Design and performance of spectroscopic filter of rotational Raman temperature lidar for absolute measurement
Authors: Li Qi-Meng  , Li Shi-Chun  , Qin Yu-Li  , Hu Xiang-Long  , Zhao Jing  , Song Yue-Hui  , Hua Deng-Xin  ,   School of Mechanical and Precision Instrument Engineering, Xi'an 710048, China
Published: December 2020

Rotational Raman temperature lidar for absolute measurement is an important method to directly detect the atmospheric temperature profile by using active remote sensing technology. Compared with the rotational Raman temperature relative measurement, the absolute measurement can avoid the systematic error caused by the calibration process, but its high-precision requirements of rotational Raman spectroscopic filter restrict the development of absolute measurement technique for atmosphere temperature. In order to achieve the absolute measurement technique of rotational Raman temperature lidar, the fine resolution of single rotational Raman line and the effective suppression 60-70 dB for the elastic scattering signal are the key factors for directly retrieving the atmospheric temperature by using the relationship between the single rotational Raman line and temperature. Based on the operational principle of grating, a two-stage parallel multi-channel Raman spectroscopic filter with one-order blazed grating and fiber Bragg grating is designed, and the parameters and optical path structure of the core cascade device (micron-level fiber array) are optimized. The optical path of the primary spectroscope is simulated, the wavelength difference between the rotational Raman lines of adjacent even rotational quantum numbers of nitrogen molecule (N2) gradually decreases from 0.4506 nm to 0.4475 nm. Compared with the average of approximately 0.4494 nm, its floating interval is -0.0012-+0.0019 nm, and the maximum centrifugal distortion of the rotational Raman spectra is approximately 0.0031 nm, which means that the centrifugal distortion ratio is 0.69%. Under the different values of incident angle , the diffraction position difference between adjacent rotational Raman lines varies from 124.43 m to 125.51 m, with a variation interval of -0.57-+0.51 m compared with a fixed value of 125 m. In order to test the matching consistency between rotational Raman spectra and the multi-channel fiber array, and to obtain the out-of-band suppression and channel coefficient of each fiber channel, an experimental system which consists of a first-order blazed grating, a convex lens and a fiber array is set up, and the atmospheric echo signal is simulated by using a broadband light-source and a semiconductor laser (LD). The experimental results show that the channel coefficient of the rotational Raman channels of the primary spectroscope is above 0.75, and the maximum deviation between the measured wavelength of extracted spectrum and the theoretical value is approximately 0.0398 nm, which means the the deviation degree is 8.86%. Each channel can provide more than 27 dB effective suppression to elastic scattering signal, and then by combining with the second spectroscope of fiber Bragg grating, the suppression at least is up to 62 dB. Therefore we can fine extract single rotational Raman line of even rotational quantum number.