO₂-laser synchronization have been experimentally studied. It has been shown that the utilization of simple phase-shifting components incorporating the reflection optics extends substantially the possibilities of these methods and improves their characteristics. A simple analytical estimation of allowable phase aberrations in coupling channels is presented, the totally synchronized generation mode being still retained.

A numerical investigation was made of the influence of the number and packing density of a linear array of periodically arranged coherent sources on the efficiency of redistributing the radiation power from the side lobes to the main lobe of the angular distribution of the emitted radiation by using a binary phase corrector mounted in the image-doubling plane. The results are given of experimental investigations of a new device for improving the radiation pattern of phase-locked laser arrays.

The arrangements for formation of single-lobe beam intensity of waveguide CO₂-laser synchronized arrays using diffraction optics have been discussed. The results of some experiments have been presented to show these arrangement possibilities.

A high-power waveguide CO₂ amplifier with injection locking was constructed. Its energy and frequency characteristics were investigated. Single-frequency radiation of 50 W power was generated under quasi-cw operation conditions.

A regime of synchronized excitation of the fundamental modes of oscillation of diffraction- coupled lasers is discussed. An example is the elements of a semiconductor laser with `emitting mirrors.' The diffraction-coupling coefficient of two open resonators is calculated, and the stability conditions are found for a synchronized regime of excitation of the lowest even and odd modes of oscillation.

A numerical investigation is made of a two-dimensional model of a composite unstable resonator with three mirrors. The relevant parabolic equations are solved using a three-layer scheme and a fast Fourier transformation algorithm. A strong dependence of the distribution of the intensity of the output wave and of the distribution of the radiation in the far-field zone on the displacement of a reference mirror over distances of the order of one wavelength provides an opportunity for the control of optical characteristics of a system.

The results of analytic and numerical investigations of collective modes in two coupled unstable optical resonators are reported. The dependencies of the Q factors of the modes and of their frequencies on the mismatch between the resonator lengths and on the phase shift (advance) in the coupling channel are derived. It is shown that an important role is played by the difference between the distributions of the fields in the resonators and by their inhomogeneity, as well as by the effects of diffraction at the edges of mirrors and coupling apertures.

We describe the performance of two lasers with homogeneously saturated gain, coupled through a central hole in a common mirror of symmetric unstable resonators. A new iterative procedure is proposed based on combining the Fox-Li and Prony methods. The suggested procedure allow us to calculate several resonator modes and find a stable phase-locked operating region for various sizes of coupling hole and small-signal gain values. Far-field patterns from combined lasers are computed for various operating regimes.

The influence of a phase shift in a channel coupling two unstable-resonator CO₂ lasers on the band width of stable phase locking of the laser fields was studied experimentally for the first time. The fraction of the radiation affecting the optical coupling was determined experimentally and found to vary in the range 10^-3 - 10^-2 for different coupling apertures. The maximum mismatch of the resonator lengths which did not disrupt the coherence amounted to (Delta) L approximately equals (lambda) /20. A method developed for calculating the diffraction made it possible to determine numerically the stable phase locking range of the output radiation from two lasers with unstable resonators. The calculated and experimental parametric dependencies of the phase-locking band width were in qualitative agreement.

Measurements were made of the phase difference between the output beams of two coupled CO₂ lasers operating in a coherent regime. The results of the experiments are in qualitative agreement with calculations performed on the basis of a point model of two coupled lasers, taking the gain dynamics into account. The dependencies of the phase difference on the mismatch of the resonator lengths and on the length of the coupling channel should be useful when constructing a phasematched set of lasers and an optical guidance system linked to it.

An analysis is made of the possibility of locking of lasers with independent resonators as a result of a four-wave interaction in a nonlinear cell. Conditions for phase-locked lasing are determined theoretically and the stability of such lasing is investigated.

Frequency locking of two TEA CO₂ lasers in a common nonlinear element was achieved for the first time. The frequency locking arrangement was based on a four-wave interaction of light in SF₆. The mechanism of nonlinear coupling between the CO₂ laser resonators is considered.

Radiation from two quasi-cw waveguide CO₂ lasers was phase-locked by a four-wave mirror. The coefficient of coupling between the lasers was determined.

Bifurcation and chaotic lasing regimes have been observed for the first time ever in a system of optically coupled CO₂ lasers. The regions of stable laser mode locking are determined analytically and numerically. The dynamic regimes in the absence of mode locking are calculated. Agreement between experimental and numerical results is obtained.

An analysis is made of the characteristics of the formation of fields in a resonator with a composite mirror consisting of inverting elements and in a resonator with a corner-cube reflector. The lowest oscillation modes are found numerically. It is shown that the nature of the fields which are formed and their diffraction losses are governed by phase relationships between the beams reflected from the mirror elements and by the pattern of their interference in the corner-cube reflector. The diffraction losses in a resonator with a composite mirror are usually less than in a subresonator or a resonator with a plane mirror equivalent to the composite one. It is pointed out that dephasing of the beams reflected from the mirror elements, due to a shift of these beams or due to optical inhomogeneities, alters significantly the diffraction losses only if the Fresnel numbers are sufficiently small.

A theoretical analysis is reported of the radiation produced by an array of periodically distributed lasers with effective coupling between the elements. The coupling is based on the reproduction of the periodic field structure. The possible mode types, losses, and eigenfrequencies are determined for infinite and finite sets of lasers assembled in triangular rectangular arrays. The influence of mirror misalignments, saturation of the active medium, and random phase spread among the individual channels on the modes and their Q-factors is examined.

The mode structure is considered for a set of periodically placed lasers with close component coupling, which reproduces the periodic field structure. Possible mode types, losses, and natural frequencies are considered for infinite and bounded laser sets forming triangular or square arrays.

An investigation is made of the influence of the difference between individual lasers and of the optical coupling parameters on the number of lasers which can be phase-locked in an array. The estimates are confirmed by a numerical experiment.

An investigation is made of the possibility of stabilization of phase-locked operation of a set of lasers as a result of nonlinearity of the refractive index of the active medium.

In this paper we present the results of analytical and numerical studies into synchronization of a large number of optically coupled resonators with random detuning of eigenfrequencies. We consider the `nearest-neighbor' coupling model with allowance made for gain saturation. Assuming that the coupling coefficient has zero imaginary part, the problem of steady-state phase distribution over the aperture of the two-dimensional array reduces to the problem of temperature distribution in a 2-D medium at the presence of heat sources and coolers with random powers. It is shown that the steady-state phase distribution over the aperture has the domain-like form. Phases in adjacent domains differ by values of the order of (pi) . Combining the scheme with nearest-neighbor coupling and external signal injection, we can easily attain synchronous lasing from large arrays.

A theoretical investigation is made of the phase locking of a laser array in the case of different types of multibeam intracavity interaction in nonlinear media. The conditions are found under which a long-range coupling of the `all with all' type is established between the lasers and also when only the nearest neighbors interact (short-range coupling). The influence of the number of lasers, frequency offsets of their resonators, and of the coupling coefficients on the phase- locking band is considered. Expressions are obtained for determination of the threshold values of the gain and of the frequency characteristics of cophasal and noncophasal operation of a laser array under long-range and short-range coupling conditions. A study is made of the influence of the parameters of a resonantly absorbing medium on phase locking of a set of lasers and it is shown that in the case of the optimal long-range coupling the phase-locking band is independent of the number of lasers.

A review is given of the main results obtained in the last few years in the course of development and studies of integrated injection laser arrays. The main configurations of such arrays are considered: they differ in respect of the structure of the active elements and methods used to couple the elements optically. The main approaches to calculations of the mode compositions of the output radiation are described. The experimental energy, spectral, spatial, and dynamic parameters of laser arrays are reported to illustrate the attained levels of their technical characteristics. An analysis is made of phase locking of the radiation from laser arrays by injecting an external phase-locking optical signal, and also by using a shared external resonator, spectral-spatial selectors, and other external optical components.

Optically coupled lasers are a striking example of a class of self-organized nonlinear dynamic systems which are currently attracting much interest. From a practical standpoint, the coupling of lasers makes it possible to increase relatively simply both the output power and brightness of a light source. The present review gives an account of the current status of the theory and experimental results obtained for two or more optically coupled lasers. The stress is on the structure of collective modes and their stability. Frequency and phase locking of laser arrays is considered. Optical coupling systems tested experimentally are discussed and the agreement between the experiment and theory is considered. A description is given of the operation of systems in which lasers are detuned by an amount exceeding the width of a locking band. Methods for increasing the brightness of the output radiation of systems with a synthesized aperture are also discussed.

In this paper we wish to call attention to a new instability mechanism, which results in the development of a transverse structure in the radiation in lasers with a plane-parallel resonator containing a cell with a saturable absorber. We analyze the structure which arises as a result.

The recent developments of a theory for phase locking in optically coupled laser arrays are reviewed with an emphasis on problems still remaining unresolved. The system of two optically coupled lasers and large laser arrays are discussed separately.