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A recent measurement campaign at Vandenberg Air Force Base, Calif. involved taking simultaneous observations with a VHF radar and high-data-rate (1-micron diameter) platinum wires to sense optical turbulence (from temperature fluctuations). The radar observations produce profiles of the refractive index structure parameter (C 2 n ), the turbulent kinetic energy (σ 2 t ), the eddy dissipation rate (e), the inner scale (l o ), the outer scale (L o ) of turbulence, and wind speed and direction to an altitude of 20 km AGL. The fine wire measurements were taken from the surface with several sensors mounted on a balloon-ring platform sampling in excess of 3 kHz to balloon burst altitudes (typically above 25 km AGL). The main objectives of this effort are to compare the two measurement techniques and to obtain observations that can address several fundamental turbulence issues of the real turbulent atmosphere related to laser beam propagation. To date, modeling and simulation of laser beam propagation through atmospheric turbulence have relied upon a traditional theoretical basis that assumes the existence of homogeneous, isotropic, stationary, and Kolmogorov turbulence. Results presented from the radar observations include C 2 n , σ 2 t , e, l o ,and the standard deviation of vertical velocity (σ w ). A comparison of the profiles of C 2 n obtained from the two measurement techniques is shown and discussed. A time series of temperature data obtained from a fine wire probe traversing one radar range gate is presented and discussed. Future measurement and analysis efforts are presented.

Methodology is presented using observations from a radar and new measurement system to address several fundamental turbulence issues related to laser beam propagation that impact high energy laser (HEL) and laser communication systems. The successful design and operation of these laser systems require high-fidelity realistic laser beam propagation models coupled with a thorough and comprehensive knowledge of the real turbulent atmosphere. To date, modeling and simulation of laser beam propagation through atmospheric turbulence have relied upon a traditional theoretical basis that assumes the existence of homogeneous, isotropic, stationary, and Kolmogorov turbulence. The approach and methodology is discussed to assess the impact of real atmospheric turbulence on laser beam propagation. Analysis will include effects of non-classical turbulence as well as inner (l o ) and outer scale (L o ) effects. Data will be obtained from a new measurement platform using a free-flying balloon that lifts a ring with a boom upon which are mounted fine wire (1μm diameter) sensors to measure high-speed temperature and velocity fluctuations from which the turbulent quantities can be calculated including the refractive index structure parameter (C 2 n ) and the eddy dissipation rate (e). The “ring” is actually 8-sided with a diameter of 30 feet and trails the balloon with several risers. This design eliminates contamination of the balloon wake that plagues conventional systems.

Understanding the turbulence along a propagation path is required to evaluate new methods for tracking, pointing, and compensation of laser beams, studying image degradation, and interpreting remote sensing observations. This paper presents observations using a new balloon-ring platform equipped with multiple fine wire probes (1 μm diameter) at various separations for sensing both temperature and velocity fluctuations. These measurements provide profiles of the temperature structure parameter ( C T 2 ), refractive index structure parameter ( C n 2 ), eddy dissipation rate (e), inner scale ( l o ), and outer scale ( L o ) values. Of particular interest is the path variability of C n 2 , e, l o , and L o , and the relationship of these parameters to atmospheric stability. Since all “raw” data is archived, spectra are shown and discussed as to how often the atmosphere truly follows the “Kolmogorov-5/3,” or under what atmospheric conditions for which the structure function is represented by the r 2/3 law. Also the concerns of local isotropy and intermittency can be addressed. The interpretation of these results to propagation effects is discussed. Salient features of the new measuring system are presented as well as the rationale for its implementation, including the longstanding concern of wake contamination on conventional measuring systems tethered under a single balloon.

This paper present new methodology to address critical refractivity turbulence issues for laser propagation using a new measurement system-a portable balloon-ring platform with multiple fine wire sensors at several separations. All raw data is transmitted to a ground station-allowing spectra to be calculated. The new platform is discussed and preliminary examples of observations, including artifacts, are shown and discussed. This new platform provides capabilities during the daytime as well as nighttime-unlike conventional thermosondes that are used only at night. Such all time observations are important due to the pronounced diurnal variation in the planetary boundary layer where many laser systems are operated. Plans to address the longstanding concern of wake contamination on systems suspended below a balloon quantitatively will be presented. The objective of this effort is to develop the capability that can address several questions related to laser propagation such as: 1) Is the atmospheric isotropic for the scales of interest? 2) Is the turbulence Kolmogorov under various atmospheric conditions, or how often is the structure function represendted by the r2/3 law? 3) what are the profiles of inner and outer scale? 4) To what degree does wake contamination affect conventional thermosonde measurements? 5) Does fine structure within the scattering volume sensed by radar affect refractive index structure parameter (Cn2) and eddy dissipation rate (epsilon) estimates? These questions and concerns will be addressed by making the appropriate observations using the balloon-ring platform. Many of the measurements will be taken at Vandenberg AFB since the Western Test Range operates a ground receiving station, balloon launch facility, VHF radar, boundary layer radars, sodars, and instrumented towers that will enhance this effort. This effort provides an observation platform that will ultimately lead to the development and validation of conceptual/statistical/physical models.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We discuss aspects of the development of the Airborne Laser Laboratory. Our discussion is historical in nature and consists of the text from a speech given on the occasion of the Airborne Laser Laboratory leaving Kirtland Air Force Base (AFB) to fly to Wright-Patterson AFB to become an exhibit at the National Museum of the United States Air Force. The last part of the discussion concerns the inception of the study of aero-optics as an area of research and some of the milestones in the understanding of the causes and prediction of aero-optical effects.

A joint radar/aircraft optical turbulence measurement is discussed. The joint measurement was made over White Sands, New Mexico with a KC135E aircraft equipped with fiber film temperature probes and a ground-based atmospheric profiler. General meteorological conditions were such that the turbulence on the day of the experiment was fairly weak. Emphasis was placed on the 36,000 ft to 42,000 ft altitude regime. Results showed good agreement between the atmospheric profiler and the aircraft-based measurements.

We document the concept and the design method for dome lenses that have a compressive pressure applied to the periphery. Dome (meniscus) lenses can eliminate both birefringence and tensile stresses if used in evacuated spatial filters where a pressure difference is present across the lenses. Calculations show that spatial filter birefringence is an insignificant 3 nm within the portion of a 740-mm Nova spatial filter dome lens illuminated by the laser beam. Membrane stresses are everywhere compressive, varying from -1.2 to -2.4 MPa, and are nearly uniform over the illuminated portion of the lens. Eliminating sources of birefringence on the Nova laser could increase the energy on target with Type 11/Type II frequency conversion from '-50% to the '70% level attainable with Type I/Type II frequency conversion. Eliminating tensile stresses may reduce the damage and catastrophic failure that has occurred in some Nova spatial filter input lenses. The major disadvantage of using dome lenses on Nova is the cost of new lenses which could range up to $50,000 per lens for the material and $20,000 per lens for finishing.

The propagation of intense 3rd harmonic light (0351 μm) through large optical components of the Nova laser results in fracture damage of the center of the component. This damage is caused by an intense acoustical wave brought to focus in the center by reflecting off the circular edge of the optic. The source of this wave is light generated by transverse stimulated Brillouin scattering (SBS). By taking into account the transient gain characteristics of the SBS, we can correctly predict the pulse energy that would cause damage for any time variation in intensity in the pump beam, and predict the relative intensity of the Brillouin light. Our model is based on the transient behavior of a first order linear system.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-powered glass-laser systems with multiple beams, frequency-conversion capabilities, and pulseshaping flexibility have made numerous contributions to the understanding of inertial confinement fusion and related laser-plasma interactions. The Nova laser at Lawrence Livermore National Laboratory is the largest such laser facility. We have made improvements to the Nova amplifier system that permit increased power and energy output. We summarize the nonlinear effects that now limit Nova's performance and discuss power and energy produced at 1.05-, 0.53-, and 0.35-microm wavelengths, including the results with pulses temporally shaped to improve inertial confinement fusion target performance.

Over the past year, the Nova laser at the Lawrence Livermore National Laboratory has been undergoing major refurbishment. Concurrently, an extensive research program has been undertaken to characterize and understand, in detail, the characteristics of Nova and the factors which limit its performance. As a result of these combined efforts, Nova now exceeds its original performance goals. Among the topics discussed are the substitution of platinum free laser glass in the power amplifier section of the Nova beamlines; increased frequency conversion efficiency, and large optic damage resulting from transverse stimulated Brillouin scattering. When we have implemented the necessary changes on all 10 Nova beamlines, we will soon be able to routinely deliver 120 kilojoules of energy at 1.053 μm, and 75 kilojoules at 0.351 μm.

Using 2.4-nsec pulses at 350 nm, we have observed transverse stimulated Brillouin scattering (SBS) in fused-silica optical components. Transverse SBS sets in when the product of laser fluence and growth time for the scattered optical wave exceeds ∼2.3 J nsec/cm2. An increase in laser bandwidth to 8.3 GHz suppresses SBS losses up to approximately twice the SBS threshold. We review the theory of transverse, broadband SBS and its scaling with experimental parameters.