Donald L. Walters

Measurements of r 0 and θ 0 : two decades and 18 sites

We present a statistical analysis of r0 and θ0 measurements collected over the last 2 decades at 18 different sites. Although the site altitudes varied from sea level to 3 km, the major distinguishing feature between average r0 values was the presence or absence of a turbulent, atmospheric boundary layer above the surface. For locations without a strong boundary-layer inversion, the mean coherence length, r0, was 93 ± 6 mm and the mean isoplanatic angle, θ0, was 9.4 ± 0.5 µrad. Where a boundary-layer inversion was present, the corresponding values for r0 and θ0 were 53 ± 2 mm and 6.5 ± 0.5 µrad, respectively.

Atmospheric inner-scale effects on normalized irradiance variance

We have investigated five types of atmospheric optical-turbulence inner scales for their effects on normalized laser irradiance variance in the Rytov and early saturation regimes: (1) zero inner scale, (2) Gaussian inner scale, (3) Hills viscous-convective enhancement inner scale, (4) Frehlichs parameterization of the viscous-convective enhancement, and (5) turbulence spectrum truncation because of the discrete grid representation. Wave-optics computer simulations yielded normalized irradiance variances within 2% of the results from numerical integrations of the Rytov-Tatarskii predictions. In the Rytov regime a Gaussian inner scale reduces the normalized irradiance variance compared with the zero-innerscale case, and the viscous-convective inner scale first raises, then lowers the irradiance variance as the inner-scale size increases. In the saturation regime all inner-scale models increase the intensity variance for a spherical wave.

Measurements of optical turbulence with higher-order structure functions

Higher-order structure functions have been used to extract atmospheric optical C(n)(2) profiles from a vertical sequence of temperature data collected by a single probe carried by a meteorological balloon. This technique circumvents trends and fluctuations in the atmospheric mean temperature and simplifies the equipment and complexity of measurement collection compared with traditional, horizontal differential-probe pair systems.

Instrument comparison: corrected stellar scintillometer versus isoplanometer

The scintillation pattern from a single star can be utilized to provide information on the refractive turbulence along the line of sight. Instruments that provide refractive turbulence parameters are the isoplanometer and the stellar scintillometer. Attention is drawn to the fact that the National Oceanic and Atmospheric Administration theoretical treatment and implementation of the stellar scintillometer is incomplete. The theory is corrected for spectral effects and finite aperture. A comparison is made of simultaneously obtained isoplanometer values and stellar scintillometer-derived values for isoplanatic angle. The measurements are obtained from an electro-optical/meteorological experiment conducted at Pennsylvania State University in April and May 1986. An atmospheric drop-off model is used to extrapolate the scintillometer measurements beyond the heights probed. Agreement between the two instruments is significantly improved after the appropriate corrections are applied to the scintillometer data. These data were obtained during widely varying meteorological conditions that provided the opportunity for comparisons over a wide range of isoplanatic angles (3 to 14 µrad). Over the 5 days that data were obtained, relative percent departures of mean isoplanatic angles derived from the corrected stellar scintillometer are within 10% of the mean isoplanometer isoplanatic angle values. The uncorrected departures range from 16% to 24%.

Infrared Spatial Interferometer

The Infrared Spatial Interferometer (ISI) is an interferometer installed on Mt. Wilson and operating in the 10 μm wavelength region, using heterodyne detection and two movable 1.65 m telescopes. Its general technology and characteristics, recent changes, and observational results are broadly discussed. Some compensation for atmospheric path length fluctuations is demonstrated. Stellar observations show, among other characteristics, that many stars emit gas and dust episodically with times of 10-100 years between events, and that stellar diameters measured in the mid-infrared region are about 10 percent larger than those measured with interferometry using visible light.

Refractive turbulence profiling using an orbiting light source

The possibility of obtaining vertical profiles of refractive turbulence C(2)(n)using an orbiting monochromatic light source is examined. The method employs spatial and temporal filtering of the observed scintillation pattern arising from density fluctuations in the atmosphere to measure C(2)(n). The impact of atmospheric motion on the method is discussed along with ways to mitigate its effect. Single and array receiver configurations are examined and the multiple response problem inherent in array configurations is corrected by tuning the individual array elements to the array response. The method is expected to be significantly better than the existing stellar scintillometer method.

Saturation And The Zenith Angle Dependence Of Atmospheric Isoplanatic Angle Measurements

A 10-11 cm aperture stellar scintillometer can measure atmospheric isoplanatic angles reliably provided the normalized variance and the zenith angle of the data are sufficiently, restricted. Within these constraints, the normalized variance data has a zenith angle power law that is quite close to the theoretical prediction. This is a self-consistent indication that the instrument is actually measuring the isoplanatic angle.

Huygens-Fresnel wave-optics simulation of atmospheric optical turbulence and reflective speckle in CO2 differential absorption lidar (DIAL)

The measurement sensitivity of CO2 differential absorption LIDAR (DIAL) can be affected by a number of different processes. Two of these processes are atmospheric optical turbulence and reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. The effects of this phenomenon include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of CO2 DIAL. We have previously developed a Huygens-Fresnel wave optics propagation code to separately simulate the effects of these two processes. However, in real DIAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. In this work, we briefly review a description of our model including the limitations along with a brief summary of previous simulations of individual effects. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements and show good agreement. In addition, simulation studies have been performed to demonstrate the utility and limitations of our model. Examples presented include assessing the effects for different array sizes on model limitations and effects of varying propagation step sizes on intensity enhancements and intensity probability distributions in the receiver plane.

Optimal sites for optical interferometry

It is pointed out that the performance of speckle imaging or optical interferometer systems increases with (r sub 0/D) exp n, where r sub 0 is the atmospheric coherence length, D is the aperture size, and n is between 2 and 4. It has been determined that, since r sub 0 is about 10 cm at visible wavelengths and D may be several meters, selecting a site with a large r sub 0 becomes critical for 30-100-m baseline systems. A unique problem for such optical systems is the need for a relatively large, flat, approximately 100-m site; however, this is inconsistent with the atmospheric dynamics that produce optical sites. Albuquerque and Chilao Flats results indicate that katabatic flows produce r sub 0 values of 30-50 mm; on the other hand, large mountain tops tend to have large 50-200 m inner layers, making r sub 0 extremely sensitive to the surface heat flux and wind speed. It is concluded that few locations can achieve this; those along the California Pacific Coast and Mauna Kea are two such regions.

Characterization Of Asymmetric Self-Defocusing And Centrosymmetric Scattering In Barium Titanate

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.