Wind and refractive-turbulence sensing using crossed laser beams

Atmospheric turbulence strength distribution along a propagation path probed by longitudinally structured optical beams

Atmospheric turbulence can cause critical problems in many applications. To effectively avoid or mitigate turbulence, knowledge of turbulence strength at various distances could be of immense value. Due to light-matter interaction, optical beams can probe longitudinal turbulence changes. Unfortunately, previous approaches tended to be limited to relatively short distances or large transceivers. Here, we explore turbulence probing utilizing multiple sequentially transmitted longitudinally structured beams. Each beam is composed of Bessel-Gaussian ([Formula: see text]) modes with different [Formula: see text] values such that a distance-varying beam width is produced, which results in a distance- and turbulence-dependent modal coupling to [Formula: see text] orders. Our simulation shows that this approach has relatively uniform and low errors (<0.3 dB) over a 10-km path with up to 30-dB turbulence-structure-constant variation. We experimentally demonstrate this approach for two emulated turbulence regions (~15-dB variation) with <0.8-dB errors. Compared to previous techniques, our approach can potentially probe longer distances or require smaller transceivers.

First retrieval of vertical profiles of turbulence characteristics and horizontal wind velocity from solar transmission measurements at 212 and 405 GHz

We report on the investigation and successful application of vertical profiling of the structure parameter C2m and of the outer scale L0 of absorption fluctuations and of the horizontal wind velocity (vector) during daytime by the analysis of solar transmission measurements. The method is relatively simple and straightforward so that the presented (or a similar) technique could be used in the routine remote sensing of daytime C2m, L0, and wind profiles. It requires multiple beams pointing in different directions at the Sun. The retrieved profiles are consistent with the current knowledge of atmospheric physics. Simultaneous in situ wind velocity measurements agree with the retrieved wind velocity in the lowest 100 m above ground within the measurement uncertainties of less than +/-2 m/s. The derived values of C2m at 200 m above ground are in good agreement (within a factor of 1.5) with the findings of an earlier investigation at the same test site. Finally, it is shown that irradiance fluctuations of millimeter and submillimeter waves are dominantly affected by humidity fluctuations, even at a dry and elevated site.

Remote sensing of atmospheric turbulence and transverse winds from wave-front slope measurements from crossed optical paths

In the theory of atmospheric turbulence, the strength of the spatial variations of the index of refraction n is proportional to a parameter known as the atmospheric-structure constant. The atmosphericstructure constant is denoted C(2)(n)(z) and is a function of position along the optical path z. The characteristics of the temporal variations of the index of refraction are related to both C(2)(n)(z) and to the transverse wind velocity V(z). Current optical techniques for remotely sensing C(2)(n)(z) and V(z) rely primarily on the spatial or temporal cross-correlation properties of the intensity of the optical field. In the remote-sensing technique proposed here, we exploit the correlation properties of the wave-front slope measured from two point sources to obtain profiles of C(2)(n)(z) and V(z). The two sources are arranged to give crossed optical paths. The geometry of the crossed paths and the characteristics of the wave-front slope sensor determine the achievable resolution. The signal-to-noise ratio calculationsindicate the need for multiple measurements to obtain useful estimates of the desired quantities.

Sensing refractive-turbulence profiles (C(n)(2)) using wave front phase measurements from multiple reference sources

A new technique for sensing refractive-turbulence profiles is described. The technique is based on performing a spatial correlation of the measured wave front phase from two reference sources. This technique is unique in that the correlation properties of the wave front phase are used instead of the more common approach of using optical scintillations. The geometry between the reference sources and the two wave front sensor apertures is arranged such that the two optical paths cross at some point in front of the sensor plane. The wave front phase for each reference source is reconstructed from the measured wave front sensor data. A spatial correlation of the two reconstructed phase maps is performed. From this correlation we are able to extract a measure of the structure constant of the refractive-index fluctuations C(n)(2). The resolution of the technique depends on the angle between the optical paths, the spatial frequency response of the wave front sensors, as well as the size of the wave front sensorapertures. For sensing the vertical profile of C(n)(2), we can obtain resolution of the order of 100 m by using sources separated by 1.15 degrees .

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.

Effect of refractive dispersion on the trichromatic correlation of irradiances for atmospheric scintillation

Formulations of the trichromatic correlation of scintillating irradiance are corrected by introducing the cospectrum of the two different refractive indices in place of the refractive index power spectrum. Receiver aperture averaging is included in the formulation. The effect of dispersion on the bichromatic correlation coefficient is studied and quantified for four specific experimental cases. Dispersion of the water vapor refractive index between wavelengths in the visible and 10 microm window is of particular significance in this study. In most cases, the effect of dispersion on the bichromatic correlation coefficient is much less than on the ratio of monochromatic irradiance correlations, and is negligible for practical considerations. The exception is the case of such strong humidity fluctuations that they (as opposed to temperature fluctuations) cause most of the scintillation of the midinfrared radiation.

Remote sensing of atmospheric winds using speckleturbulence interaction, a CO(2) laser, and optical heterodyne detection

Speckle-turbulence interaction can be utilized to measure the vector wind in a plane perpendicular to the line of sight from a laser transmitter to a target. A continuous wave source of around 1 W and operating at 10.6 microm, in conjunction with an optical heterodyne receiver, has been used to measure atmospheric winds along horizontal paths. A theoretical basis, the experimental apparatus, processing techniques, and experimental results are presented. The technique has been demonstrated for remote sensing of atmospheric winds along horizontal paths but also has potential for global remote sensing of atmospheric winds and for onboard wind shear detection systems for aircraft. The results show that rms accuracies of the order of 0.5 m/s are possible with averaging times as short as 2 s.

Localized measurements of refractive turbulence using spatial filtering of scintillations

Remote measurements of refractive turbulence strength with high spatial resolution are demonstrated. The technique uses spatial and temporal filtering of scintillation from a spatially filtered incoherent optical source. Spatial resolution as fine as 4.5 m was observed at the center of a 110-m propagation path. An analytic approximation to the theory agrees very well with the data. This theory predicts the spatial resolution of a system of this type to be in the vicinity of the path length divided by the total number of cycles in the transmitter and receiver spatial filters.

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.

Finite aperture optical scintillometer for profiling wind and C(2)(n)

A new optical technique is described for measuring the path profiles of crosswind and of a refractive-index structure parameter C(2)(n) along a line-of-sight path. Different sizes of transmitters and receivers are used to control the path-weighting function so that it will peak at different path locations. Various linear combinations of these measurements yield the path profile of crosswind and C(2)(n). A prototype instrument has been built and tested. Experimental results show good agreement with the theoretical predictions.

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 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.