in this paper, we have reviewed the design principles and numerical formalisms applicable to push the average power limits of state-of-the-art solid-media lasers from typically 1 kilowatt to hundreds of kilowatt. Several device architectures are considered, taking into account utilization of off-the-shelf technology, and inclusion of realistic values of propagation loss. We demonstrate that once the beam propagation problem is addressed properly, laser devices employing solid-media are capable of producing 100 kW to 1 MW of average output power in cw operation at 6 to 12% wall plug efficiency. This will open up applications for solid-media lasers where gaseous and chemical lasers are currently used.

Several kW-class solid-state lasers at TRW are described with an emphasis on the performance modeling used to aid development of high brightness operation. Comparisons of results and analysis are presented for key aspects of high power, diode pumped, Nd:YAG lasers and amplifiers that use zigzag slab configurations to minimize thermal effects. Devices described include multi-kW power oscillators suitable for high power machining, welding, and material processing; and phase conjugated master oscillator/power amplifiers (MOPAs) which provide short pulse, high brightness beams for active tracking, photolithography, or remote sensing. Laboratory measurements are in good agreement with predictions of diode pump profile and absorption efficiency; slab extraction efficiency and thermal load; and slab OPD.

A model to predict the optical efficiency and beam quality obtained from a diode pumped high-power thin disk laser is presented. The laser consists of a linear resonator with the quasi-three-level laser active material Yb:YAG placed as a thin disk on a cooling finger. The laser crystal is pumped by a near flat-top pump beam profile in a multiple pass configuration. Cw-output powers of 255 W at pump powers of 519 W have been achieved experimentally. Within the model the temperature distribution inside the crystal is calculated, taking into account the mutual dependence of temperature (and thus level population) and pump light absorption. The pump beam power distribution is modeled by Monte-Carlo-ray-tracing. The resonator is simulated by self-consistently circulating the power distribution within the resonator via transformations into a suitable mode system and back, using simple matrix multiplication. Thermal lensing due to thermo- optical and mechanical deformations is considered, using the results of finite-element-modeling. The resulting optical efficiency and beam quality have been calculated. The dependence on variables like temperature, dimensions of the pump beam source, number of pump beam passes and crystal thickness is discussed.

A general model has been developed for the output power optimization of fiber-coupled laser-diode end-pumped lasers by including the thermal effect into the analysis. The optical path difference (OPD) distribution has been derived as a function of the pump-beam quality, focus position of pumping light, and pump radius at the focal plane under the assumption that the end faces of the crystal are thermally insulated. With the derived OPD, the diffraction losses arising from thermal-induced spherical aberration have been estimated by the strehl intensity ratio. The practical example of a Nd:YVO₄ laser pumped by a 1.2 W fiber-coupled laser diode is considered to illustrate the utility of the present model. Experimental results have shown a fairly good agreement with the theoretical predictions.

The ability of the laser crystal to withstand the damaging effects of thermal stresses and strain is an important factor in the design of high power solid state laser systems. To compute these effects, a set of cylindrical coordinates differential equations relating 3-dimensional thermal stresses and strains relevant to a Cr:LiCAF rod was derived. The c-axis of the end-pumped rod was assumed to be perpendicular to the symmetry axis of the cylindrical crystal. Expressions for spatial temperature rise in such a configuration was obtained and the approach of stress calculations was presented.

We describe the spatial mode-matching of a diode-pumped Nd:YAG laser to a ring mode-cleaner cavity with an efficiency as high as 99.8%. Spatial-mode coupling including thermal-lensing effects in the cavity are analyzed.

We present results from a theoretical model that has been developed to simulate the 3-micrometer laser transition in Er³⁺ doped Y₃Al₅O₁₂ (YAG), Y₂Sc₂Ga₃O₁₂ (YSGG), LiYF₄ (YLF) and BaY₂F₈ (BaYF) host crystals. The rate equations for the lowest seven energy levels of Er³⁺ were solved numerically and laser action was simulated under cw, gain-switched (pulse pumped) and Q-switched operation with optical pumping at wavelengths of 975 nm and 795 nm. The relative performance of each laser crystal was compared under identical pumping and cavity conditions to establish the optimum crystal host, doping concentration and pump wavelength for each mode of operation. Some unexpected saturation effects were investigated that could limit the maximum practical pump fluence used for high energy Q-switched systems. We investigate possible additional multi-ion energy transfer processes that may cause the decrease in efficiency that is observed experimentally at high Er³⁺ ion concentrations. In addition, lower laser level deactivation by co-doping with Pr³⁺ in BaYF was simulated and compared with singly doped Er:BaYF for a range of Er³⁺ and Pr³⁺ concentrations. It was found that co-doping was not as effective as the cooperative upconversion process present in singly doped Er³⁺ crystals for efficient laser operation.

A numerical model for high power diode laser-pumped Q-switched Nd:YAG laser is presented. Number of models based on analytical methods for describing the performance effects of diode-pumped solid-state (DPSS) lasers have appeared in the literature. But very few numerical models have so far been reported. Our model based on the numerical method takes into account all operational characteristics of pump laser diode at 808 nm, all opto-geometric and spectroscopic parameters of Nd:YAG crystal, the cavity parameters and various losses including Q-switch loss. Only the thermal lensing effect is excluded in the model. The model has been described to be simple, flexible with on-line graphics facility. It has been tested in case of Q-switched Nd:YAG laser experimental results of which are in close agreement with those predicted by the model. The model can efficiently and accurately compute the Q- switched pulse energy, energy stored in the gain medium, pulse width, etc. Efforts are now being made to generalize and improve the predictive capability of the model so that it can be extended to other solid-state laser crystals optically pumped by laser diodes. It is expected that generalized mathematical model will be highly proficient and interactive to accurately predict the laser output.

A source flow gain model for optical extraction from the chemical oxygen-iodine laser medium is presented. In this model the gas dynamics of the reactive flow through the optical cavity, transverse to the optical axis, is described by the two-dimensional boundary-layer approximation to the full Navier-Stokes equations. The appropriate mass continuity, species, momentum, and energy conservation equations in cylindrical polar coordinates are presented and discussed. The gas kinetics are described by a reduced set of fourteen reactions among nine chemical species and includes pumping of the upper laser level by O₂(¹(Delta) ), deactivation by water and energy pooling with O₂(¹(Delta) ), and the Heidner molecular I₂ dissociation mechanism. Stimulated emission on the 3 YLD 4 hyperfine transition of atomic iodine is described by a generalized version of the Zagidullian gain model which includes finite hyperfine relaxation of the ²P_1/2 and ²P_3/2 iodine sublevels, velocity cross-relaxation of the iodine atoms, and allows for incomplete I₂ dissociation. The model is illustrated by application to a laboratory-sized device and the effects of the boundary layer upon the gas flow, I₂ dissociation, O₂(¹(Delta) ) fraction, small signal gain, optical extraction, and the medium homogeneity are examined.

When numerically modeling optical extraction from a high energy laser it is necessary to interpolate the loaded gain to determine the gain to be applied to the optical field propagating around the optical resonator. In high gain laser systems direct interpolation of the loaded gain can cause aberrant intensity spiking near the edges of the field because both loaded and unloaded points of the gain distribution will be used to obtain the gain to be applied to the field. A new interpolation method which significantly reduces these intensity artifacts is described and illustrated. The method may be used with both geometric and wave-optic propagation algorithms.

The reaction processes in chemical oxygen-iodine laser nozzle flows are investigated analytically. In the transport equations for the reacting species, order-of-magnitude arguments are applied to retain the dominant terms. The effects of local flow properties on the chemical kinetics are found in terms of a transformed coordinate which is a function of the nozzle shape and inlet flow conditions. Approximate closed-form solutions for the iodine dissociation, oxygen yield, and the dissociation cost are derived. The results indicate that the chemical processes occur predominantly in the subsonic section of the nozzle where the pressure is high and velocity is low.

A detailed engineering model for chemical oxygen-iodine laser (COIL) performance modeling and design predictions has been developed. In this model, mixing between the primary oxygen flow and the secondary iodine injectant is treated using a two-stage/three-stream model based on the flow characteristics of the transverse injection mixing scheme. Iodine dissociation, excited state pumping and quenching are treated using the standard Phillips Laboratory COIL kinetics package. Stable resonator optical extraction is described by a rooftop geometric optics model. These models have been incorporated into the two-dimensional advanced cavity code for COIL (AC³). The validity of the mixing, kinetics, and optics models used in this code has been tested by comparing the predictions of the model with the iodine dissociation, laser small signal gain, and optical power data measured using the high pressure RotoRADICL device. Selected small signal gain and output power measured using the low pressure RotoCOIL were reproduced by the models. Modeling of the high efficiency RADICL data obtained with various nozzle throat heights using this model shows good agreement with power. The good agreement with the data obtained from various devices encompassing a broad range of experimental parameters lends credibility to this model.

We propose and analyze a novel low-threshold amplification scheme by bichromatically coupling three upper states including a metastable state. We numerically show that it is possible to amplify VUV emission in Kr gases without population inversion at the probe transition. It is shown that the origin of low-threshold gain is the population trapping of upper states dressed by two intense coupling lasers, and there is a possibility of forming large population inversion in optically dressed systems.

The Thomas Jefferson National Accelerator Facility (formerly known as CEBAF) has embarked on the construction of a 1 kW free-electron laser operating initially at 3 microns that is designed for laser-material interaction experiments and to explore the feasibility of scaling the system in power and wavelength for industrial and Navy defense applications. The accelerator system for this IR demo includes a 10 MeV photocathode-based injector, a 32 MeV CEBAF-style superconducting radio-frequency linac, and single-pass transport which accelerates the beam from injector to wiggler, followed by energy-recovery deceleration to a dump. The electron and optical beam time structure in the design consists of a train of picosecond pulses at 37.425 MHz pulse repetition rate. The initial optical configuration is a conventional near-concentric resonator with transmissive outcoupling. Future upgrades of the system will increase the power and shorten the operating wavelength, and utilize a more advanced resonator system capable of scaling to high powers. The optical system of the laser has been modeled using the GLAD^R code by using a Beer's-law region to mimic the FEL interaction. Effects such as mirror heating have been calculated and compared with analytical treatments. The magnitude of the distortion for several materials and wavelengths has been estimated. The advantages as well as the limitations of this approach are discussed.

Dynamics of a constricted filamental discharge in a discharge- excited ArF excimer laser have been examined using a two dimensional fluid model. An entire process of the filamental discharge, from its initiation and development to extinction, is shown. The filamental discharge is triggered at protrusions which would always exist on cathode surfaces, and develops in the direction of the anode assisted by the high field induced by space charge. The gas temperature in the filamental discharge in the vicinity of the cathode is found to rise. This temperature is shown to play an important role in development of the filamental discharge. The effect of preionization electron density n_e0 is also examined, and significant development of the filamental discharge toward the anode is seen when the n_e0 becomes lower. The results are examined properly in connection with experimental observations.

We have analyzed the energy transfer process of a XeCl laser with a spiker-sustainer circuit using a zero-dimensional plasma model. The calculated relation between an electrical input energy and a laser output energy reveals approximately linear characteristics as an experimental result. Furthermore, main physical parameters determining a slope efficiency and threshold energy of these linear characteristics are clarified. These results show us practical solutions to realize 2-kW laser.

Test stand for function optimization (incomposition of gas- dynamic circuit (GDC) of operating characteristics of full- size discharge chamber of flowing TEA carbon-dioxide lasers (power up to 100 kW) was created in Samara State Aerospace University (former Kuibyshev Aviation Institute). Test stand includes an inside-type GDC, low inductive generators of voltage pulses of preionization and main discharges, two-flow rate system of gas supply and noise immunity diagnostic system. Module construction of units of GDC, power supplies of preionization and main discharges allows to change configuration of stand's systems for providing given properties of gas flow and its energy supply. This test stand can also be used in servicing of laser system. The diagnostic system of this stand allows us to analyze energy properties of discharge by means of oscillographic measurements of voltage and current with following processing of discharges' volt- ampere characteristics by means of a computer; rate of non- stationary gas-dynamic disturbances in discharge gap of discharge chamber was measured by means of pulse holographic system (UlG-1M) with data processing of schliren- and interferogram (density fluctuation sensitivity approximately 10^-2) and sensor measurement system of gas-dynamic shock and acoustics process with resonance frequency exceeding 100 kHz. Research results of process of plasma plate wave and channel structures interaction with mediums, including actuation non-stationary gas-dynamic flows, cavitation erosion of preionization electrodes' dielectric substructure, ancillary heating of channels by main volumetric discharge are presented as well.

The influence of thermally induced phase-mismatch in nonlinear crystals for frequency doubling caused by absorption of laser power is described. A numerical model is developed, which considers the spatial temperature distribution in the crystal and the corresponding wave-vector mismatch. For the temperature profile an approximate analytical expression is derived from the the heat-transfer equation in cylindrical symmetry. The conversion efficiency is calculated by solving the basic differential equations for frequency doubling with a spatial dependent wave-vector mismatch. Because the absorption coefficients are rather different at the fundamental and second harmonic wavelength, the heat-density in the crystal depends on the conversion efficiency and vice versa. Therefore an iteration method has to be used to calculate self-consistent solutions. Experimentally a Q- switched oscillator-amplifier-system with an average power of 200 W at 1064 nm is frequency doubled to an output of 103 W at 532 nm using a KTP crystal. The numerical calculations are in good agreement with the experimental results.

A novel, wide-range, high-speed, and tunable wavelength conversion scheme, 'a fiber Raman converter,' is proposed, in which an externally injected high power pump laser and the associated Stokes laser are used to assist the Raman conversion process of signal light coded with optical information. In order to get a large frequency difference between two carrier frequencies, this fiber Raman process is cascaded twice. However, since the common external pump laser can be used in two cascaded Raman processes as long as phase- matching conditions are attained, the entire configuration is still simple. We numerically demonstrate that wide-range wavelength conversion from 1.31 micrometer to 1.55 micrometer for optical fiber communication is feasible at up to 5 Gbit/s.

We performed computer simulation of the amplifier tract of a terawatt picosecond laser with pulse compression by means of our 3-D computer code PIKA. Parameters of the laser system on Nd-glass for obtaining subpicosecond pulses were determined. Our computer code VERA was used for simulation of a compressor based on holographic metallized gratings with dielectric coating and increased damage threshold.

The model developed allows us to know the temperature field in a cw Nd:YAG laser joining piece. To realize the model, a physical approach has been used for defining the heat sources. First, from a pressures balance the geometry of the keyhole is determined. Then, an analysis of the plume existing at the surface of the target shows that the beam is scattered by the plume and consequently one more heat source must be considered. The spectrometric characterization indicates that no ionized particles exist in the plume and certainly in the keyhole. So Fresnel absorption must be the predominant effects for energy absorption in the keyhole. This assumption has been verified owing to Fresnel absorption coefficient measurements.

The numerical modeling on pulse discharge for a high power carbon-dioxide laser was developed to describe and predict laser operating characteristics. The influence of turbulence and convection on the output of a high power fast axial flow carbon-dioxide laser is especially considered in this paper. An obvious requirement is that the pulse has a precisely specified time shape in order to obtain well-controlled optical pulse. The active medium is described by assuming a five-temperature model and balancing the quantum densities of vibrational states of the CO₂ and N₂ molecules. The conclusions are very helpful to realize pulse operation in cw carbon-dioxide lasers, especially in the laser equipment which is applied to drilling, welding and cutting.

The propagation of pulsed laser radiation along the atmospheric ground paths containing droplet aerosols (fog, drizzle, rain) belongs to a class of problem in nonlinear optics in which the multiple-factor nature and nonadditivity of the processes affecting the transmission of the radiation channel are strongly pronounced. The interaction of the radiation with the medium along the propagation path strongly depends on the type and microphysical properties of the specific meteorological formation as well as on the energy parameters of the beam, the structure of the beam, the temporal regime, and the conditions of focusing. The purpose of this work is to analyze the affect of these factors on the optical characteristics of the channel (the integral transmission) under conditions of explosive vaporization of droplet media.

The present paper is devoted to the application of the phase conjugation method for the thermal blooming compensation. Analysis of the numerical experiment data has shown that the appearance of continuous auto-oscillations in adaptive system is connected with the occurrence of dislocations in the reference beam. The use of the Hartmann sensor with low spatial resolution and modal estimation of the phase results in smoothing the phase estimate and damps the AOS oscillations.