Study On Transmitting And Receiving Optical System Of Doppler Wind Lidar

The transmitting and receiving optical system is a vital part of Doppler wind lidar.The subject mainly analyzes the selection of optical parameters, adjustment of the optical system, echo signal reception, collimation of the output light from the optical fiber by numerical calculation and experiment research, which provides an important basis and guarantee for Doppler frequency shift discrimination and echo signal processing in subsequent work.The theory research mainly includes the following two aspects: The analysis of the main optical parameters in the transmitting and receiving system of Doppler wind lidar, which presents the optical design requirements for the 8 times beam expander for the laser beam and the multimode fiber with 62.5μm core diameter on the basis of the lidar equation, cassegrain telescope model and the theory of lens imaging. The influence of the beam divergence from the fiber on F-P etalon frequency discrimination, which mainly analyzes the influency on the angel-scanning transmission curve and its envelope, the selection of the initial operating point and the sensitivity of frequency shift discrimination in the angle-tuning F-P etalon working mode.The experiment research mainly includes three aspects:The optical adjustment on the coaxial transmitting and receiving system by the CCD; The precise optical adjustment on coupling the echo signals to the fiber by the 532 nm and 1064 nm laser; The collimation of the light from the fiber by a 23.5 mm focal length aspheric lens or a GRIN lens. From the experiments, the optical coaxial error is 15.86μrad when set the image centroid error being 5 pixels. When the transmitting and receiving system is adjusted to be optical coaxial in this method, the echo signal from 0 km to 9km can be detected availably by a photon counter and data acquisition system. The multimode fiber coupling efficiency can only reach 17.54% under the best adjustment, as an influency of the laser beam parameters and the adjusting itself. From the measurement of the beam divergence, a small error about 2.73μrad can be reached when placing the CCD in the focus of the long focal length lens precisely. The change of the beam divergence and the distribution of spot can be observed real-timely in the beam collimation adjustment process by this method. The beam divergence collimated by a 23.5 mm focal length aspheric lens can reach 3.25 mrad measured by the CCD. And the angle-scanning transmission curves of the F-P etalon are obtained when the beam divergence is 7.34mrad, 3.25 mrad and 1.42 mrad, from which the impact of the beam divergence on the shape of the curve and its envelope, which is in good agreement with numerical analysis. The beam divergence collimated by a GRIN lens and the 4 times defocused system can reach 2.75 mrad. For further 3 times expansion, the beam divergence can be compressed to 1.09 mrad, Comrared with the single-lens collimator, the GRIN lens can acquire a better collimation and the spot intensity distribution is improved in some degree.