We have found the gain of a Raman amplifier to be almost completely suppressed when the amplifier is pumped by a multi-longitudinal mode laser and seeded with an initially uncorrelated Stokes beam that is nearly as intense as the pump.

We have studied two-wave mixing in resonant media. Illumination with two beams of the same frequency leads to the formation of a spatially local volume grating with no exchange of energy between the writing beams, each beam merely reducing the absorption experienced by the other beam. Nondegenerate two-wave mixing leads to gain for one beam and additional absorption for the other beam as they propagate through the medium. Starting from first principles, the explicit forms of the two-beam coupling constants are derived using third-order perturbation theory, and their dependence on material and experimental factors is examined.

We examine the diffraction efficiency of an acousto-optic device illuminated by an intense optical beam. The electrostrictive coupling within the acousto-optic medium is shown to give rise to enhanced diffraction efficiency.

The transient phase conjugate character in the resonant absorbing meaia, BDN-toluene solution, is theoretically studied. The quantitative relations of the pulse duration and reflectivity or the pulse phase conjugate wave versus the physical parameters of degenerate four wave mixing (DFWM) set up are obtained in the experiment. It is proved that the phase conjugation can compensate for the pulse speading due to the dispersive effect in the fibers. The experiment has been done using the short multimode fibers and the result shows that the approach is feasible.

Multiple-short-pulse stimulated Brillouin scattering (MSP SBS) is a means to obtain phase-conjugation of pulse trains characterized by pulse durations and inter-pulse separations both short relative to the Brillouin lifetime. The Brillouin lifetime of the SBS medium provides a memory for pulse-to-pulse build-up of the acoustic grating. For these short-pulses and for pulse energies below the thresholds of competing non-linearities, a single pulse by itself would not be above threshold. It is the cumulative addition of in-phase acoustic gratings created by successive pulses that results in the above-threshold operation required for the high reflectivities observed.

Threshold measurements of stimulated rotational Raman scattering in hydrogen have been performed for the case of a linear and circularly polarized pump, with and without atmospheric-like phase distortions. Since the effects of Stokes-anti/Stokes (SAS) coupling disappear with circular polarization, the comparison of threshold measurements with each polarization provides a direct measurement of the effective gain reduction caused by SAS coupling combined with turbulence induced intensity scintillations. The results are in good agreement with Raman code calculations.

We have demonstrated simultaneous multiple-line stimulated Brillouin scattering (SBS) from a common focus in the 2.8 to 3.5 gm spectral region using a pulsed hydrogen fluoride chemical laser. Total SBS reflectivity agrees well with a model that assumes no interactions between lines. Both common-focus and spectrally-dispersed multiline input produce the same SBS reflectivity and phase conjugate fidelity.

The laser source for this set of experiments was a photolytically pumped iodine laser. The pulse had an energy of 5 joules and a time duration on the order of 100 microseconds. The pulse shape generally consisted of two primary parts: the first peak of approximately 10 microseconds followed by a 20 microsecond period and a final large peak of 40 microseconds full width at half maximum. The other predominant feature was relaxation oscillations. High speed detectors were set up with transient digitizers that monitored the pump laser power, throughput laser power, and SBS return power. The SBS medium was carbon disulfide. The laser beam was focused into the medium and then recollected on the throughput side. SBS was observed for all portions of the pulse above threshold and for continuous periods as long as 40 microseconds. There were two new effects observed in this set of experiments. The first is a decay tail several microseconds long present in the return SBS after the pump signal had dropped below SBS threshold. Secondly, the SBS threshold appears to be correlated with the laser power as a function of time during the pulse. The observed effects are attributed to the additional presence of Stimulated Thermal Brillouin Scattering (STBS) on the basis of qualitative arguments. There was no evidence for forward SBS as a competing process.

Dye molecules held rigidly in solid hosts display unusual polarization properties that can be exploited to produce nearly perfect vector phase conjugation with high sensitivity.

We have investigated the growth, the pulse energy statistics, the spatial modes, and the spectra of the first and second Stokes light generated in transient stimulated Raman self-generator experiments in hydrogen and deuterium. In particular, we have observed two distinct regions of second Stokes growth. The first region is characterized by a low signal level due to 4-wave mixing. The second region appears to be one of stimulated growth seeded by spontaneous scattering at the second Stokes wavelength, an assumption supported by the rapid growth of the second Stokes signal and the observation of pulse energy statistics indicative of growth from noise. We have measured the second Stokes threshold as a function of gas pressure. At high pressures the threshold is consistent with measured first Stokes thresholds, indicating that the second Stokes is not being seeded by the second Stokes light generated from 4-wave mixing and is due entirely to stimulated scattering from quantum noise. At lower pressures, however, the second Stokes threshold is lowered, indicating some seeding of the stimulated process due to 4-wave mixing. This change in the effectiveness of seeding may be caused by changes in the coherence length of the non-phase-matched 4-wave mixing signal as the pressure is changed. Measurements of the first Stokes spectrum indicates that there is substantial spectral broadening at pump energies corresponding to the second Stokes threshold. This spectral broadening is larger for higher pressures. We postulate that the spectral broadening and shift are due to the AC Stark effect and that this may also reduce effective seeding by 4-wave mixing due to spectral mismatches.

Mutually-Pumped Phase Conjugation (MPPC) in a transparent Kerr medium is considered. A pair of mutually incoherent light beams pump each other to generate the phase-conjugate replica of the other beam via Nondegenerate Four Wave Mixing (NFWM) in the nonlinear medium.

The use of angle-tuned phase matching can greatly reduce limitations on the pump laser bandwidth in crossed-beam Raman amplifiers. Potential advantages over collinear broadband amplifiers predicted to be attainable with this technique include improved intensity homogeneity, reduction of intensity-phase print-through, simplified beam combining, and avoidance of competing nonlinear processes. Expressions for allowed bandwidth are given.

Measurements on soliton decay in stimulated Raman scattering have led to the study of frequency fluctuations in the single shot Stokes linewidth. These spectral measurements show that the spectrum of a single shot can be considerably narrower than the gain narrowed profile predicted by quantum mechanics for the ensemble average. However when spectra of many shots are averaged together the predicted result is recovered. The fluctuations seen are thought to be related to the quantum fluctuations associated with the initiating spontaneous emission. Theoretical results based on coherent mode theory are shown to be in agreement with the data.

Measurements of two-wave mixing gain and phase-conjugate response in barium titanate using GaAlAs diode laser sources emitting at 830 nm are discussed. Gain coefficients as large as 18 cm-1 have been obtained with optimized mixing geometries. With an optically isolated barium titanate ring passive phase-conjugate mirror we have obtained phase-conjugate reflectivities as large as 56% (uncorrected for Fresnel reflection losses) and response times on the order of tens of seconds. These results represent significant improvements over corresponding values previously reported in the literature.

Transient laser-induced gratings have been proposed as a means of controlling high power beams at a different frequency. Molecular absorption which shows an unusual sensitivity to buffer gas pressure has so far prevented observation of the desired grating.

One of the most troublesome problems encountered when using diode laser arrays is that their beam quality is typically quite poor. In this paper, we present a possible solution to this problem. By using the photorefractive properties of materials such as barium titanate (BaTiO3) it is possible, through two-wave mixing (TWM) and phase conjugation, to obtain near-diffraction-limited Gaussian beams from diode laser sources.

The linearized theory of thermal blooming instabilities is reviewed. The dispersion relation is derived for unstable modes with and without correction. Perfect phase-only correction is unstable at all spatial scales, while perfect field conjugation is stable at all scales. Various atmospheric phenomena such as turbulence, diffusion and wind shear may mitigate or dampen the instabilities. Full nonlinear time-dependent computer simulations of laser atmospheric propagation agree with the dispersion relation for small perturbations, and show saturation and nonlinear mode beating at long times.

Comparisons are made of the transverse coherence length (ro) and isoplanatic angle (θo) derived from measurements taken with the Flatlands very high frequency (VHF) radar, an ro system, an isoplanometer, and temperature fluctuation sensors mounted on a thermosonde. The measurements were conducted at the Flatlands location near Urbana, Illinois, from 7-13 June 1988. The site was selected because of the lack of orographic effects on airflow and the refractive index structure parameter (Cn2). The radar was operated with alternating beams in the north and east directions at 20° zenith angle in order to avoid contamination by specular reflections. Contributions of the refractive index structure parameter (Cn2) from different altitudes above ground to the integrated-path ro and θo values are presented and discussed. Diurnal variations of ro and θo obtained from the suite of instruments are shown with particular emphasis on the "neutral" events. Measured results are compared to results from the Aeronomy Laboratory's theoretical model of Cn2.

Nonlinear propagation phenomena such as thermal blooming, thermally amplified scintillations, and breakdown often have thresholds below which their respective effects are negligible. Depending upon the gain associated with the phenomena, the threshold may be either "hard" (i.e., the onset is sudden and dramatic as the threshold is passed) or "soft" (i.e., the effect grows steadily more important). Critical propagation parameters, such as the distortion number (ND) and coherence length (r0) are often computed from atmospheric data to represent these phenomena. Thresholds for the onset of the nonlinear effects and adaptive optics correctability are commonly expressed in terms of these parameters. This paper examines practical considerations in computing and applying critical propagation parameters for realistic slant-path propagation scenarios in an inhomogeneous atmosphere. Issues such as correlation of wind speed and direction at different altitudes, location of blooming and turbulence layers, and the effect of averaging techniques on the computed parameter values are discussed. Calculations are presented contrasting predictions for model atmospheric profiles with actual meteorological data from specific rawinsonde launches.

The short time behavior of turbulent diffusion in the planetary boundary layer for various atmospheric conditions is reviewed. The magnitude of turbulent diffusion is derived for the Von Karman spectrum (modified Kolmogorov spectrum), which is widely used for propagation analyses. The effect of turbulent diffusion on thermal blooming is evaluated by using the time-dependent, turbulent diffusivity model. The results show that turbulent diffusion introduces significant defocus to the phase front of the laser beam. The length and time scales of turbulent diffusion are also assessed for various atmospheric conditions. The finite scales may play an important role in alleviating growth of thermal blooming instability, but the effect of turbulent diffusion on instability is not assessed.

A model has been developed to simulate the effects of a 314.5 nm laser pulse on creating electrons and ions in an atmospheric path. This wavelength was selected to exploit a 3+1 multiphoton ionization resonance with the 3p5 4s 1P°1 excited state in naturally occurring argon. Using a system of coupled rate equations and a detailed collection of atmospheric chemistry reactions the model determines the concentrations of electrons and ions of major atmospheric constituents as functions of altitude, time and laser energy. When considering the propagation of a laser beam of this wavelength through the atmosphere, results provided by the model indicate large peak electron densities can be obtained, without causing optical breakdown, depending on the beam irradiance and distance the beam has propagated.

Neutral krypton atoms were excited from the ground state 4p6 1S0 to the 4p5 6p[3/2]2 state by a resonant two-photon absorption from a line- narrowed ArF excimer laser operating at 193.41 nm. A third photon, absorbed while the atom is in the excited state, ionizes it. Excited state and ion densities were theoretically computed using a stvdard,rate-equation analysis. The irradiance levels used (1-5x10 8 W/cm2) were too low for significant ground and excited state ac Stark and Rabi effects. The photon detection system was calibrated with a standard tungsten lamp. Ion signals wel3 measured with known electrical components. The resonance results were compared with predictions of non-resonant ionization based on a standard formulation. The ion and excited state densities have been used with a modified electron beam propagation code (IPROP) to model such propagation in a low pressure laser-excited krypton channel. The modifications included the effects to field ionization of the excited krypton atoms. Implications for guiding of e-beams using ArF excited krypton are discussed.

One of the major obstacles to the propagation of intense electron beams in low pressure media is the occurrence of the "ion hose instability" which arises when such a propagation takes place in the so called "Ion Focus Regime" or IFR. In the simplest model of this instability, namely, the rigid beam model, the instability is known to be absolute, i.e. it grows at every point in space. When the radial motion of the ions in the propagation channel is included in the simple model, it is also known that the IFR instability transforms from absolute to connective, i.e. the wave grows as it propagates. In this paper we re-examine this problem by incorporating the effects of collisions that take place between the beam particles and those of the channel medium through which they move. Specifically we include the effects of impact ionization by the beam front as well as the avalanche ionization generated by the ions in the channel. We treat both the beam and channel ions as rigid but bendable, and solve the appropriate set of coupled equations that describe the displacements of the beam and the ions. We find the IFR hose instability to remain connective and its spatial growth rate to be reduced by about 50% for the same charge neutralization factor when avalanche ionization is superimposed on impact ionization. When an ad-hoc damping factor that simulates a spread in the betatron frequency is introduced we find that the damping rate varies inversely with the charge neutralization factor.

The interaction of high energy laser beams with aerosols in the atmosphere produces non-uniform temperature variations in the medium surrounding the aerosols. These non-uniformities in the ambient index of refraction act as scattering centers for the laser beam. The basic thermodynamics and conservation equations are solved numerically for pulsed high energy lasers interacting with aerosol water droplet systems. The resulting temperature variations at the surface of the aerosol and in the surrounding medium are found for a variety of input variables including absorbed laser intensity, pulse repetition rate, pulse time, and particle radius. The laser beam scattering is calculated through the Born approximation utilizing the computed temperatures. The range of variables for which laser beam scattering by aerosol aureole systems may be important is discussed.

Tropospheric and middle atmosphere measurements using lidar systems at the University of Maryland are described. Measurements include depolarization by clear atmosphere and high altitude cirrus clouds, cloud morphology for cirrus and tropospheric haze and clouds, and rapid variations of the cloud free line-of-sight. Rayleigh scattering from the stratosphere and mesosphere is employed to derive temperature and density profiles. A photon counting system has been developed for extending temperature measurements into the mesosphere. We are also constructing a transportable lidar system for clear air, aerosol, cirrus cloud, mesospheric cloud, and sodium measurements at geographical locations of interest for atmospheric propagation studies. The lidar measurements will bear directly on selection of sites for tests and operation of ground based high energy lasers, and on estimation of extinction in long range optical detection and tracking of targets.

Statistics on cloud cover and the attenuation of radiation by clouds are being compiled from satellite and lidar data. The multispectral data from the GOES/VAS satellite allow the definition of cloud altitudes and infrared emissivities. Thin transmissive cirrus clouds can be detected with this technique which have been mis-identified in other studies as low altitude clouds or completely undetected because of the complications of underlying clouds or land backgrounds. A summary of cloud cover statistics over the last two years is presented in Section 3. The cloud infrared emissivity derived by this technique is a measure of the attenuation of infrared radiation by the clouds. It has been theoretically related to the total vertical optical thickness in the visible spectrum by Hansen l. This relationship is predicted using a multiple scattering model of cirrus cloud sized particles. With this relationship we plan to convert our cloud emissivity data to optical thickness estimates so that radiative attenuation estimates can be made in both the visible and infrared parts of the spectrum. We are in the process of verifying the relationship using lidar and satellite data. The first results of this verification study are shown in Section 4.

Atmospheric variability affects the utility of most existing and planned DOD weapons systems. The goal in assessing the impact of the atmosphere is to establish the frequency and extent of the system performance degradation caused by the environment and identify both design changes and operational employment procedures which will minimize the atmospheric impact and thereby optimize the system performance. With proper understanding of the effect of the atmosphere on system performance coupled with reliable evaluations of the spatial and temporal occurrences of the constraining environmental phenomena, optimization of system performance is possible. In the case of high energy beam propagation through the earth's atmosphere, clouds are a limiting atmospheric phenomena. With current technology, clouds in the line-of-sight are "show stoppers". Hence to maximize the availability of high energy weapons, sites with low frequencies of cloud occurrence should be selected. For several reasons we have been collecting and archiving meteorological satellite imagery over the western U.S. for use in evaluating cloud conditions at the local scale. Visible light data have been archived during daylight hours and thermal infrared data collected 24 hours each day from the GOES satellite. Using these data, composite cloud climatology images have been constructed which give the frequency of occurrence of cloud cover by hour and month over the western U.S. with a spatial resolution on the order of the satellite pixel resolution. This paper will describe the techniques used in the construction of these composite cloud climatologies and their use in local area studies. Additional applications of this new satellite-derived cloud climatology will be discussed.

The attenuation of visible and infrared radiation by atmospheric ice particles is influenced by the wavelength of the radiation, the particle size, the detailed particle shape, and its orientation with respect to the incident beam. Our studies have revealed the importance of soluble ionic salts and growth rate on the detailed shapes of ice crystals. The mechanisms of crystal growth from the vapor are intimately linked to crystal morphology. Thus, from the study of morphological details of ice crystals, the crystal growth mechanisms are deduced.

The question of laser beam propagation in a disturbed atmosphere has been considered. In this paper an overview is presented of the sensitivity of beam propagation to turbulence in the neutral atmosphere. Using the lateral coherence length as a measure of beam modification, the variation of the coherence length is considered as a function of a variety of fluid parameters that characterize the disturbed and turbulent atmosphere. It was found that subsonic turbulence occurring with densities representative of the ambient atmosphere at 80 km could produce significant effects on the propagation of a laser beam.

Electron density fluctuations in the earth's ionosphere can adversely affect SDIO systems which involve electromagnetic wave propagation, e.g., laser beams. This is particularly true for severely disturbed ionospheric conditions produced by high altitude nuclear explosions (HANEs). In this paper we present a detailed analysis of the lower-hybrid-drift instability; an instability which is likely to be excited in the ionosphere following a high altitude burst and can generate small scale electron density fluctuations (i.e., λ ≤ 10's m). We focus on the HANE environment at early times (t ≤ min) because of its importance to SDIO systems and because it is an unusual plasma regime: high β and collisional. Both the linear and nonlinear behavior of this instability are discussed. We describe how these results can be applied to potential SDIO laser systems in a HANE environment.

In order to statistically represent the turbulence encountered in a structured environment, the established procedure has been to employ simple mathematical generalizations of models that are known to perform well for atmospheric turbulence. However, for the non-Kolmogorov regimes of interest, this approach encounters theoretical as well as practical difficulties of a fundamental nature. These will be described in detail, along with new analytical methods better suited to the particular problem. Finally, results of wave optics computer codes implementing these techniques will be presented for specific realizations of the environment as computed by the hydrodynamic models.

In this study we discuss the interaction of a high energy laser beam with the ionospheric space plasma environment. We consider the plasma to be inhomogeneous, underdense, collisional, and embedded in a magnetic field. The density and temperature of the plasma are such that koλD ≥ 1 where ko is the wavenumber of the radiation beam and λD is the plasma electron Debye length. Using linear theory we compute the growth rates for stimulated Raman scattering and beain filamentation parametric instabilities using several values for laser beam intensity and disturbed ionospheric space plasma conditions.

Using a fully nonlinear two and one half dimensional fluid simulation code, we have performed a series of simulations of the effects of Raman scattering on the propagation of an intense laser pulse through an underdense plasma. The growth of the electrostatic plasma wave and the backscattered electromagnetic wave are in good agreement with theoretical predictions.

Communications at radio frequencies are adversely affected by natural and man made disturbances of the ionosphere and atmosphere. Signals are absorbed and refracted as well as distorted by amplitude, phase and frequency dependent scintillation effects which can be very severe. Techniques have been developed to evaluate the affect which these propagation disturbances have on radio frequency communications links.