Direct-detection Doppler lidar employing a CCD detector: Design and early measurements (Fabry-Perot etalon)

A high efficiency lidar system has been developed at the University of Michigan which employs charge coupled device (CCD) detection. This lidar is a direct-detection Doppler lidar, which measures winds in the atmosphere by observing backscattered light from a high-powered laser. A Fabry-Perot interferometer with a 30-cm plate spacing is used to look at the high spectral resolution return from aerosol particles in the atmosphere. A comparison of light from the unshifted laser with this return light reveals the Doppler shift caused by the motions of these particles, and hence the velocity of the wind carrying these particles.; The ability to detect light is dependent on a number of factors. One way to increase the amount of detected signal is to employ a detector which has a high quantum efficiency. Previous measurements with lidar systems have involved mostly photomultiplier-type devices, which have quantum efficiencies of ∼4%. The system described here employs a CCD detector, which has a quantum efficiency at 355 nm of ∼40%—an order of magnitude improvement. Patented technology has allowed the practical use of this device in a direct-detection system for the first time. Further significant gains in the overall light collection efficiency have been realized by a unique imaging system. Off-axis imaging allows the reflected light from the 30-cm etalon to be captured and directed through an etalon with a 0.8-cm spacing (optimized to resolve molecular-backscattered light) and imaged onto a second CCD detector. The reflected light from this etalon is also captured and detected in this system, thus employing nearly all of the light that enters the detection system.; Early wind measurements demonstrate the capabilities of this system. Winds measured from aerosol-backscattered light show good agreement with a rawinsonde launched nearby. Wind measurements from aerosols are presented up to ∼6 km altitude, which is four times higher than winds obtained typically with the previous University of Michigan Doppler lidar. Measurements of the aerosol loading profile are also shown which are obtained concurrently with the wind profile. Example returns of molecular-backscattered light are further presented in this work to demonstrate the potential of measuring winds through the entire lower atmosphere.