A development history is presented for the last 25 years of research into the phenomenon of thermal blooming associated with high energy laser (HEL) beam atmospheric propagation. Attention is given to the problem of convection-dominated thermal blooming, which is the most important blooming-associated problem encountered by CW and repetitively pulsed HELs. Important results and scaling parameters are presented for the characterization of steady-state, convection-dominated whole-beam thermal-blooming effects in the cases of (1) a collimated beam in a homogeneous atmosphere, (2) focused-beam propagation, encompassing the effects of wind and beam slewing and finite attenuation, and (3) ground-to-space propagation, with and without phase-compensation.

The 'thermal blooming' nonlinearity associated with lasers' atmospheric propagation causes several other propagation instabilities which limit the maximum power transmissible by the atmosphere; these are stimulated thermal Rayleigh scattering, the closed-loop instability, the phase-compensation instability, and, in the case of a repetitively-pulsed laser, the stimulated thermal Brillouin scattering instability. These instabilities, which are excited by optical turbulence along the atmospheric path and by noise of the laser beam, grow through the creation of three-dimensional filament or ribbon structures in the atmosphere which are correlated to disturbances of the laser beam. Phase and intensity compensation can be implemented in principle, via special arrangements of such phase-only correctors as deformable mirrors.

Objective: A predictive theory for the correctability of thermal blooming in the presence of turbulence that is appropriate to very high Fresnel number beams and that does not rely solely on nonlinear wave optics code calculations. Approach: Exploit the facts that the physics of such beams are both local and linear. Calculate an MCF analytically using linear theory and compare with fully nonlinear numerical result. Infer whole beam Strehi from either.

In this paper we show how the electric field spectrum for a high energy laser beam propagating through a uniform atmosphere can be calculated using the linear theory of small fluctuations. The beam is modeled analytically as an infinite plane wave which propagates through a medium with constant absorption and no transverse wind. Return-wave phase compensation is modeled as a filter in the transverse Fourier domain. The linear theory describes the growth of small fluctuations on the beam and accurately predicts the evolution of the electric field spectrum until the magnitude of the fluctuations approach the original beam irradiance. The accuracy is tested by comparing with the spectrum calculated using ORACLE, a full wave optics thermal blooming code.

Thermal blooming limits the HEL power transmitted through the atmosphere at a given beam diameter. The role of refractive index and velocity turbulence has been studied. When the absorption is due to the aerosols, one can reduce the blooming by vaporizing the particles and clearing the beam path.

This paper discusses the stimulated rotational Raman scattering (SRRS) of N2 in the atmosphere and its effect on propagation of high energy laser beam through the earth's atmosphere. The calculation of the gain coefficient, and threshold for several wavelengths are given. The effects of the transient stimulated Raman scattering (SRS) on the gain coefficient and threshold are discussed.

The present evaluation of recent progress in the analysis and computer modeling of adaptive optics hardware applicable to compensation for thermal blooming gives attention to an analytical theory of phase-compensation instability (PCI) that incorporates the actuator geometry of real deformable mirrors, as well as to novel algorithms for computer simulation of adaptive optics hardware. An analytical formalism is presented which facilitates the quantitative analysis of the effects of the adaptive-optics control system on PCI, and leads to both a universality theorem for PCI growth rates and the realization that wind exerts a greater influence on PCI growth rates than previously suspected. The analysis and algorithms are illustrated by the results of the time-dependent adaptively-compensated laser propagation code for thermal blooming, MOLLY, which has been optimized for the Cray-2 supercomputer.

Many of the advances in high energy laser beam propagation through the atmosphere have been made or have been influenced by computer-aided analysis. The WJSA Omega code is being used to analyze the performance and sensitivity of ground-based high-energy laser beam propagation. This very flexible code has been configured to model the uplink/beacon beam propagation through the atmosphere and to model the beam control system. A discussion of the special requirements placed on a four-dimensional atmospheric propagation code is presented here. Also, some results comparing several modeling methods are given with an analysis of their respective limitations.

The sefu1 power of lasers transmitted from ground to space is affected by the level and sophistication of the adaptive optics (AO) correction used. The adverse effects of thermal blooming, turbulence, and their interaction can be reduced by varying the spatial and temporal characteristics of the AO. Results of an analytical study are presented where we vary AO simulation methods and hardware parameters. Since various modeling methods show similar results, we conclude that the problems, such as high spatial frequency instabilities, are of a physical nature rather than numerical artifacts or constraints in the modeling assumptions. Variations of AO hardware parameters, such as actuator location, bandwidth, and deformable mirror influence functions, lead to scaling laws useful to the technologist. The analysis concludes with suggestions for optimizing the adaptive optics approach to the ground-to-space propagation problem.

An adaptive optics (AO) correction system is generally required to compensate for beam degradations caused by interactions between a high energy laser (HEL) beam and the atmosphere. The GRAND propagation code includes a model of a realistic AO system representing many features of a state-of-the-art beam control system. This AO system includes models of a wavefront sensor, a tilt mirror, a focus (secondary) mirror, and a woofer-tweeter deformable mirror arrangement. This paper reports the results of a study to assess the impact of the realistic AO system on the correctability of HEL-atmosphere interactions. The GRAND code results compare the performance of the low-pass filter model and the realistic AO system model in the presence of turbulence and moderate-to-severe thermal blooming. In addition, the effects of low frequency Kolmogorov turbulence were studied in terms of its impact on the AO system requirements.

In view of the possibility that turbulence-thermal blooming interactions (TTBI) in laser beam atmospheric propagation may yield rapidly growing small scintillations, and ultimately lead to severe beam degradation, attention is presently given to a Born approximation/perturbation transmitter system in a turbulent atmosphere without net wind or beam slew, but with the important effect of 'smearing' in high spatial frequency wavefront distortion due to a random (zero mean) atmospheric wind velocity. The perturbed high energy beam, as well as the perturbed probe beam sensed by the adaptive optics system, are shown to satisfy a system of first-order linear ordinary differential equations. It is found that the presence of the small random wind plays a major role in limiting the growth of TTBI effects.

The standard FFT, optical propagation computer code-applicable method for calculating realizations of phase screens to simulate the effects of atmospheric turbulence is presently modified to include such LF aberrations as tilt. It is shown that the modified phase screens yield superior statistical representations of Kolmogorov turbulence, and comparisons of simulations employing such modified phase screens with the results of theoretical calculations for Strehl ratio with finite outer-scale effects are presented. The modified method is noted to be highly memory-efficient, relative to the phase screens used for the propagation simulations.

An investigation is conducted of the effect of the disturbed nuclear environment on the propagation of 1-micron laser beams, proceeding from a physical model for a high altitude nuclear explosion environment, anticipated to be pertinent to SDI battle scenarios, whose primary components are the striation density and length scales, in conjunction with the turbulent density fluctuations associated with the striations. Attention is given to the propagation model used to ascertain the effect of this environment on laser beam propagation. The results obtained indicate that the laser beam can be degraded by filamentation, broadening, and steering in a nuclear environment.

A two-dimensional hydrodynamic model incorporating turbulent mixing is derived by averaging the three-dimensional hydrodynamics equations over slabs of thickness L in the direction of propagation. The resulting equations are identical with those for two-dimensional, incompressibl flow with additional terms representing the average effect of velocity fluctuations within each slab. These equations depend only parametrically on the coordinate in the direction of propagation. The extra terms have a simple physical interpretation but must be modeled in order to close the equations. Correlation times of scintillations are computed from a numerical simulation of plane-wave blooming using this model. A comparison with the correlation times from simulations with only wind shear indicate that the effect of turbulent mixing is relatively small for typical values of the Kolmogorov inner scale.

We have numerically investigated the effect of random fluctuations on uncompensated (open-loop) and phase-compensated (closed-loop) small -scale thermal blooming instabilities of a collimated beam propagating through refractive index turbulence. We used the ORACLE time-dependent, three space-dimensional, wave-optics code on a Cray X-MP. The Monte Carlo random wind fields, v(z), were exponentially correlated along the propagation direction, z. The small scale instabilities are present up to a threshold value of the rms random wind, beyond which beam propagation appears to be stable; the threshold value is nearly independent of the correlation length as long as the latter is much shorter than both the length of the thermal blooming region and the Rayleigh range of dominant perturbations. We describe our results with a dimensionless shear parameter, S, that is directly proportional to the ratio of the turbulence scintillation rate to the thermal blooming rate. S is defined as: (formula available on paper) is the one-axis variance of the random wind and N is the time derivative of the thermal blooming optical path difference (OPD) in waves/sec. The open-loop calculations use a plane wave “beam” and a uniform medium. The Strehl ration at 50 waves of thermal blooming OPD remains approximately constant while S ≥ 2.3 and then it decreases rapidly as S decreases. The closed-loop calculations use a large Fresnel number finite beam, a non-uniform medium of length sL, absorption - exp(-z/L), and a Hufnagel-Valley typ On2 profile whose r0 Fresnel number was 24. For 10 is less than or equal to Np is less than or equal to 40 we see no evidence of the closed-loop instability at a wind clearing time (20 waves) for S ≥ (54 ± 2)/Np, where (formula available on paper) is twice the actuator spacing simulated by a Fourier filter. This rresult suggests that the ration of the turbulence scintillation rate to the closed-loop gain for a uniform wind determines the threshold.

High power laser propagation through the turbulent atmospheric is difficult due to thermal blooming effects. Current atmospheric turbulence models assume that the turbulent structure is transported across the laser beam diameter at the mean wind speed. This frozen turbulence model is valid if the laser length is short compared to the characteristic time scale associated with the evolution of the turbulence. However, for laser systems with very long inter-pulse spacing, the turbulent structure may have sufficient time to evolve dynamically between laser pulses. The turbulent structure early in the pulse train may be quite different from the turbulent structure late in the pulse train. In this study, the results from a two dimensional code used to model the time evolution of atmospheric turbulence are presented. Calculations of the turbulent diffusion and the auto-correlation time of the turbulence as a function of eddy size and velocity structure constant are also given.

The present consideration of the atmospheric sensitivities of high-energy lasers intended for meteorological studies gives attention to the absorption effects of deuterium fluoride and CO2 lasers for several atmospheric gaseous species and aerosols, cloud and precipitation effects, and optical turbulence. Such nonlinear effects as thermal blooming and thermal shock waves are characterized, and the measurement characteristics of modulation transfer function devices, stellar scintillometers, isoplanometers, thermosondes, RF radars, FM-CW radars, molecular absorption/extinction lidars, wind lidars, and passive microwave temperature and humidity profilers, are presented for state-of-the-art devices.

The present evaluation of high energy laser beam propagation sensitivity to turbulence effects is based on both analytical results and the far-field calculation results of the Four-Dimensional Wave Optics Code with Adaptive Optics. Attention is given to the way in which turbulence profile intermittencies affect propagation calculation results, as well as the effect of the Kolmogorov inertial subrange model on tilt variance. A statistical analysis was performed on an ensemble of turbulence phase screens, and the results were compared to Sasiela's (1988) theoretical curve. Good agreement is obtained, and the ability to create screens with accurate tilt-variance characteristics appears to be confirmed.

Sensitivity analyses for high-energy laser atmospheric propagation have been performed on the basis of a four-dimensional wave optics propagation code which defines the requirements for atmospheric dynamics measurements. Initial estimates are presented for range, spatial and temporal resolutions, and accuracy. It is noted that these analyses have considered parameter fluctuations as uniform throughout the atmosphere; the determination of frequency requirements in different regimes of the atmosphere entails a more sophisticated analysis encompassing height-dependent fluctuations.

A theory of aerosol thermal blooming has been developed in which atmospheric aerosols being irradiated by a high energy laser (HEL) beam heat the air resulting in distortion of the laser beam. Other models of atmospheric thermal blooming assume the heating from aerosols at a given height to be uniform in the transverse direction. This model differs in that the discrete, particulate nature of aerosol heating is taken into account. It assumes that the only significant cooling of the irradiated particles is by conduction loss to the surrounding air. Results from the new aerosol blooming model are presented and its impact on the atmospheric propagation of HEL beams is assessed. For laser beam intensities typical of strategic defense scenarios and for aerosol characteristics of the desert southwest, it is found that the amount of aerosol-induced blooming is not significant.

We report the first clear experimental demonstration of large amplification of smallscale spatial perturbations by stimulated thermal Rayleigh scattering (STRS) of a CW laser beam propagating through an absorbing medium in a context normally associated with thermal blooming. A single-mode argon-ion laser beam with = 488 nm was propagated vertically downward through a 1 .2 m cell filled with CC14 that was doped with an absorber to have optical depths in the range 0.5-2.3 . A shear-plate interlerometer near the cell input generated the perturbation. Fringe growth was rapid and visually obvious, as was competing growth from dust specks, etc. The measured growth rate is in good agreement with the asymptotic rate from analytic STRS theory.

An experimental system is presented for measuring a pulsed laser beam's atmospherically-induced phase and amplitude distortions, using a fixed receiver and mobile transmitter. The noninterferometric measurement method operates over ground paths up to 2 km, and has as its primary advantage over MTF and scintillometric methods the gathering of explicit phase information. System operation involves the transmission of 10-20 nsec laser pulses, at either 532 or 1064 nm, from the 200-m aperture of the transmitter to the 200-mm aperture of the receiver. The pupil and focal planes are related by the two-dimensional Fourier transform; by iteratively analyzing the irradiance distributions in these planes, the computer can ascertain the phase and amplitude of the entering wavefront.

High-resolution Fourier-transform spectrometry is presently used, in conjunction with a specially designed multipath absorption cell, to obtain molecular absorption spectra in the 1.30-1.32 micron range for water vapor and isotopically substituted water vapor. A range of water vapor number densities, including the foreign gas-pressure broadening of water vapor by air, were studied in order to extract line-shape parameters required for atmospheric laser-propagation modeling. Molecular line shape parameters are summarized for the 1.3-micron region, and estimates are presented for ground-to-space laser propagation at several representative wavelengths. The majority of line strengths are smaller than the corresponding values in the USAF Geophysics Laboratory HITRAN molecular-parameter data base.

Chemically generated CO laser pulses at 10.6 im have been used to clear a 5cm diameter hole through a stratus-like cloud in a laboratory cloud chamber. The results show that 100% clearing can be achieved. The mechanism is shown to be droplet shattering followed by evaporation. Under the conditions of the experiment, the channel closure is dominated by turbulent mixing and not droplet recondensation.

An experimental study of velocity and thermal turbulence generation is presented. These studies are directed to simple and compact laboratory simulation of atmospheric turbulence. The velocity and temperature turbulence was produced by injection of heated air into a main uniform flow of air. The turbulence velocity and temperature flow field was mapped and the one dimensional energy spectra and integral scales were measured. Homogeneous and isotropic turbulence with 18.5 percent streamwise velocity fluctuation intensity was achieved 8 mesh sizes downstream of the injector. At this location the turbulence was homogeneous within 5 percent and the ratio between the longitudinal and the perpendicular fluctuation components was 1.26. The average integral scale of the streamwise velocity turbulence was 18 mm, 0.18 units of mesh size, and the ratio between the velocity and temperature integral scales was 1.04.

The Gound-Based FEL Technology Integration Experiment's high power laser requires a beam director-dome window to protect the optics from dust accretion and reduce the optical aberrations occurring in the air interface between the dome interior and exterior. One half of the dual aerocurtain designed for this purpose is heated 18 C above ambient temperature, while the other is not heated; this results in the development of two mixing zones, (1) between the heated and unheated halves, and (2) between the latter and the room air. This aerocurtain configuration is shown to reduce the Strehl ratio loss through the window to several percent.

A detailed calculations of the optical quality of a large aperture telescope mounted aboard an aircraft is presented. The open fuselage platform flies at high altitude, at near sonic speed. The optical degradation of the telescope performance due to the surrounding is evaluated.

The cross-sections of high-speed aerodynamic phenomena are presently measured by means of a system that interferometrically ascertains optical path length changes with high spatial resolution. Attention is given to the optical disturbance through an aerocurtain, and to the shock wave produced by a hypersonic projectile. While the slowest repetition rate of the laser, camera, and data recorder determines the time-frame rate, the combination of the camera and data recorder determines the spatial resolution. The technique is the spatial equivalent of phase-shifting interferometry. The real-time hardware employed is discussed.

Small-scale thermal blooming is investigated with a linear theory, a fully non-linear wave optics propagation code, and a laboratory experiment The linear theory and the wave optics code show excellent agreement. The laboratory experiment shows excellent agreement with the modelling for the high spatial frequency modes.

Evidence is presented which suggests the existence of turbulent atmospheric beam-propagation environments that are not adequately described by a smooth Kolmogorov cascade. Attention is given to a model which alters the Kolmogorov spectrum for refractive index with a 'bump' at high spatial frequencies, as well as to a method for computing the model's phase spectrum in the case of one transverse dimension. Analytical results indicate that, if the total power in the index spectrum is held constant, the total power in the phase spectrum is reduced by the bump's presence; the power is linearly proportional to the fraction of power not contained in the bump. Reduced scintillation is noted in the beam intensity, together with reduced beam centroids.

The linearized equations of thermal blooming for an infinite beam in a uniform atmosphere and wind are analytically solved as a perturbation series in blooming for the case of compensated and uncompensated propagation. A Feynman diagram representation of the series is presented. The propagators are used to compute the mutual coherence function (MCF) and Strehl also as a perturbation series in blooming. The dependence of the results on the actuator Fresnel number is discussed along with the relative roles of the phase compensation instability and stimulated thermal Rayleigh scattering. A brief comparison is made with nonlinear numerical simulations to show that the nonlinearities may be neglected.