G. R. Ochs

Saturation of optical scintillation by strong turbulence

The diffraction theory of optical scintillations has so far failed to describe the propagation of light over paths where the integrated amount of refractive-index turbulence is sufficient to cause saturation of the scintillations. We present a simple, physically based elaboration of the first-order perturbation theory and compare it with observations. Our theory reproduces in detail the observed saturation curve and the observed spatial covariance of the scintillations. In particular, we show why the fine-scale structure of scintillations persists deep into the saturation regime.

Use of scintillations to measure average wind across a light beam.

We report the successful construction and testing of an optical wind sensor that uses the motion of the scintillation pattern to measure the transverse component of wind blowing across a laser beam. As is done for measuring ionospheric and interplanetary winds, we use a correlation method. However, in our application, the slope at zero lag of the time-lagged correlogram proves to be more useful than the more commonly used delay to the peak. The reason is that, in the atmosphere, irregularities are distributed along the entire propagation path. We use a detector spacing of 0.33 of the diameter of the first Fresnel zone to obtain a nearly uniform weighting function along the path, though the center of the path is still more effective than the ends. The sensor has been used extensively over 1-km and 15-km paths, and field tests of various applications are planned.

Measurements of Atmospheric Turbulence Relevant to Optical Propagation

The aspect of atmospheric turbulence of interest to optical-propagation studies is the variation of refractive index. We demonstrate the application of high-speed temperature sensors to the direct measurement of this variation at optically important scale sizes, as small as a few millimeters. The thermometers, used in pairs with spacings ranging from 3 mm to 1 m, disclose that the turbulence near the ground frequently differs substantially from the Kolmogoroff model, and that the temperature difference does not follow the gaussian probability-distribution function. A model of the turbulent atmosphere containing sharply bounded regions with stronger than average turbulence agrees well with our observations. We also demonstrate the use of a single sensor mounted on an airplane to observe refractive-index variations at heights up to 3 km.

Measuring surface-layer fluxes of heat and momentum using optical scintillation

An experiment is described showing that an optical scintillation instrument gives reliable values of heat and momentum fluxes in the surface layer, subject to the usual restrictions of homogeneity and steady state. This instrument measures the turbulence inner scale and refractive-index structure parameter, which are used to obtain the fluxes from Monin-Obukhov similarity relationships. The instrument gives space-averaged values over a propagation path that can range in length from tens to hundreds of meters. The history of the use of optical propagation to estimate fluxes is reviewed.

Wind measurements by the temporal cross-correlation of the optical scintillations

Various methods of correlation analysis that have been used to deduce crosswind from a drifting scintillation pattern are briefly described and then compared with regard to their immunity to noise and their accuracy when faced with nonuniformities along the propagation path or changes in the characteristics of the turbulence. Of the techniques considered, none is ideal; but a new technique, using complete knowledge of the cross-covariance function, proves to be advantageous in a wide variety of situations.

Refractive-turbulence profiles measured by one-dimensional spatial filtering of scintillations

Stellar scintillations, when appropriately analyzed, yield information about the turbulence throughout the atmosphere. We describe an instrument involving a 36-cm telescope and an on-line minicomputer that provides, after 20 min of observation, the refractive-turbulence profile of the atmosphere. The height resolution is sufficient to divide the atmosphere into about four independent regions. The principal limitation to greater accuracy and resolution is the nonstationary behavior of the atmosphere during the 20-min observing period.

Optical-scintillation method of measuring turbulence inner scale

An optical technique is described that uses coherent and incoherent optical scintillation to measure the path-averaged value of the turbulence inner scale. The technique is verified by comparison with an in situ measurement, and inner scale values obtained 1.5 m above the ground over a 24-h period are shown.

Fine calibration of large-aperture optical scintillometers and an optical estimate of inner scale of turbulence

Large-aperture optical scintillometers [Ting-i Wang et al., J. Opt. Soc. Am. 68, 334 (1978)] lose their calibration if they are sensitive to portions of the spatial spectrum of temperature fluctuations where (K)(-11/3) fails to hold. The model temperature spectrum having the bump [R. J. Hill, J. Fluid Mech. 88, 541 (1978); R. J. Hill and S. F. Clifford, J. Opt. Soc. Am. 68, 892 (1978)] is used to find conditions under which the scintillometers maintain their calibration. We find that the aperture size D should be at least twenty times the inner scale l(0) if the contribution of the spectral bump is to be ignored. For application in the surface layer, one needs the height above ground of the optical path to be much greater than three times the aperture size if outer-scale effects are to be negligible. It is shown that the inner scale dependence of a scintillometer having D/l(0) approximately 2.0 and the lack of such dependence for a scintillometer having D/l(0) approximately 20.0 can be used to estimate both l(0) and C(2)(n) if the two systems are used simultaneously on the same path. A preliminary experiment was performed in the atmospheric surface layer with scintillometers having aperture sizes of 2.0 cm, 5.0 cm, and 15.0 cm; the results are consistent with the existence of the spectral bump. The inner scale is estimated by comparing data from the 2.0-cm and 15.0-cm systems. Using this inner scale, the C(2)(n) values from the 5.0-cm and 15.0-cm scintillometers are corrected for the spectral bump; the corrected values are in agreement. Other turbulence parameters are not deduced from the l(0) and C(2)(n) estimates because the l(0) values have been found to be insufficiently accurate.

Phase Variations in Atmospheric Optical Propagation

Temperature structure in the atmosphere, transported by the wind across a laser beam, produces time variations in the optical path length. Using a He-Ne laser (0.6328 μm) on a 70-m propagation path, we measured the optical phase variations at four different spacings, ρ≤30 cm. Simultaneously, a midpath measurement of wind velocity and temperature structure parameter, CT2, provided the necessary meteorological measurements to compare the observed phase structure function with Tatarski’s theoretical curve. We obtained excellent agreement between theory and experiment. Direct measurements of the outer scale of turbulence, taken continuously over a 24-h period at a height of 1.6 m, indicated an average outer scale of 1.3 m with diurnal variations of ±20%. The frequency spectrum of the received phase difference at each of the four spacings is plotted and its implications for the data-sampling rate are examined. The curves obtained exhibit excellent agreement with the predicted spherical-wave phase-difference frequency spectrum.

Optical wind sensing by observing the scintillations of a random scene

We demonstrate the feasibility of using a naturally illuminated scene, such as a hillside or forest, as a passive optical source to measure the path-averaged crosswind between the scene and the observer. The resultant path weighting function for the crosswind cannot be varied arbitrarily, but we can obtain a useful range of weighting functions by adjusting the geometry of the receiver.