This experimental research program is designed to assess the possibility of using gas-phase optically pumped lasers (OPL) as efficient, frequency-agile mid-infrared sources. The eventual goal will be to incorporate efficient diode lasers into the pumping step, either by direct frequency stabilized diode pumping or by pumping with diode-pumped solid state lasers. In this paper, we discuss experiments on a optically pumped hydrogen fluoride laser. Rotation- vibration transitions in the (2,0) band around 1.3 micrometers are pumped, and lasing is observed on (2,1) band transitions near 2.7 micrometers .

In this paper we describe a new energy transfer concept to produce excited electronic states of molecules that are candidates for short wave chemical lasers. Vibrational excitation within the ground state is used to enhance the production of excited electronic states via energy transfer from metastable species such as O₂(a) and NF(a). We describe experiments on the molecule IF as a prototype system.

The dark reactions between the perfluoroalkyl iodides, CF₃I, C₂F₅I, and C₃F₇I and F₂ have been investigated over a wide range of pressures, flow velocities and temperatures. In addition, the effects of surface and the presence of oxygen have been investigated. The reactions are characterized by an induction time followed by an exponential decrease of the iodide over time. The temperature dependences have been measured and Arrhenius parameters have been calculated. Conclusions concerning the mechanism of the reactions are presented and discussed.

We discuss two approaches to create electronically inverted Na based atomic and diatomic configurations: (1) highly efficient near resonant intermolecular energy transfer and (2) highly efficient and selective fast direct chemical reaction.

Excited iodine atoms I(²P_1/2) are created when ICl is injected into a stream of NCl(a¹(Delta) ). A population inversion between the I(²P_1/2) and I(²P_3/2) states of atomic iodine was observed using an optical double-resonance technique.

The radiative lifetime and the quenching rate constants of the PF(A³$PI_0,1,2) state have been measured using a microwave discharge to generate PF(X³(Sigma) ^-) in a flow reactor incorporating laser-induced fluorescence. A radiative lifetime of 4.2 +/- 0.2 microsecond(s) has been determined for a 300 K Boltzmann distribution of rotational and spin- orbit states of PF(A,v' equals 0). The two-body quenching rate constants for PF(A³$PI) by diatomic and polyatomic molecules and rare gases were determined at 300 K from the pressure dependence of the first-order decay constants. Electronic quenching by He, Ar, CF₄ and SF₆ is inefficient and upper limits to these deactivation rate constants are 2 - 4 X 10^-14 cm³ molecule^-1 s^-1. Except for highly fluorinated molecules, the quenching constants for most molecules are in the range of 0.05 - 4.0 X 10^-10 cm³ molecule^-1 s^-1. The available data suggest that the PF(A³$PI_0,1,2) state has some promise as a potential UV laser candidate, providing that an efficient excitation method can be discovered.

Kinetic modelling of a short-pulse optically pumped hydrogen fluoride laser has been undertaken. This work utilizes scaling laws to obtain many of the rate constants describing the numerous energy transfer reactions. Because of the numerous differential equations and energy exchange reactions that must be considered in a complete model, a truncated version has been constructed which includes the processes which are most important in the first several nanoseconds: an optical pump term which includes saturation effects and rotational relaxation kinetics during the buildup of the laser pulse. This model demonstrates the importance of rotational relaxation processes to the efficiency with which energy is coupled into the laser.

Output characteristics of a transverse discharge pumped atomic Ne laser (585.3 nm) using H₂ as a penning partner has been studied. The laser energy is decreased with increasing the operating pressures. Lasing is terminated at 250 Torr. Peak intensity of the laser is saturated at high excitation rates (1 MW/cm³(DOT)atm). Mixing He as a buffer gas with Ne/H₂ mixtures leads to depletion of the laser energy.

This work addresses the possibility of a high average power coherent light source in the 4.8 micrometers atmospheric window. The approach is to frequency-double the 9.55 micrometers CO₂ laser line, yielding 4.775 micrometers . In order to achieve efficient conversion, the intensity of the laser is increased by intracavity resonant modulation, or 'mode-locking'. Efficient conversion in available lengths of doubling material requires an intensity of at least 20 MWcm^-2. However, the most efficient CO₂ lasers operate with a pulse duration of at least 1 microsecond(s) ec, the characteristic time for energy transfer from N₂ to CO₂ at 1 atmosphere. Without modulation a fluence of at least 20 Jcm^-2 would therefore be needed for high overall system efficiency. With modulation at a 1:10 or better mark-to-space ratio, this fluence is reduced to the 2 Jcm^-2 range that typifies the surface damage threshold of available materials. The doubling material chosen for this work was AgGaSe₂, silver gallium selenide. A relatively long crystal (35 mm) was used in Type 1 phase-matching. A mode-locked pulse train was generated using a TEA CO₂ laser pumped for a duration of 3 microsecond(s) ec, containing 1 nsec pulses spaced by 40 nsec. The 9.55 micrometers line was selected either by injection or by the use of an intracavity grating.

Textron Defense Systems (Formerly Avco Research Laboratory) has developed a kilowatt class dye laser. The device is a transverse flow, flashlamp pumped laser that operates at greater than 100 Joules per pulse, and at a repetition rate up to 10 Hz. Operating at 10 Hz, an average power of 1.04 kW was obtained at 585 nm using rhodamine 590 in a methanol/water solvent mixture. The output power was increased to 1.4 kW by adding the triplet quencher cyclooctatetraene to the solution. Under these conditions, the measured efficiency (average laser pulse energy/energy stored in flashlamp capacitors) was 1.8%. A limited series of experiments using alternative dyes was also carried out. Comparable energies and average powers were obtained at 610 nm using rhodamine 610, and 660 nm using sulforhodamine 640.

A vacuum path of about one kilometer is needed in the optical amplitude (or intensity) correction system only using one deformable mirror. This system is too large to be applied. The compression path algorithm given in this paper could make one to two orders of magnitude reduction in length for the vacuum path.

Alpha is a megawatt-class ground demonstration of a hydrogen fluoride, continuous wave, space-based chemical laser. The laser operates in the infrared at 2.8 microns. The basic device consists of a cylindrical combustion chamber that exhausts radially outward through circumferential nozzles into an annular lasing area. An annular ring resonator is used to extract the laser energy from this area. Technical firsts include: (1) use of aluminum combustion chamber/nozzle ring modules, (2) diamond turned, water-cooled optics made of molybdenum for low thermal distortion with good heat transfer, (3) use of uncooled silicon mirrors in a megawatt-class laser system, (4) an optical bench made of aluminum honeycomb, and (5) active controls to adjust alignment of selected mirrors and the optical bench.

The hydrogen fluoride (HF) chemical laser is the baseline concept for SDIO space based laser (SBL) weapons systems. Ground based tests at power levels appropriate for this application have been demonstrated. Because the brightness of a laser beam projected to the far field is inversely proportional to the square of the wavelength, shorter wavelengths are desirable to enhance brightness on target. Development of the HF overtone chemical laser ((lambda) equals 1.3 - 1.4 micrometers ) as a growth technology for SBL applications has proceeded rapidly during the past several years. This paper reviews the parametric characterization and optimization of the Hypersonic, Low-Temperature (HYLTE) nozzle concept for HF overtone and HF fundamental performance. The experiments utilize advanced multilayer dielectric coatings on uncooled silicon substrates. The experimental results reported include laser power, small signal gain, mode footprint, and spectral content. The design of a multiple nozzle HYLTE module as a building block to an advanced high power HF chemical laser device is presented. Design philosophy emphasizes traceability from an intermediate size linear module to a full scale cylindrical gain generator for SBL applications. The key issues addressed are power scalability, fabricability, regenerative cooling capability, and thermal/structural performance.

The rate of generation of O₂(¹(Delta) _g) using the chlorine-basic hydrogen peroxide reaction is a key element to predict the performance of the chemical oxygen iodine laser. O₂(¹(Delta) _g) carries the energy in the laser, thus is one of the prime determinants of power in the flow. To predict the performance of O₂(¹(Delta) _g) generators requires the prediction of the utilization of chlorine, the yield of excited oxygen, and the concentration of potential contaminants in the chemical exhaust of the generator. In essence, this is a standard problem in two phase flow. This paper is in the spirit of a review and describes the author's approach to the analysis to O₂(¹(Delta) _g) generators.

An end-to-end system model has been developed to model the performance of a rotating disk reactor-based Chemical Oxygen Iodine Laser (COIL) system. The model consists of a system of nonlinear algebraic equations combined into modules by subsystem and is well suited for system trade studies and subsystem data analysis. The model treats the three main components of the COIL system including the rotating disk oxygen generator, the nozzle/cavity/resonator system and the delivery duct system that connects the two previous systems. The oxygen generator module was developed by linearizing the governing partial differential equations for a wetted wall oxygen generator assuming the O₂H^- ion concentration is constant across the chlorine/basic hydrogen peroxide reaction zone. The linearized equations are solved analytically and averaged over the flow path to produce a set of nonlinear algebraic equations for chlorine utilization and singlet delta oxygen yield. The output of the oxygen generator is passed to the delivery duct module where the transit loss of O₂ (¹(Delta) ) and the singlet delta loss during iodine dissociation are determined from analytic solutions of the O₂(¹(Delta) ) species conservation equation. The model compares favorably with experimental laser performance data over a range of operating variables including chlorine flow and diluent ratios, disk rotation rate, iodine-to-chlorine ratios, nozzle throat area, and resonator outcoupling fractions.

The rates for deactivation of singlet oxygen, O₂(b¹(Sigma) _g⁺), upon collision with pyrex, Teflon, aluminum, copper, and nickel surfaces have been determined in a steady-state flow tube reactor. Numerical and approximate analytic solutions of the radial diffusion equation with coaxial cylindrical boundary conditions are presented to obtain wall deactivation probabilities from observed first order decay rates. The probability for deactivation at nickel surfaces is large, (gamma) equals 0.026. However, it is unlikely that surface deactivation of O₂(b¹(Sigma) _g⁺) in Chemical Oxygen-Iodine Lasers has any significant effect on laser performance.

A small UDOG was developed and tested over a range of geometries and operation conditions. Analysis and numerical modelling were used to characterize uniform droplet oxygen generators. The numerical model included mass, momentum, energy conservation equations for the droplets and gas, and species rate equations for the gas, gas-liquid interface, and liquid phase chemistry. A relatively simple analytical model was also developed to allow solution of the equations which govern chlorine and O₂(¹(Delta) ) concentrations in the reaction zone. Measurements were made of most inflow and outflow properties, including the O₂(¹(Delta) ), O₂(¹(Sigma) ), Cl₂, and H₂O concentrations. The chlorine utilization, O₂(¹(Delta) ) yield, and generator efficiency were measured as affected by several parameters. Predicted dependencies were observed over a wide range of test conditions. The UDOG efficiency was significantly higher than has been achieved by the other generator types, when compared at the laser cavity condition.

A model for the production of singlet delta oxygen, O₂(¹(Delta) ), following the reaction of gaseous chlorine, Cl₂, with liquid basic hydrogen peroxide, BHP, is described. The model includes diffusion of the Cl₂ gas into the liquid, diffusion of the hydroperoxy anions, HO^-₂, to the surface, reaction of the Cl₂ with the HO^-₂ ions at a finite-rate, heterogeneous deactivation of the O₂(¹(Delta) ) within the liquid, and homogeneous deactivation of the O₂(¹(Delta) ) molecules in the gas. Transport equations are written for the chlorine, oxygen, and HO^-₂ species concentrations in the liquid while ordinary rate equations are written for the chlorine and oxygen species in the gas. The appropriate initial and boundary conditions for these coupled, nonlinear equations are discussed. Several assumptions and approximations, justified because of the existence of several widely disparate temporal and spatial scales associated with the convection, diffusion, and reaction of Cl₂ with BHP, are discussed and applied to simplify these coupled equations.

Over the past decade, the Mid-Infrared Advanced Chemical Laser (MIRACL) and SeaLite Beam Director (SLBD) have been completed, moved to the High Energy Laser System Test Facility (HELSTF) at White Sands Missile Range, New Mexico, and integrated into the largest and most powerful operational high energy laser system in the U.S. The MIRACL/SLBD system has since been used for the development and demonstration of high power optical components and beam control techniques. High power propagation, tracking and beam control experiments over horizontal and near vertical beam paths have been conducted. Successful demonstrations of the vulnerability of both subsonic and supersonic in-flight targets to high energy laser radiation have also occurred.

The low gain nature of HF overtone chemical lasers has heretofore limited the devices to low magnifications on the order of 1.5. In this paper analyses are presented that show a phase step mirror resonator can enhance modal feedback in these devices such that operation at magnifications of at least 3 is possible. The mechanism for this feedback enhancement is destructive interference of the output wave causing confinement of the beam around the resonator axis. It is shown that the radius of the phase step for optimum performance is directly related to the resonator geometry. It is also seen that the phase step technique increases sensitivity of the resonator to misalignments. The analyses include both two and three-dimensional physical optics calculations complete with rotational non-equilibrium chemical laser gain.

The performance of combustion driven cw HF chemical lasers using the HYpersonic Low- TEmperature (HYLTE) nozzle at the fundamental (2.8 micrometers ) and overtone (1.3 micrometers ) wavelengths was investigated. A pseudo-two dimensional, rotational equilibrium, finite rate, chemical kinetic, mixing laser simulation code, Blaze II, was used to model the HYLTE nozzle. The fluid dynamic profiles from Blaze II were used as input to the computationally efficient, rotational nonequilibrium model, ORNECL, which was used for power and small signal gain (SSG) calculations. The Blaze II fluid dynamic profiles and the ORNECL SSG profiles compared very well to the Direct Simulation Monte Carlo (DSMC) calculations performed by TRW. ORNECL was used to calculate the fundamental and overtone powers and SSG's for various flow conditions. Good agreement with experimental data was obtained. Calculated powers, SSG's and power spectral distributions (PSD's) are reported. The ORNECL model, which was baselined to experimental data, can be used to predict the performance of the HYLTE nozzle in advanced gain generator configurations. Blaze II and ORNECL (capable of power and SSG calculations) provide an inexpensive simulation of the HYLTE nozzle.

The proof of principle for a new technique to determine the laser beam spatial profile on a surface was demonstrated. The technique proposed herein involves the use of a target surface with a back surface mounted array of thermocouples to obtain temperature history data of the back surface. These data provide input to a one-dimensional transient space marching inverse conduction code to solve the inverse problem and determine the absorbed flux and temperature conditions at the front surface. The packaged code allows for temperature dependent thermophysical properties of the material. The results of this technique are compared, in a qualitative sense, with the spatial beam profile derived using staring array infrared cameras viewing a diagnostic scatter plate. These two different measurement methods yielded consistent results. In addition, an absorbed temporal beam profile and total integrated absorbed energy for the surface were calculated.

This paper describes the preliminary results of an ongoing study to characterize the nature and sources of sub-micron aerosols in the Phillips Laboratory small scale supersonic COIL device. Heterogeneously nucleated aerosols from both sub- and supersonic flow regimes were sampled and characterized using the University of Missouri-Rolla, Mobile Aerosol Sampling System (MASS). Under all operating conditions where the oxygen generator discs were rotating, significant concentrations of aerosols were detected. Typically these aerosols had peak dry diameters of < 0.05 micrometers and nascent wet diameters of < 0.08 micrometers . Their total number density increased with increasing rotating disc velocity and with the addition of chlorine. A maximum number density of < 3000/cc was observed at maximum chloride flow rates when the initial generator mixture had been heavily depleted (i.e. neutralized with chlorine). The chemical compositions of these aerosols were found to be exclusively solution droplets of KOH and KCl. The critical supersaturation spectra for a KOH, KCl, I₂ have been measured and compared to theoretical calculations thus permitting growth calculations based on laser operating conditions. Experiments to simulate, in a pure gas phase supersonic nozzle facility, cavity pressure increases indicative of homogeneous nucleation of aerosols were successful. Experiments to directly detect the presence of homogeneously nucleated aerosol under these clean simulation conditions are planned for the next phase of this program.

A heuristic approach for predicting the available laser power per nozzle area will be presented and discussed as a function of reduced mass flow rate and oxygen pressure. The different loss mechanisms due to limited oxygen generator performance, gas transport, iodine dissociation, water deactivation, and laser threshold will be clearly identified. A comparison with available experimental data shows that the model describes subsonic and supersonic experiments, as well. However, a limited validity of the simple model is found for further pressure scaling. This relates in particular to the usual description of the water deactivation losses. The goal of this paper is to derive a new expression for those losses which are still valid at higher gas pressures. The new approach presented in this paper starts from the basic rate equations and takes into account the steady state situation between the excited iodine atom and the excited oxygen molecule. The newly derived expression is included in the general heuristic approach predicting the available laser power. Numerical results are discussed for various laser parameters. Special emphasis is given to the relative importance of gas transport vs. water quenching losses as a function of oxygen pressure and mass flow rate in the laser.

The problem of imaging distant objects through atmospherically-induced aberrations has attracted considerable attention is recent years. Such imaging often requires active laser illumination for two reasons: the received signal must be enhanced, and specific spatial and temporal coherence properties are required to obtain an image. Many different active illumination techniques are available: these include Doppler, range, and angle-angle imaging, as well as techniques from stellar speckle interferometry and holography. The various techniques are compared in terms of illuminator power and waveform requirements.

For large zenith angles, the atmospheric density gradient causes differential refraction of beams at different wavelengths. As a result, the wavefronts of two different lasers differ not only because of diffractive effects but also because of the different pathway through atmospheric turbulence. The anisoplanatic effects of this pathway separation differ from a strict angular divergence of the beams because of the cumulative nature of the refractive bending. Differential equations describing ray trajectories and analytic expressions for piston- tilt removed anisoplanatic degradation will be presented. Analytic results will be compared with predictions of wave optics calculations. Explicit piston and tilt removal make the phase variances a direct measure of the optical quality usable directly to calculate strehls.

FEL power beaming has broad application to space operations. The Rocketdyne Division of Rockwell International Corporation has examined the commercial applications of beamed power from Earth to space using the Radio Frequency LINAC Free Electron Laser (RF FEL) and has determined that there is a substantial addressable market. Rocketdyne's experience in developing and demonstrating FEL technologies, optics and atmospheric compensation and advanced power and power distribution systems ideally positions the Division to conduct the initial demonstration to prove the feasibility of using a FEL to beam power to space platforms.

This paper describes an in-house experiment that was performed at the Avco Research Labs/Textron to test a proprietary atmospheric phase compensation algorithm. Since the laser energies of interest were small enough that thermal blooming was not an issue, it was only necessary to simulate the effect of atmospheric turbulence. This was achieved by fabricating phase screens that mimicked Kolmogorov phase statistics. A simulated atmosphere was constructed from these phase screens and the phase at the simulated ground was measured with a digital heterodyne interferometer. The result of this effort was an initial verification of our proprietary algorithm two years before the field experiment.

The LCT/LOWKATER transmitter is a compact, medium power, repetitively pulsed, UV- preionized discharge, CO₂ laser currently producing a pulse 'chirped-tone' waveform intended for ground, airborne, or space-based ladar fire-control applications. Straightforward modifications can extend its applicability to higher resolution ladar ranging and imaging, generation of additional wavelengths via CO₂ isotopic selection and non-linear processes, and chemical detection and identification. For example, addition of an AO modulator will produce a coherent, pulse-burst waveform suitable for precision ladar ranging and imaging and efficient frequency multiplication into wavelength regions having good atmospheric transmission. Further addition of a rapidly tunable grating will yield a wavelength agile transmitter suited for chemical detection and identification. This device represents a versatile source of coherent radiation suitable for diverse applications sharing a common need for waveform fidelity from a compact transportable device.

The analytical theory governing the propagation of spiral laser beams in nonlinear media is developed. The stability of a circular symmetry structure of the beam is investigated. It is found that critical autoguide power depends on the topological charge of the spiral beam.

A diagnostic system was designed, fabricated and tested for measuring a Nd:YAG laser's performance using a five-axis laser machining center. Far field beam profile, beam quality, beam tilt and beam power were measured at high frequencies while holes were simultaneously percussion drilled in aerospace materials used in gas turbine engines. The system allowed determination of effects of varying laser and machine operating parameters on materials processing performance. Variations were made in laser pulse length, pulse energy, pulse repetition rate, drilling standoff distances from part, incident angles and other parameters. Measured laser beam performance was compared directly with drilled holes. A number of identically drilled holes provided hole-to-hole variance, allowing the machine center operators to optimize materials processing by tuning the laser operating parameters. The results also gave the operator additional understanding of variations of processing performance.

Rate constants for collisional transfer between highly excited levels of I₂(X) have been measured. A single vibration-rotation state (v' equals 42, J' equals 17) was populated by stimulated emission pumping. The prepared level, and collisionally populated levels, were detected by laser excitation of the D-X system. Relaxation kinetics were determined by variation of the delay between the dump and probe laser pulses. Collisions with He, Ar, I₂, and O₂ were investigated. Rotational energy transfer rate constants were measured for all collision partners. Vibrational energy transfer ((Delta) v equals 1) was observed for He, and O₂, but this process was immeasurably slow for Ar and I₂.

We have been investigating kinetics issues associated with the development of a new chemical laser in the green based on the b-X transition of NF at 529 nm. The proposed scheme involves energy pooling of NF(a¹(Delta) ) with I^*(²P_1/2) to form NF(b¹(Sigma) ). The kinetics of NF(a) and NF(X) have a strong impact on the relative populations of NF(X) and NF(b) and therefore play a central role in this system. We have measured the rate coefficients for the reactions of NF(a) and NF(X) with several relevant species including O₂, NF₂, N₂F₄, and I₂. We have also carried out a series of measurements on the energy pooling of I^* with NF(a) by photolyzing mixtures of HI and NF₂ at 193 nm to produce H atoms and NF(a). Using a flashlamp-pumped I^* laser, we optically prepared I^* to create a steady-state ratio of [I^*]/[I] during the 30 microsecond(s) laser pulse. This greatly simplifies the kinetics and allows us to extract rate constants for: NF(a) + I^* yields NF(b) + I, NF(b) + I yields NF(a) + I^*, and NF(b) + I yields other products.