Prospects for laser remote sensing of refractive turbulence

There is currently no active, single-ended optical technique for remotely sensing the parameters of atmospheric refractive turbulence, such as the structure characteristic C_n² and the inner scale l₀. Researchers in basic atmospheric physics currently use radar and acoustic sounders to measure turbulence, but both of these techniques are sensitive to water vapor fluctuations as well as temperature fluctuations. An optical technique would be sensitive to refractive index fluctuations, which are almost completely due to temperature fluctuations in the optical spectral region. An optical remote sensor for C_n² could also be used for horizontal, path-averaged measurements, to infer fluxes of heat and momentum. Scintilometers are currently used for such measurements, but they have the disadvantage of a fixed optical path, and they require long averaging times. A single-ended sensor could be pointed in any direction, so it could be used over the sea surface, and fast spatial averaging could be accomplished by scanning in azimuth angle. Any optical sensor for turbulence must make use of some atmospheric optical phenomenon caused by turbulence. Three different lidar-type techniques have been recently proposed by the present authors. These techniques are based on the following phenomena: enhanced backscattering, residual turbulent scintillation, and image distortion. Each of these approaches is reviewed in terms of its advantages and disadvantages for various applications, and some considerations for practical systems are also discussed