Our approach concerning the development of hard target return CO₂ DIAL transmitter/receiver systems is two phased: (1) through analysis and experiment, develop a fundamental understanding of the transmitter/receiver physics specific to DIAL systems and (2) apply these fundamentals in the development of optimal performance DIAL transmitter/receiver systems. We present our progress and results towards these objectives with the following topics addressed: a general overview of the DIAL transmitter/receiver system characterization effort with a focus on transceiver noise processes. The effects of correlated noise on DIAL performance, especially those effecting statistical convergence over long sample structures, is introduced. And, preliminary measurements of a low-noise, `white' receiver prototype are presented.

A combined experimental and computational approach utilizing CO₂ infrared gas lasers and chemometric multivariate analysis was employed to detect chemicals and their concentrations in the open atmosphere under controlled release conditions. Absorption spectra of four organic gases were collected in the laboratory by lasing 40 lines of a Synrad 15 W CO₂ laser in the 9.3 to 10.8 micron range. Several chemometric calibration models were constructed based on this IR data using the Partial Least Squares computational technique. The chemometric models were used to analyze in near real time the field DIAL data acquired over this exact wavelength range at round trip distances of 7 and 13 km. It will be shown that the ability to predict the chemicals and their respective concentrations depends on a variety of factors. In 39 of the 45 experiments, the identities of the released chemicals were correctly identified without predictions of false positives or false negatives. Under the best field conditions, we achieved predictions of absolute concentrations within 30% of the actual values.

The ambient atmosphere between the laser transmitter and the target can affect CO₂ differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with CO₂ DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.

Mode-locked CO₂ lasers have been developed which can produce long coherent pulse trains consisting of many narrow subpulses. This laser waveform may be used to numerically generate range-Doppler images (inverse synthetic aperture radar images) of a target wherein the Doppler spread of a spinning target is used to create a synthetic cross-range target dimension. The narrow micro-pulse temporal width provides good range resolution, and the long coherent pulse train provides good frequency resolution of the (cross-range) target Doppler spread. In this paper we examine the algorithms and imaging capabilities of this waveform as implemented for the FLD and Hi-CLASS laser radar (ladar) systems which are now being installed in the AMOS facility on Mt. Haleakala, Maui and in an aircraft testbed.

Preliminary scoping exercises indicate that remote-sensing lidar can play a useful role in missions that involve determining regional weather patterns and atmospheric transport conditions. Both meteorological modeling and local atmospheric sensing should be employed. Satellite-based remote sensing systems, using an incoherent Doppler wind-sensor, seem feasible.

Reflection of laser light from a diffuse surface exhibits a complex interference pattern known as laser speckle. Measurement of the reflected intensity from remote targets, common to `hard-target' differential absorption lidar, requires consideration of the statistical properties of the reflected light. We have explored the effects of laser speckle on the noise statistics for CO₂ DIAL. For an ensemble of independent speckle patterns it is predicted that the variance for the measured intensity is inversely proportional to the number of speckle measured. We have used a rotating drum target to obtain a large number of independent speckle and have measured the predicted decrease in the variance after correlations due to system drifts were removed. Measurements have been made using both circular and linear polarized light. These measurements show a slight improvement in return signal statistics when circular polarization is used. We have conducted experiments at close range to isolate speckle phenomena from other phenomena, such as atmospheric turbulence and platform motion thus allowing us to gain a greater understanding of speckle issues. We have also studied how to remove correlation in the data caused by albedo inhomogenuties producing a more statistically independent ensemble of speckle patterns. We find that some types of correlation are difficult to remove from the data.

The High Performance CO₂ Lidar Surveillance Sensor system (HI-CLASS) is a state-of- the-art coherent ladar system which will provide precision tracking and high resolution imaging at the Air Force Maui Optical Station (AMOS). System development is occurring in 3 phases representing increasing hardware/software complexity and system capability. The recently-completed Phase 1 HI-CLASS system employs a compact, pulsed, coherent CO₂ oscillator, a heterodyne receiver, and signal recorder coupled to the AMOS 0.6 m Laser Beam Director to demonstrate target (satellite) acquisition and tracking, illumination, return signal detection, signal recording, and off-line processing for range and range rate extraction and range-amplitude imaging. This paper will discuss the Phase 1 HI-CLASS hardware configuration, test objectives, and test results to date employing cooperative, retro-equipped satellites and describe the Phase 2 hardware currently undergoing initial testing.

The Simulation and Optimization Numerics for DIAL (SONDIAL) code is described. An overview of the code structure is given consisting of individual models for the lidar hardware linked to a set of models for the natural environment through which the laser energy must traverse. The affects of the environment are contained in the field models. A detailed description of the field models is given including the effects of the atmosphere, plume and target on the return statistics of the radiation. The atmosphere attenuates and scatters the laser light. Atmospheric extinction has both spectral and temporal structure that can cause bias and correlation in the DIAL measurements. The results of FASCODE calculations utilizing the HITRAN database are used to assess the magnitude of these effects. Atmospheric turbulence spatially modulates the intensity footprint at the target and modifies the statistics of the return light collected by the receiver aperture. The roughness of the target produces speckle at the receiver, the statistics of which are given by the intensity distribution on the target surface. An albedo model that includes the effects of (macroscopic) spatial reflectivity variations of typical natural targets on the system single-shot signal-to-noise ratio is described. To address the effect of footprint size and position on effluent detection sensitivity, an overlap calculation between a Gaussian plume model and the optical footprint is performed. Validation comparisons between the model and experimental data are given.

Symmetrical edge and face cooling geometries have been evaluated as a thermal control technique for a high power acousto-optic modulator employed as an intracavity mode locker inside a pulsed TE, 0.5 atm, mode-locked CO₂ laser oscillator with a 0.5 - 1 kW average output power. Both cooling methods have proved effective in minimizing transverse thermal gradients generated in a germanium crystal of the acousto-optic modulator operating in the Raman-Nath regime.

A large-aperture, high-Q, germanium standing-wave AO modulator for mode locking a pulsed high-energy CO₂ laser is described. By operating it in the Raman-Nath regime, optical absorption-induced thermal lensing effects are minimized. Due to a smaller round-trip transit time and desired Bessel function transmission characteristics, only a single set of mode locked pulses are generated.

Both EO and AO modulators can be used to extend the spectral coverage of CO₂ lasers in the 9 - 11 micrometers region. For laser radar local oscillator application, the spectral purity of the frequency shifter output must be high and can be achieved with an AO frequency shifter using a special modulator configuration and a molecular absorption technique. A 500 MHz AO frequency shifter was designed, tested and shown to have a high spectral purity at an output power level of over 50 mW with a conversion efficiency of a few percent.

HI-CLASS (FLD) Laser Radar Produces 2D range-Doppler images, range-amplitude images, azimuth and elevation offsets and whole body range and Doppler estimates of rapidly moving space based targets. The Receiver-Processor converts the return signature on two selectable waveforms into a heterodyned signal which is subjected to conditioning and normalization in the receiver (e.g., extraction of gross Doppler) and then operated on within the processor, primarily convolution and Fourier transformation, to produce the output products in real time at the operator's workstation. This paper will show the instrument design, and provide a look at some of the parametric trade-offs considered in arriving at the final operating configuration. Details of the hardware, and software architectures will be presented, and the algorithms required for acquisition, tracking, and imaging will be explained.

Laser performance of an electric discharge, plasma cathode preionized, atmospheric pressure HF chemical laser was investigated and geometrically optimized. The preionization electron density was measured at atmospheric pressure in helium and in a mixture of SF₆ + H₂ diluted by helium. The measured preionization density is n_e approximately equals 6.7 X 10⁶ - 2 X 10⁷ cm^-3, which is higher than the minimum critical density required for a self-sustained, homogeneous discharge of atmospheric-pressure electro-negative gasses. We have found improvement in laser specific energy for a diluted gas mixture of SF₆ + H₂ due to reduction in SF₆ concentration. The effects of inter-electrode gap height and plasma cathode grid width on laser performance were also investigated and will be reported.

X-ray photo-triggered discharges have been used to energetize Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ gas mixtures. For a discharge volume of 312 cm³ an output energy as high as 3.0 J has been obtained in the ethane mixture, while the best laser performance achieved in the hydrogen mixture is only 1.8 J. The physical reasons which could explain these differences, i.e. different kinetic pathways leading to the formation of the HF molecule, or the onset of discharge instabilities, have been investigated. It is shown, experimentally and theoretically, that the production of atomic fluorine weakly depends on the gas mixture type. A time resolved imagery of the interelectrodes space emphasized that the Ne/SF₆/H₂ active medium degenerates into a spatially inhomogeneous plasma, whereas a very stable homogeneous discharge is obtained in the Ne/SF₆/C₂H₆ mixture. As a result the onset of a discharge instability is responsible for the laser emission collapse at low hydrogen partial pressure and high initial applied electric field values.

The proposal about using COIL not only for industry but also for reverse-industry such as decommissioning are discussed.

We describe several diode laser-based instruments that can detect important species in chemical oxygen iodine lasers (COIL). Species detected include: water vapor, atomic iodine, and ground state oxygen. The sensors allow non-intrusive, real-time measurements from which one can determine small signal gain and the singlet delta oxygen yield. The water vapor concentrations can also be continuously monitored. The sensitivities of the sensors are sufficient for all the conditions found in typical COIL devices. The room temperature diode lasers are miniature and fiber coupled. Data for all three species are presented.

The determination of small signal gain in candidate laser systems, especially low gain chemically pumped lasers, is a challenging problem. In this paper we discuss a new approach to this problem using tunable diode lasers as sensitive probes for optical gain. The gain diagnostic was verified on optically pumped molecular iodine. Small signal gains of 10⁴ were detectable with signal to noise exceeding 10. The gain diagnostic was also applied to a candidate chemically pumped laser system, the oxygen pumped IF laser.

The Airborne Laser (ABL) under development by the United States Air Force is described. The ABL consists of a multi-megawatt Chemical Oxygen Iodine Laser and multimeter beam director installed on a widebody aircraft. The ABL Demonstrator will demonstrate the ability to integrate scalable/traceable technologies into a useable weapon system shortly after the turn of the century. The legacy of the ABL from the earlier Airborne Laser Laboratory (ALL) is discussed as well as the advances in technology which have occurred since which enable the ABL to achieve much larger effective ranges than were achieved using the ALL. The integration of the ABL into the theater battle is also discussed.

Energy transfer processes of highly vibrationally excited I₂(X) v equals 23, J equals 57 have been investigated using stimulated emission pumping techniques. Inelastic collisions with O₂, H₂O, Cl₂, and Ar have been characterized. These energy transfer processes are important in the dissociation of I₂ in the COIL system. Spectrally and temporally resolved measurements of vibrational and rotational energy transfer have been carried out in room temperature static cells. The results of these experiments have been used to develop a kinetics model to simulate the experimental results and to predict rate constants. This model is based on an energy corrected sudden approximation and includes vibrational energy transfer.

An end to end model was developed to predict the performance of a rotating disk Chemical Oxygen Iodine Laser (COIL). The model includes coupled routines to model the singlet oxygen generator (SOG) including ducting and cold trap, the nozzle, and the laser cavity and resonator. The SOG model is based on a quasi 1D two phase wetted wall approach. It includes an effective resistance chlorine/oxygen mass transfer model, a local basic hydrogen peroxide HO₂ diffusion model including property variations both in the flow direction and within the film, thermal analysis for both the liquid and gas phases, and evaluation of O₂(¹(Delta) ) detachment yield, surface deactivation, and gas phase deactivation. A mass addition routine based on variable specific heat and molecular weight influence coefficients is used to establish nozzle initial conditions. The results of the mass addition calculation are used iteratively to assess SOG stagnation pressure and if necessary SOG calculations are repeated. The primary and secondary streams are then separated into a number of stream tubes. The scale of the primary and secondary stream tubes is determined by the specific mixing scale and their stoichiometries by the initial flows. Mixing is assessed based on an approximate bimolecular diffusion coefficient analysis. Concurrent calculations are made to evaluate the effects of area expansion and chemistry. Once the calculation reaches the mirror location the computations can be performed iteratively with either a roof-top or Fabry Perot resonator to achieve self consistency between the mode characteristics and gain medium. The model is described and representative results presented.

Numerous surface modifications take place during the irradiation of an alumina ceramic by a KrF excimer laser. So the aim of this work therefore is to investigate these modifications in order to control and improve the final properties of the irradiated material for possible industrial applications. Furthermore, the understanding of fundamental processes involved in the interaction is necessary to explain the phenomena observed. Here we describe the surface topography showing the modifications versus irradiation conditions. However, depending on the irradiation parameters and the surrounding atmosphere, the laser treatment leads to quite different results. One of these is the metallization of irradiated surfaces with an electrical conductivity which is not negligible on these regions. The formation of conducting patterns allows an electroless deposition with an excellent adherence and hence demonstrates potential uses in microelectronics applications.

Experiments were carried out using excimer lasers in order to improve the properties of some metallic alloys widely used in the automotive industry as well as aeronautic or space. The main results presented in this paper concern the modifications of the mechanical properties (roughness, hardness, residual stresses,...) as well as physico-chemical properties (surface composition, microstructure,...) and corrosion resistance.

The feasibility of coupling in and transmitting high average power UV radiation down fused silica fiber without damage has been demonstrated in burst mode. The approach we have followed to transmit high average XeCl laser powers is to use modest laser energies at very high repetition rates, because the induced attenuation coefficient at high repetition rate is likely to be less than that which would occur using a high fluence modest repetition rate approach. We have also chosen to utilize a long optical pulse duration XeCl laser made possible using a magnetic-spiker excitation circuit. For a given laser fluence coupled into the fiber, the low peak power long pulses reduce the probability of intensity dependent effects such as catastrophic surface damage and color center formation. The influence of pulse duration, repetition rate and laser beam quality on fiber optic transmission will be discussed. An output average power of 75 W (in a burst mode) of XeCl laser radiation has been transmitted through a single step-index fused silica fiber using a high repetition rate (820 Hz), long optical pulse (180 ns (FWHM)) magnetic-spiker excited XeCl laser.

A new high repetition rate laser module on the basis of our latest metal-ceramic laser tube technology is developed. This laser can be operated up to 1 kHz with practically constant output energy of 30 mJ at 248 nm and up to 15 mJ for ArF and XeF. Gas lifetime and window cleaning interval can be extended to 500 million pulses. We present our latest test results for ArF, KrF, and XeF operation. All industrial applications require excellent pulse energy stability. In order to meet this demanding feature we modified the power supply and the high voltage circuit with respect to pulse energy stability. We achieved excellent pulse energy stability at all important laser wavelength with our new high repetition rate laser module, for example a pulse energy fluctuations of Sigma 0.8% at 1 kHz KrF operation.

Uniformity and stability of discharge process in a high Pulse Repetition Frequency (PRF) long pulse XeCl laser are investigated for three different cathode materials (aluminium, copper, brass) versus PRF (f <EQ 700 Hz) and burst duration (1 - 100 shots). The discharge quality evolution is experimentally studied with a fast gated CCD video camera providing 200 - 400 ns time-integrated discharge photographs. This discussion also benefits from a theorical approach of the macroscopic properties of the different cathode materials and from the development of a simple code calculating the shot to shot electrode temperature increase due to the successive input energy releases.

Results of the experimental study of powerful lasers pumped by a radially convergent electron beam and powerful excilamps are presented. Laser radiation energies of 110, 90, 100, and 50 have been obtained at (lambda) equals 308, 249, 1730 and 2030 nm, respectively. Excilamp average power of 130 W have been obtained at (lambda) approximately 222 and 208 nm, the efficiency was up to 14%.

The increasing of stagnation pressure and Re number of gas flow is a very important for supersonic oxygen-iodine laser (COIL). This goal can be achieved with the aid of high pressure singlet oxygen generator (SOG) and high dilution of oxygen with buffer gas of high molecular weight downstream of SOG. The study of COIL operated with jet type SOG at 10 and 20 mmole/s of chlorine flow rate and 50 torr output of pure oxygen is presented. Two experimental set-up were tested. In the first one the mixing of chlorine with buffer gas was provided upstream of SOG gas inlet. In the second one the pure chlorine was injected into SOG and oxygen was mixed with buffer gas downstream of SOG outlet. The stability of jet SOG in the first set-up strongly depended on partial buffer gas pressure and its molecular weight: at higher pressures and molecular weight the stability of SOG operation was worse. In the second set-up the operation of SOG didn't depend on buffer gas pressure and its molecular weight. COIL output power was highest for first set-up with dilution of chlorine by buffer gas until SOG stable operated. In the second set-up the output power was in twice less and strongly depend on type and position of buffer gas injector between SOG and iodine injector. This dependence strongly demonstrated the importance of gas mixing to molecular level for achieving highest COIL power. Another problem considered in this work is connected with BHP heating that important for recirculation of liquid in long time duration COIL operating system. The correlation of BHP heating and O₂(¹(Delta) ) yield is presented. It is shown that nacsent O₂(¹(Delta) ) yield is close to 100%.

This presentation is an historical overview of the activities related to the Chemical Oxygen- Iodine Laser conducted in the Czech Republic. It includes small scale basic research, both the experimental and the theoretical, with a view to particular topics of this laser system in the cw and the pulsed regime of generation.

The Air Force Phillips Laboratory has developed a small-scale supersonic Chemical Oxygen- Iodine Laser (COIL) test stand (VertiCOIL) in order to acquire COIL performance data quickly and inexpensively. The VertiCOIL device has demonstrated a chemical efficiency of 26.7%, the highest efficiency ever reported for a supersonic COIL. VertiCOIL uses a continuously-cooled basic hydrogen peroxide flowing loop which allows run times of greater than one hour.

A high power chemical oxygen-iodine laser (COIL) at the Air Force's Phillips Laboratory was evaluated to determine its cutting capability in thick section stainless steel. The results of this experiment were used to validate a new model that describes the effects of using rectangular- shaped beams for thick section materials processing. That model, while derived in a way similar to a model developed by researchers in Japan, predicts a completely different behavior when cutting extremely thick metal sections. A detailed understanding of the thick section metal cutting is necessary for the development of new commercial applications of very high power lasers, including nuclear power plant dismantlement, shipbuilding, and heavy equipment manufacture. Such applications depend on the demonstration of high power delivery of a laser beam through fiber optics. A case is made for the capability of delivering a COIL beam over 10 kilowatts, and an outline for a possible demonstration using off-the-shelf components is described. Areas requiring further research in high power materials processing technique are described, including advanced assist gas nozzle development, laser focal spot shaping to optimize processing rates and large area surface modification. A description is included of the cooperative research and development agreement program to describe how laser application developers can gain access to testing at the Phillips Laboratory COIL test facility.

Experiments were conducted in order to determine the heat effects which accompany the reactions between gaseous chlorine and strong, concentrated aqueous base solutions. The bases used were potassium hydroxide (KOH) and potassium hydroperoxide (KO₂H). Solution concentrations ranged up to 7 M. Chlorine gas (Cl₂) was bubbled into the base solution which was thermally isolated from the surroundings in a Dewar vessel. From the experimental data, enthalpies of reaction and enthalpies of formation for the concentrated basic solutions were calculated.

This paper describes an effort in which a new generation of HF/DF/HF-OT laser nozzle technology has been developed. Among its objectives is to demonstrate scalability in intermediate laser module size through the Overtone Research Advanced Chemical Laser (ORACL), demonstrate a high power HF Overtone device that can operate in the HF fundamental and DF modes, and to demonstrate uncooled resonator optics at each level. The advancement in the laser nozzle technology improves the laser power density (kW/cm₂), improves the laser specific power (kW/kJ reactant) and simplifies manufacturing, An intermediate sized module has been completed, called ORACL, that could serve is a water cooled laser workhorse along with its associated uncooled optics for the resonator. An operational concept for a regeneratively gas cooled module has been analyzed an is ready for design. This paper will specifically discuss the design, completion and availability of the ORACL module along with its associated uncooled optics.

Laser performance at power levels up to 5 kW have been demonstrated for run times extending to 1 minute. Parametric studies of chlorine utilization and excited oxygen yield are presented for a wide range of operating conditions. Substantial degradation of the utilization is found as a function of accumulated KCl in the generator. Important water vapor density is inferred from pressure increase in the cavity when power extraction is inhibited. A new small scale disk generator driven by BHP flow only, is presented. Evaluation of processing costs shown industrial potential of COIL.

A CO₂ laser with fast axial gas flow was set up and operated with a maximum cw output power above 30 kW. The laser makes use of 8 rf-excited discharges which were optimized regarding to the gas-flow, to the discharge homogeneity and to the optical properties of the gain medium. Results of experimental investigation of these topics are described as well as performance characteristics of the laser system equipped with a stable and an unstable resonator, respectively. With an unstable resonator and an aerodynamic window for the extraction of the beam the laser system gives a beam quality which is close to the diffraction limit for this type of resonator. Disregarding the difficulties which are related to the definition and measurement of beam quality for unstable resonators, the beam quality could be described as M² equals 3. Measured far field intensity profiles in the focal plane of a focusing optics are presented as well as the beam propagation behavior near focus. First results of applications in materials processing are discussed.

CO₂ electrical discharge laser produce CO and oxygen by discharge dissociative of the CO₂. It is well known that oxygen is deleterious to the operation of the discharge. In this paper we evaluate methods of controlling the buildup of the oxygen by gas additives as well as by various in-line and off-line catalysts.

The results of systematical investigation in the field of production of the sealed-off TEA-CO₂ lasers are reported. The main regularities of a stable volume discharge in CO₂ laser mixture, containing the products of plasmachemical reactions are determined. Some versions of the sealed-off TEA-CO₂ lasers operating at the pulse repetition rate up to 5 kHz are realized.

A laser has been developed on the basis of the CW CO₂ laser with DC pumping and cross gas flow. An optical resonator of the laser has been developed on the SFUR (Self Filtering Unstable Resonator) scheme. High beam quality is provided by this scheme (full angle divergence is 0,7 mrad on 0,86 power level at beam diameter 30 mm). The laser generates 3 kW output power without any additional equipment. Besides, it is equipped by four accessory units in a feedback arm to provide a variety of operating modes. (1) Single-frequency CW mode with capability of monotone line tuning in bands 9,4 microns and 10,4 microns (about 70 lines in all) with maximum output power up to 2,6 kW at line 10P20 and per-unit frequency instability of the order of 10 to the minus 7th. As it has been shown, there is a possibility to reduce a laser line width to 50 kHz. (2) A mode with rapid tuning of the generation line (with about 1 microsec transition time) in combination with an opportunity to form any given spectral-temporal sequence. The generation spectrum and output power are the same, as in mode 1. (3) Repetition-rate Q-switching mode with high peak power (up to 800 kW) and the average one (up to 2,8 kW) with pulse repetition rate control in the range of 1 - 100 kHz. (4) The repetition-rate Q-switching mode in combination with line-tuning (such as in mode 1) has been developed for selective photochemistry and laser isotope separation. The results of the applications of the laser with repetition rate mode to producing of a new variety of gas discharge-powerful optical pulsating discharge are presented.

The possibility and conditions of single mode laser operation for lasers with resonator Fresnel number up to 10 and substantial intracavity astigmatism employing unstable resonator with uniform semitransparent output coupler (URUSC) have been investigated experimentally. The influence of astigmatism may be significant in multipass resonators with spherical folding mirrors. The experimental condition of single mode operation in URUSC was for the resonator to be in an unstable region if only for one plane of symmetry determined by astigmatism. At the same time in the other plane the resonator might be stable. Comparison of the far field measurements with numerical simulations was performed. It revealed that even if equivalent Fresnel number in `unstable plane' was greater than unit the laser operation was still single mode.

Degenerate four-wave mixing (DFWM) and phase conjugation (PC) of pulsed CO₂ laser radiation on transient gratings inside its own laser medium have been studied both experimentally and theoretically. The operational mode of the e-beam controlled discharge CO₂ laser has been chosen in such a way (CO₂:N₂:He equals 1:2:4, p equals 0.28 atm, t_in equals 30 microsecond(s) ) that the laser pulse length ((tau) _out approximately 10 - 20 microsecond(s) ) is comparable with an effective relaxation time of the upper laser level ((tau) _rel approximately 15 - 30 microsecond(s) ). The time-history of the CO₂ laser and PC signal pulses, and PC reflectivity have been thoroughly investigated for different cavity Q factors and specific electrical energy inputs. It has been shown, that the formation of the PC signal under the intracavity DFWM on gain (amplitude) and thermal (phase) gratings is characterized by a complicated time history that reflects the main relaxation processes taking place inside the inverted medium. A feature of the transient PC process inside the laser active medium is the dependence of the effective relaxation time of the upper laser level on the intensity of pumping waves. PC reflectivity has been obtained by numerical calculations. A comparison of the theoretical and experimental data confirms the contribution of two different mechanisms of grating formation under DFWM inside the active medium of the CO₂ laser.

Cryogenic high-power CO lasers having an efficiency in the range of 30% have a good promise for many applications, including a material treatment (welding, cutting, hardening), selective chemistry and so on. Because of a shorter wavelength its radiation has a number of advantages over the CO₂ laser radiation. Among those are: the better focussability; less plasma shielding effect; availability of transparent materials withstanding a high-power density; reasonable optical fiber transmission. The disadvantage is the cryogenic gas temperature because of which the CO laser technology is less advanced in comparison with CO₂ lasers. CO lasers have rather high energy efficiency, however not so much is known about the optical quality of the high-power laser beam. Our experimental studies on the laser efficiency as a function of the input power, the gas composition, and gasdynamic conditions allows us to verify the kinetic model. This kinetic model was employed further for predictions of the laser beam quality for different optical cavity parameters, and energy loading in the gas flow. For this purpose 2 2D models were formulated: the first describes the self-consistent distributions of the multiwavelength wave field, gain, and index as functions of coordinates along the gas flow and along the cavity axis; the second describes the spatial distributions of the gain and index approximately as functions of the coordinates along the gas flow and along the discharge current. Both models predict the strong influence of the refractive index non- uniformities on the beam quality, which may explain, in part, the experimentally observed poor beam quality. The methods to improve it will be discussed, including the phase conjugation in the active medium.

A frequency selected (FS) Q-switched (QS) e-beam controlled-discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses ((tau) _out approximately 1 - 10 microsecond(s) ) having different spectral contents including single line one within 4.95 - 6.50 micrometers spectral range. A special optical scheme has been chosen, which has enabled us to study an influence of spectral contents of CO laser radiation `locked' or `blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths. The influence of laser mixture contents, gas pressure and temperature, laser pulse length and spectral width upon the laser characteristics also has been studied. A use of frequency selection and Q-switching decreases the laser efficiency dramatically from approximately 30% for non-selected (NS) free-running (FR) mode of operation down to approximately 0.5% ((tau) _out equals 5 microsecond(s) ) for optimal wavelength ((lambda) equals 5.3 micrometers ) and to 0.1% at the long wave edge of laser spectrum ((lambda) approximately 6 micrometers ). Output energy and laser efficiency strongly depend on the number of short pulses in a train for a single electrical pumping pulse. For instance, eight short pulses lasing ((tau) _out approximately 5 microsecond(s) ) has increased the output energy up to approximately 0.5 J (0.6 J/l Amagat) and efficiency up to 0.6%. For some wavelengths the output energy and laser efficiency of FS QS CO laser do not strongly depend on temperature within 100 - 150 K interval. Comparative analysis of NS FR, FS FR, NS QS and FS QS mode of operation of carbon monoxide laser has been done.

The highly selective Na₃ + Br reaction creates a continuous electronic population inversion on Na₂ transitions which demonstrates optical gain through stimulated emission in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10^-3 cm^-1 for an individual rotational level at approximately 527 nm). With a focus to increasing amplifier gain length and amplifying medium concentration (conversion to oscillator), we extend these initial studies employing the intersection of sodium and bromine flows formed from slit source expansions to create an extended reaction-amplification zone. Initial results with this extended reaction zone provide the first example of chemically induced Raman pumping. Unique Raman signals are induced by and correlate with emission from the NaD-line components (D₁, D₂) formed in the chemical reaction zone primarily as a result of the Na₂ + Br yields Na^* (3P) + NaBr reaction. The NaD-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulation of the experimentally observed spectra predict an unusually large scattering linewidth close to 4 cm^-1. We describe initial laser based studies on a pure sodium expansion to probe the mechanism of the unusually strong Raman scattering and to understand the origin of the large scattering linewidth. These experiments demonstrate that the Raman process can be induced only upon excitation of the NaD-line components, the Raman scattering being insufficient or nonexistent for wavelength shifts (Delta) (lambda) > 1 nm from the D lines. The scattering linewidth for the reactive process is not readily correlated with the line-broadening of the NaD-line and may result from the nature of the Na- Na₂ interaction in the reactive environment. Initial double pass measurements indicate the possibility of optical gain associated with the Raman process.

In this paper I will review work in the development of Singlet Delta Oxygen Generators. I start out with an historical background as several recent reviews were from different perspectives than mine. The basic chemistry and generator performance for chlorine-BHP production of O₂(a¹(Delta) g) is covered. Finally, I discuss how generator yield influences laser performance and some questions from recent work.