We discuss the unusual diffraction properties of laser speckle and related structures generated by stimulated rotational Raman scattering (SRRS) from nitrogen in long air paths with collimated, large aperture beams. We have obtained convincing evidence for the four-wave parametric gain suppression of SRRS and enhancement of nonlinear diffraction by Raman-induced Kerr effect in this high Fresnel number geometry. These phenomena significantly influence the beam filamentation and speckle structures observed in the near and far fields. We contrast present observations with previous SRRS experiments at Lawrence Livermore National Laboratory using the Nova Laser.

We have examined the time dependence of laser light for propagation parallel and antiparailel to the c-axis of a poled single crystal of barium titanate. A type of symmetric beam fanning was found to be more important in determining the transmission of our "best" crystal than two-beam coupling between the transmitted wave and that reflected from the back face of the crystal. This finding contradicts with previous reports of asymmetric transmission where effects of beam fanning were assumed small. Observation of the fanning motivated the more detailed measurements reported here.

We have demonstrated efficient stimulated Brillouin scattering (SBS) of both single-line and mul-tiline hydrogen fluoride laser radiation near 3µm in high pressure xenon gas. Power reflectivity of >80% was observed in the single-line case. The expected 100-MHz Brillouin shift of the return was experimentally confirmed. High fidelity phase conjugation was also demonstrated using the single-line source. For both single and multiline experiments, the apparent Brillouin reflectivity was shown to significantly decrease if the SBS was not well isolated from the laser source. Preliminary SBS reflectivity results using the quasi-simultaneous -20-line HF laser source are presented.

We present a comparison of the theoretical predictions of a multimode broadband model with the experimentally measured gain enhancement in a Raman amplifier. The results show that the multimode theory with fixed and totally random phases is in agreement with the data obtained from an excimer laser pumped Raman amplifier. Additionally, this theory indicates that the correlated gain can be larger than the gain for a monochromatic laser, as might be expected for a model with amplitude modulation.

We have earlier described a detailed mechanism of self-pumped phase conjugation in a barium titanate crystal [1]. The proposed mechanism was motivated and supported by observations of light paths in the crystal, and differs in some respects from better-known previous explanations of similar phenomena [2]. Our model attaches particular significance to a proposed resonance between two opposite reflecting corners and to the simultaneous existence of a system of cooperating transmission and reflection gratings. We show the operation of the hypothesized system of gratings, as simulated by computer analysis. The results show points of agreement and disagreement with the actual behavior of the self-pumping crystal.

The phase difference between two beams generated by stimulated Brillouin scattering of two partially overlapped laser beams is measured. The measured phase difference is compared with predictions of computer simulations. Both theory and experiment show that the phase difference is a random process whose probability distribution depends on pulse-width, phonon lifetime and the degree of the overlap between the pumping lasers.

This paper presents a design study for an ultra-high power () 1015 Watt) laser system based on multiple short pulse amplification in KrF lasers followed by coherent summation in a Raman amplifier. A gaseous Raman medium contained in a light guide is employed for pump beam averaging. Group velocity dispersion between pump and Stokes in such a system is shown to be capable of being used to advantage to obtain pulse compression in the picosecond region. The use of Raman media in the transient regime is shown to overcome problems due to second Stokes generation. A simple expression for the transient Raman threshold is developed and is shown to agree well with experiments. Thresholds for Raman generation in CH4, H2, NH3 and SF6 for 20ps pulses at 249nm are reported. In addition, preliminary results for the amplification of single KrF laser pulses of a few psec duration up to energies of ca 2.5 J are presented. These results demonstrate very effectively the unsuitability of KrF as a medium for single pulse amplification, and highlight the necessity to employ multiple-pulse extraction and beam combining techniques in order to efficiently extract energy from the amplifier and to decouple the target from amplified spontaneous emission (ASE).

Room temperature bulk GaAs and GaAs/A1GaAs multiple quantum well (MQW) samples are studied using backward degenerate four wave mixing (DFWM) at moderately high pump intensities (4 kW/cm2). The DFWM reflectivity is measured in the band gap region as a function of laser wavelength and pump intensity. When the MQW is pumped at intensities sufficient to saturate the excitonic resonance, a dispersive DFWM spectrum consistent with conduction band contributions is observed. Preliminary experiments in bulk GaAs indicate that the phase conjugate signal strength is the same as that for the MQW at approximately the same pump intensity.

Transient stimulated Raman amplification of a pulse in a pulse train can be considerably greater than that of an isolated pulse if the pulses are sufficiently close together. We describe the characteristics of gain buildup in a pulse train theoretically and report experimental observations of amplification increases of 4 orders of magnitude in 4 pulses. The effect of phase alternation of the Stokes seed on the gain buildup is also described.

We have evaluated two-beam coupling and diffraction efficiency in cerium doped BSKNN-3 during time intervals of 1 msec and 250 usec. Using a signal:pump ratio of 10-13 - 10-14, signal intensity gains of 1011 with diffraction efficiencies of 20-30% were achieved.

Results of an experimental investigation of phase conjugation by stimulated Brillouin scattering (SBS) at 1.053 μm using a focused beam geometry with very long pulse lengths (pulse length > 6700 phonon lifetimes) are reported. Scanning of the beam through the SBS medium is proposed as a means of suppressing the undesirable effects of processes which can compete with the SBS in the long-pulse regime. Data taken with and without beam scanning indicate that high-quality phase conjugation can be achieved by SBS using focused beams with arbitrarily long pump pulses.

In a Raman amplifier pumped by a multiline laser, it is well-known that dispersion in the Raman medium may decouple the Raman gains of the lines from each other. The small-signal gain of each line is then reduced by the fraction of the total pump power in each line. If these gains are very different from each other, then the overall conversion efficiency to the Stokes-shifted output may be limited by the difficulty in optimizing the amplifier cell length for all lines simultaneously. We describe here how the gains may be completely coupled together to solve these problems by adjusting the relative angles between the input beams so as to compensate for medium dispersion, and thereby eliminate the four-wave mixing phase mismatch. The theoretical basis for the effect, different geometries for achieving coupling, and computer simulation results for sample cases are presented.

Adaptive phase compensation through a Raman amplifier has been studied experimentally. Using a real-time wavefront sensor and a 69-actuator deformable mirror, the low-power Stokes seed to a Raman amplifier was phase compensated for simulated turbulence. After amplification, the beam was passed back through the simulated turbulence and its beam quality was determined. The results are consistent with determinations of amplifier fidelity and adaptive-optics performance.

We have measured the locking range and energy scaling behavior of a system of four pulsed dye oscillators coherently coupled so that they are functionally a single phase conjugate resonator. The locking range is on the order of 100's of wavelengths. Our best results show that one can obtain output energy approximately proportional to N, the number of dye gain cells. Since the N output beams are locked in phase, this implies that the peak far-field intensity would scale as N2. We also report two properties of the photorefractive phase conjugate mirror that are relevant to such a system: the fluence required for start-up and the field of view.

The nonlinear, time- dependent, one-dimensional fluid dynamic equations with the el ectrostrictive driving force arising from Stimulated Brill ioun Scattering (SBS) are solved by a finite difference method. The medium is assumed to be an ideal gas with a finite viscosity and thermal conductivity, and the electrostrictive driving force has the form of a time modulated sine wave propagating along the optic axis (+z direction) at the acoustic velocity with respect to the medium. The modulating envelope is assumed to be Gaussian, which defines the pulse shape. These calculations were carried out for pulses from a Raman shifted XeF laser (with an optical wavelength of .413 microns) incident upon a 10 amagat SF6 cell. The peak electrostrictive energy loading on the medium was taken as 944 Joules/liter, and the pulse widths were taken 50, 100, 500, and 1000 nanoseconds. The spatial density profile at maximum energy loading was plotted for each pulse width. Due to nonlinear terms in the fluid equations, these profiles have amplitudes considerably smaller than those predicted by the linear fluid theory. Also, deviations in the profile shape from that of an ordinary sine wave were noted, indicating the presence of higher acoustical harmonics. Of particular interest is the acoustical streaming predicted by these calculations. As the pulse interacts with the medium, the stream velocity increases from zero to a finite value, and remains at this value 1ong after the pulse decays. This final stream velocity increases with pulse width. Since acoustical resonances propagate at the acoustic velocity with respect to the medium, the scattered wave is Doppler down-shifted by the stream velocity as well as the acoustic velocity. Therefore, different portions of the scattered pulse receive different Doppler shifts.

Asymmetric and centrosymmetric scattering are investigated in barium titanate. The dependence of scattered intensity on beam diameter is experimentally derived and shown to be in agreement with the existing theory of stimulated photorefractive scattering (SPS). Observations on the spatial aspects of asymmetric self-defocusing within the crystal are also presented and compared with theory.

Near-resonance nonlinear dispersion is used to create a periodically modulated index of refraction in a collection of three level systems. This modulation causes the coherent combination of two incident probe beams into a single output beam.

Analytic and numerical solutions are presented of the equations describing the propagation of several pulses of different frequencies through a Raman medium. The transient case is treated, and the analysis includes Stokes, pump, and anti-Stokes frequency components. An analytic solution is found that describes the phenomenon of gain suppression in the case of perfect phase matching, Ak=0. For nonzero values of Ak, numerical solutions show that the transient gain coefficient follows a scaling rule analogous to the result known from the steady-state limit. Stokes amplification is suppressed for Ak=0 but reappears rapidly in the transient case as phase matching is lost.

Recently Hache, Ricard and Flytzanis have reported interesting observations and calculations on phase conjugation via degenerate four-wave mixing in gold colloids. The generation of phase conjugate radiation in these media arises from and reflects the creation of static index grating imposed on the electronic wave functions within the microparticies. These encouraging findings motivate us to consider the possibility of generating moving index gratings in these media with possible applications to coherent beam combination.

A theory of side beam replication in a crossing-beam geometry is reported. It is shown that side beam replication is not expected to occur when the Fresnel number of the aberrations (FNA) is large, while it is expected to occur when FNA is small, in accord with experiments. An analytic threshold is derived for the value of FNA at which side beam replication no longer occurs, and this threshold agrees well with the experiments. We propose a method for eliminating side beam replication at low values of FNA.

We present the results of an experimental and theoretical study of the gain experienced by a weak probe beam propagating through a sodium vapor in the presence of an intense pump field that is nearly resonant with the 3s→3p atomic transition. This interaction provides a means by which the outputs of two lasers can be combined coherently. The probe transmission spectrum is comprised of three distinct features, two of which result in amplification of the probe wave. The origin of these features can be traced to the modification of the atomic level structure by the ac Stark effect. The experimentally measured spectra are in good agreement with the predictions of a theoretical model based on the solution of the density-matrix equations of motion for a two-level atom and including the effects of Doppler broadening. The dependence of the induced gain on the pressure of a helium buffer gas has been measured and is in good agreement with the predictions of theory. The maximum gain observed in these experiments occurs at one of the Rabi sidebands and leads to a 38-fold increase in the intensity of the probe wave.

We examine the saturation and time response properties of an orientational Kerr medium in arbitrarily strong electromagnetic fields. For the static response, an expression for the field-induced birefringence as a function of pump intensity is obtained in closed form. The dynamic response is obtained from a nonperturbative solution to the Planck-Nernst equation for the orientation density. For comparison with experiment, we examine recent phase shift measurements of the static birefringence and time response properties of a graphite rod suspension as functions of millimeter-wave intensity and show excellent agreement with theory. The predicted response to an electromagnetic grating is also obtained. For this case, we find an optimum field strength for spatial modulation of the birefringence, beyond which bleaching of the orientation grating occurs. Potential applications to four-wave mixing and optical bistability at millimeter wavelengths are discussed.

We describe and demonstrate experimentally one-way transmission of optical images through single multimode fiber by degenerate four-wave mixing implementation of real time holography using a photorefractive BSO crystal

We show that, with favorable spontaneous emission damping rates, substantial steady state population inversions can be created in coherently pumped multilevel systems. We relate the conditions to achieve inversion to the phenomenon of effective Rabi cycling between separated levels in a ladder coupling scheme and to a detailed balance obtained with damping rates between levels mixed by the Rabi cycling. We consider the inversion process and the process of energy extraction into a cavity for a five level system with realistically chosen parameters. We show that the system behaves very much like a conventional laser, resulting in a device with output at frequency near the sum of the pump frequencies.

We have studied the phase pulling and locking of an injected Stokes pulse in a transient Raman amplifier both experimentally and theoretically. In addition, we have investigated the pulse energy statistics and related spatial modes of a high gain amplified Stokes pulse. Experiments reported here show that in the unsaturated case, the phase of the amplified Stokes is pulled almost instantaneously to that of the pump, as soon as integrated gain levels of slightly greater than unity are reached. For higher input seed-Stokes levels, such that the amplified Stokes energy is saturated, the phase locking is completed early, and the phase of the amplified Stokes then follows the phase excursion of the pump. The phase pulling has been modelled theoretically for the specific parameters of our experiment. We have also used the pulse energy statistics to study the seed-Stokes control of a high gain Raman amplifier. For an input seed-Stokes beam to control a transient, high gain Raman amplifier, operated near threshold, it must dominate the self-generated Raman which builds from quantum noise. For a near-unity-Fresnel-number Raman amplifier we have measured the number of input seed-Stokes photons needed to produce an unchanged statistical signature on the amplified Stokes pulse and have found that a surprisingly large number (109) were needed. For a larger Fresnel number Raman amplifier, a near-field camera was used to study the level at which the seed-Stokes pulse controlled the spatial output of the amplifier. Control of the spatial modes seems to have a sharp threshold with input Stokes energy and was fully established with an input of 2x103 photons per mode.

The work reported here demonstrates for the first time degenerate four wave mixing (DFWM) phase conjugation in cesium (Cs) atomic vapor. The strong, spectrally narrow transition of cesium at 852 nm gives conjugate reflectivities up to 154% with low pump beam powers. Temperature tuning of the DFWM response over a 30 GHz bandwidth is also demonstrated. An improved experimental design to determine the DFWM angular response shows a field-of-view of 20 mrad at the half response points. Self-focusing effects are observed to be similar to those in sodium vapor with an onset of increased conjugate divergence for reflectivities greater than about 25%.

Quantum phase fluctuations may be amplified into large, sudden phase shifts in the Stokes field, which subsequently give rise to solitons in the pump intensity. Ensemble simulations indicate that increased dissipation due to collisional dephasing within the medium significantly increases the frequency of soliton generation. The statistical distribution of soliton heights and positions is described.

A rectangular deformable mirror is described which meets the low spatial frequency and low bandwidth requirements associated with thermal aberrations in a solid state zig-zag amplifier. We incorporate design features which simplify the design of the mirror itself and reduce the complexity of the associated control system.

The predictions of atmospheric effects on millimeter wave radio imaging of launch vehicles can be obtained by theoretical extrapolation of measured data from propagation studies for millimeter wave satellite communication systems. Attenuation exceedence statistics versus percentage of time have been estimated for both relatively wet and dry temperate climates. Attenuation problems are minimal at 10 GHz, but increase with frequency. Escessive rain attenuation is expected at 30 GHz for a small fraction of time. Cloud attenuation becomes also significant at 100 GHz. Water vapor in clear air can often cause more than 10 dB attenuation at 300 GHz. Using the reciprocity theorem, the effects of angle-of-arrival fluctuations on beam spot resolution have been found to be negligible for a space-based telescope detecting earth-launched vehicles. Beam-coverage area averaging can reduce cloud-induced signal scintillations of several decibels by an order of magnitude. These statements are valid except at very low elevation angles approaching tangent to the earth, where the dominant problem is excessive signal attenuation.

The quality of many images are affected by unknown distortions, such as the imperfections caused by atmospheric turbulence. This paper explores a technique for estimating unknown wavefront distortions in a short period of time, possibly even during real time. A time-space algorithm has been implemented in software as a tool for estimating and canceling the unknown distortions. The algorithm uses moments of an observed point spread function and searches a database of pre-calculated moments to estimate the wavefront.

Clouds exist at all levels of the lower, middle and upper atmosphere. This paper presents results of new techniques to use software/hardware systems for the ultrafast detection and use of information on the space/time variation of water and non-water clouds. Examples are chosen from both lower and middle atmosphere situations. Our methods apply to both spaceborne and ground-based detection systems. Output from the new ultrafast methods provides input for both simulation and operation of interceptor systems.

Statistics of cloud characteristics over North. America have been calculated for the past two years. The frequency of cloud cover with the associated heights and infrared attenuation were charted using the CO2 channel radiometric data from the geostationary VISSR Atmospheric Sounder (VAS). Cloud top pressures were determined from the ratio of VAS CO2 channel radiances in a radiative transfer equation formulation. Cloud emissivities were then calculated from infrared window channel observations. CO techniques derived height and emissivity assignments have been found to be reliable in all cloud types, including thin cirrus clouds where other techniques have been inconsistent. Observations since 1985 revealed that 25% to 35% of the United States was covered with thin clouds (radiative attenuation was less than 95%), 45% was covered with thick opaque clouds, and 20% to 30% had clear sky conditions. Geographical distribution of cloud cover shows a latitudinal dependence mainly over the Pacific Ocean. Moderate seasonal and diurnal changes were also found.

Rockets containing dc probes were launched at Poker Flat, Alaska in two different campaigns in June 1983 and July 1986 to measure electron density irregularities with high spatial resolution. They were launched at times when the MST radar showed regions of intense backscatter in the mesosphere. Large changes and strong gradients in the electron density are observed in the region of most intense backscatter. The electron density profiles in general show different characteristics in the peak scattering region. Two profiles, one from each campaign, however, show a deep 'bite out' or depletion in electron density near 86 km. Spectra of the spatial density fluctuations are derived from the first campaign results. In the region of most intense backscatter, the power is up over the whole frequency range by almost 5 orders of magnitude. An intercomparison of the probe data is given, followed by a comparison between the measured radar echo power and calculated echo power based on the in situ rocket measurements.

We describe a lidar research and development program having several applications to the transmission of optical information and high power laser radiation through the atmosphere. Lidar capabilities for remote sensing of the atmosphere are reviewed. New lidar facilities and their initial operations are described, including a transportable lidar system being constructed for tropospheric and middle atmosphere measurements of constituents that can influence long range optical detection and laser transmission.

Cirrus cloud observations obtained with the University of Wisconsin High Spectral Resolution Lidar and High Performance Nd:Yag lidar are presented. These include accurate determination of the optical depths, backscatter phase functions, three-dimensional spatial structure and internal wind fields.

Investigation of several atmospheric ice crystal processes, occurring during rapid vapor diffusional growth of ice crystals, indicates that the rates of these processes are not well-characterized by theory. These processes include ice crystal aggregation, scavenging of aerosol particles by ice crystals, secondary ice formation, and the early development of cloud electrification. Additionally, ice crystal morphology (shape and form) and symmetry are characteristics arising from ice crystal diffusional growth which are not fully understood on the basis of existing theory. To explain these observations, we postulate the existence of electric multipoles in growing ice crystals, arising from a differential incorporation of contaminant ions into the ice crystal lattice. This single postulate, based on well-founded physical and chemical principles, provides a basis for understanding a variety of phenomena associated with the ice phase in atmospheric clouds.

Results of numerical calculations of internal wave breaking and turbulence production in the mesosphere are used to compute parameters describing the propagation of a laser beam from the ground upward. Peak values of C2n of about 7 x 10-18m-2/3 at a height near 40 km. are computed. The impact of the wave breaking is greatest on parameters described by integrals with height-weighted integrands; the isoplanatic angle is reduced by 75% below the value it has before breaking commences. On the other hand, the variance of the log relative amplitude is increased by 5%, and the transverse coherence length is decreased by a like amount.

This study concerns energetic dense electron beam propagation in nuclear disturbed environments. In particular, it centers on beam losses through dissipation processes. Relevant atmospheric conditions known from high-altitude nuclear explosions are summarized. Modeling of geomagnetic data as well as some new theoretical ideas for studying beam propagation are briefly described.

Parametric instabilities of laser radiation in a structured space plasma environment have been surveyed. We compute approximate thresholds and growth rates for stimulated Raman scattering, stimulated Brillouin scattering, stimulated Compton scattering, and modulation/filamentation processes.

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 briefly describe, in general terms, the HANE environment, and the various plasma instabilities which could generate small-scale 10's m) electron density irregularities. As an example, we present an analysis of a single instability, the lower-hybrid-drift instability, which is likely to be excited in the ionosphere following a high altitude burst. 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.

A wave optics computer code developed for studying high energy laser propagation in the atmosphere for tactical and, more recently, for strategic applications has been converted to include the physics of the interaction of the light beam with the special environment anticipated to be encountered in a strategic scenario. Results showing beam quality after passage through plumes of various types will be presented along with a brief discussion of the mechanism for calculation and the underlying physics and assumptions used.

The presence and effect of ionospheric structure on the propagation of electromagnetic beams are discussed in this paper. The paper addresses the upper ionospheric (> 100 km) structure that a beam might encounter. The emphasis is on development of models for the structure rather than the propagation of the beam.

We have built what we believe is the first instrument which acoustically levitates liquid drops surrounded by a continuously flowing gas stream. Drops 0.2 to 2.0mm in diameter have been levitated for periods of time up to hours in the working section of a horizontal wind tunnel. Air speeds of 25-300 cm/sec, corresponding to Reynolds numbers from 3 to 250, do not blow the drop out of the trap. Evaporation data from neat liquids indicate that the sound field has negligible effect on drop evaporation. We have observed phase separation in the evaporation of two partially miscible liquids. We have also seen evaporation stopped by the formation of a surface monolayer. Laser fluorescence from room temperature solutions is being investigated as a means of monitoring the transport of trace species between the gas and liquid phases.