Development of Portable Eye-Safe Depolarization Coherent Doppler Lidar and Observations of Asian Dust

A prototype model of portable eye-safe depolarization coherent Doppler lidar system has been developed for measuring wind and behaviors of aerosols such as dust, pollutants and so on. Figure 1 shows the photograph of the prototype model and Fig. 2 illustrates block diagram of the system. This system consists of “Optical Transmitter and Parallel Component Detection Unit (Tx/PP Unit)”, “Polarization Split and Perpendicular Component Detection Unit (Split/SP Unit)”, “Optical Antenna Unit (Ant Unit)”, “Signal Processing PC (SP PC)” including dedicated signal processing board for both parallel and perpendicular component signals, and “PC Monitor”. In this system, Tx signal (linearly polarized optical pulses) generated by Tx/PP Unit are transmitted through Split/SP Unit and Ant Unit. Received signal collected by Ant Unit is split into parallel and perpendicular component relative to the Tx signal's polarization plane by Split/SP Unit. Parallel component signal is detected by Tx/PP Unit and converted to digital sampled signal by SP PC. On the other hand, perpendicular component signal is detected by Split/SP Unit and converted to digital sampled signal by another SP PC. Finally, the digital sampled signals of both parallel and perpendicular component are converted to the frequency domain, then peak search and Doppler velocity estimation is achieved respectively. This prototype model has capabilities of measuring wind profile up to 3km at 150m range resolution, over plus/minus 30 m/s wind velocity range at velocity resolution < 0.6 m/s, mechanical scanning (1 rpm maximum) in both azimuth (plus/minus 180 degrees) and elevation (up to 90 degrees), and measuring polarization signal components (parallel and perpendicular). So this lidar is able to discriminate spherical and nonspherical element such as Asian dust, pollen and pollutants and monitor their spatial behaviors from velocity and direction calculated by VAD (Velocity Azimuth Display) method. We have been collecting various atmospheric observation data especially Asian dust (yellow dust / yellow sand) and evaluating the performance of this prototype lidar. Figure 3 shows an example of measured spectrums (a) on fine (normal atmosphere) day (14, July 2010) and (b) on Asian dust event occurred (25, May, 2010). In Fig.3, horizontal axis indicates Doppler velocity of line-of-sight (positive value mean following wind and negative value opposing wind), vertical axis backscatter signal intensity, solid (red) line indicates parallel components and dashed (blue) line perpendicular component. We can see the perpendicular component which have no peak on normal atmospheric day rises on Asian dust event day and have Doppler velocity -5.7m/s.