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1.5 μm polarization coherent lidar incorporating time-division multiplexing

Abstract:

Atmospheric depolarization ratio and wind velocity are measured simultaneously by a single versatile coherent Doppler lidar (CDL). Backscattering components at parallel and perpendicular polarization states are obtained by using a single balanced detector, adopting time-division multiplexing technique. Thus systematic error induced by the non-uniform response of different detectors in traditional lidars is avoided. The operation mode of the instrument can be switched from polarization CDL to traditional CDL by the user depending on atmospheric conditions and desired performance. As demonstrated, the perpendicular component of the backscattering, usually wasted, not only can be used to retrieve the ADR, but also can be used to improve the carrier to noise ratio in wind detection. In the traditional mode, given a tolerance of 0.5 m/s precision, a detection range of 6 km is achieved by using a 300 ns laser pulse with energy of 100 μJ, where the temporal and spatial resolution of 2 s and 60 m, respectively. Continuous wind detection of the atmospheric boundary layer over 26 hours is presented to demonstrate the robustness and stability of the system. Dynamic evolution and wind structure are recorded.

Fig. 1. Photograph of one pair of CDLs sharing one set of computer.

Fig. 2. Wind velocity retrieved from both S and P states backscattering by single balanced detector. The black points represent invalid data points where the carrier to noise ratio (CNR) is low. The inset histogram shows the velocity difference between S state and P state. The temporal and radial range resolution is 2 s and 60 m, respectively, starting at 23:50 on Mar. 17, 2017.

Fig. 3. (a). The CNR distribution of S states. The temporal and spatial resolution is 2 s and 60 m, respectively. (b). The CNR distribution of P states. The temporal and spatial resolution is 2 s and 60 m, respectively.(c). The distribution of ADR measured by the polarization CDL. The temporal and spatial resolution is 10 s and 60 m, respectively.

Fig. 4. (e). Horizontal wind speed. (f). Horizontal wind direction. The temporal and spatial resolution is 60 s and 60 m, respectively. The observation is from18:00 on Apr. 13 to 20:00 on Apr. 14 in 2017.