Dept. of Electrical Engineering
Oregon Graduate Institute of Science & Technology
Laser generated speckle and its interaction with turbulence have been studied for more than twenty years and have been utilized in atmospheric wind sensing. When a diffuse target or randomly distributed aerosols generated speckle pattern propagates back to the receiver, a scintillation pattern is created due to the speckle turbulence interaction. If a cross wind is present, the scintillation pattern will move with time across the field-of view of the receiver. As a result, the average cross wind velocity can be obtained using the time-delayed statistics of the speckle field at the receiver. Previously developed statistics of the polychromatic laser speckle field were adopted. Some development and simplifications were made on the autocovariance and Time-Delayed covariance(TDC) so that numerical analysis can be implemented for a finite receiving area. Numerical calculations were performed on the SUN work station using the Gauss Quadrature approximation. The aperture averaging effect which was simulated, corresponded to that actually observed from the experimental results. Pseudo-random code modulation made it possible to run the transmitter CW and still have range resolving capability. Semiconductor lasers have the advantage that modulation can be applied to the laser directly as a driving current. Small and reliable laser diodes and optical fibers made it possible to build a reliable and compact lidar system with potential practical applications such as wind shear detection for airplane safety. A coaxial lidar was built with an 8-inch receiving telescope. An AlGaAs laser diode with pigtailed5um single mode fiber was used as a transmitter. A single mode, multi-frequency laser working at 830 nm with a CW power of 50 mW was transmitted for better aerosol back scattering. Highly sensitive photomultiplier tubes were used as detectors and an integration detection scheme was employed. Different receiver systems were also designed and tested for the purpose of increasing the TDC and detectability. Experiments were conducted over a variety of atmospheric conditions and receiver configurations. The aperture averaging effect was observed from the experimental results and comparisons were made with the numerical predictions. Large amounts of data were taken to produce cross wind speed estimates using the Peak Shift method. Time series of cross wind speed measurements were also produced and compared with the results of other equipment. The results of a new receiver and processing scheme are presented. Suggestions for improvements and future work are also discussed.
Chen, Feng, "Range-resolved cross-wind remote sensing using a polychromatic AlGaAs semiconductor laser" (1996). Scholar Archive. 155.