Dual-polarization coherent Doppler lidar with all-fiber on/off-axis switchable design for simultaneous wind and aerosol measurements.

This paper presents an all-fiber on/off-axis switchable dual-polarization coherent Doppler lidar (DPCDL) and evaluates its performance in wind and aerosol observations. This lidar integrates the coherent Doppler and micro-pulse polarization detection techniques, and achieves the capabilities of simultaneously profiling the atmospheric wind field and depolarization ratio at the wavelength of 1550 nm. The well-developed single-depolarization coherent Doppler lidar (CDL) system typically only collects the parallel component of backscatter signals, disregarding the orthogonal component perpendicular to the polarization direction of the emitted beam, which limits its capability in measurement range and aerosol observation accuracy. The DPCDL adopts an on/off-axis switchable mode, synchronously detecting the dual-polarization component information of backscatter signals through dual coherent channels. It not only improves the signal utilization and wind speed detection performance, but also enables the high-precision real-time retrieval of the aerosol depolarization ratio. To validate the reliability of atmospheric wind speed measurements, the experiment employed a technically mature and commercially available coherent Doppler Lidar (CDL) system for side-by-side comparison observations with the newly-built DPCDL. The correlation coefficient between the wind speed measurements from these two lidars reached up to 0.982, with a wind speed deviation of less than 0.077 m/s. Additionally, the DPCDL was co-located with a micro-pulse polarization Lidar (MPL) to conduct the comparisons of depolarization ratio observations, verifying the accuracy of its depolarization detection capability. The synchronous observation results show that under clear sky and rainy conditions, the trends of the depolarization ratio at two different wavelengths are consistent, and the values are close. However, under dust storm conditions, the depolarization ratio values at 1550 nm reach above 0.4 and were significantly greater than 532 nm, indicating that the DPCDL system at the 1550 nm wavelength is more sensitive for observing coarse-particle dust aerosols. The system's ability to acquire atmospheric wind field and aerosol optical properties simultaneously facilitates the identification of large particulate pollutants, the tracing of pollution sources, and the prediction of dispersion pathways in the boundary layer. This is highly significant for environmental protection, atmospheric monitoring, and forecasting.