Effects of laser wavelength on both water temperature measurement precision and detection depth of Raman scattering lidar system

Airborne Raman scattering laser lidar technology can measure the three-dimensional (3D) distribution of subsurface seawater temperature rapidly, and it has important social and economic values. In this paper, firstly, the relationship between Raman stretching vibration spectrum peak position and excitation wavelength, and the relationship between the full width half maximum (FWHM) of Raman stretching vibration spectrum and excitation wavelength are analyzed theoretically. The results show that as the excitation wavelength increases, Raman scattering peak gradually shifts toward longer wavelength and the Raman spectrum FWHM increases noticeably. Secondly, to verify the theoretical results, the Raman spectra at different water temperatures excited by 450 nm and 532 nm lasers are measured experimentally, and the fitting analyses of them by single Gauss peak fitting method are made, the relationship between Gauss peak wavelength and temperature is obtained, and the effect of laser wavelength on the temperature measurement precision is analyzed. It is found that larger excitation wavelength can increase Raman spectrum measurement accuracy, thereby improving the temperature measurement precision. Finally, the Raman scattering lidar equation is established, the Raman scattering coefficients and attenuation coefficients of different wavelength lasers are analyzed, and the corresponding effects of laser wavelength on the lidar system detection depth are studied. Results show that the lidar system detection depth is greatly influenced by the laser wavelength, lidar system with laser wavelength below 480 nm has a good detection ability, and large wavelength laser greatly reduces lidar system detection depth. The effects of laser wavelength on both temperature measurement precision and detection depth should be considered in the desigin of Raman scattering lidar system.