Research On Seawater Temperature Remote Sensing Technology Based On Laser Scattreing

The seawater temperature is one of the most important physical parameters related to Oceanography. It is the basis of exploring and using the marine environment and resources, therefore it has significance value in both civilian and military fields. A practical remote sensing technology which can be used to measure the seawater temperature of the subsurface ocean accurately and rapidly is the goal of many researchers in marine science and technology.Currently the underwater temperature is measured with bathythermographs or optical fiber grating sensors on drifting or moored buoys. These techniques are relatively slowly and costly, cannot rapidly measure the underwater temperature in large areas. Although space-borne microwave irradiation or infrared radiation can be used to rapidly measure the sea surface temperature(SST) in large areas, it is incapable of surface penetration.For the good penetration of blue-green laser light in seawater, the airborne lidar based on scattering can be used to measure the underwater temperature of ocean. In recent decades, researchers have extensivly studied the temperature sensing technology based on Raman scattering and Brillouin scattering, therefore greatly promoted the development of these technology. But the current measurement techniques are still far from practicability. In view of the problems of underwater temperature remote sensing technology based on light scattering, we put forward this research project, in order to improve the practicality and reduce the cost of this technology. This study will lay a foundation for the Lidar remote sensing technology to be realized in measuring the three dimensional distribution of surface seawater temperature rapidly and accurately.In this thesis, we firstly theoretically studied the feasibility of measurement the seawater temperature by Brillouin scattering, analysed the characteristics of Brillouin scattering and stimulated Brillouin scattering(SBS), established the SBS numerical calculation model using the coupled wave theory, and numerical calculated the influence of focusing depth, seawater temperature properties to the pulse width and energy reflectivity characteristics of SBS. Secondly, a new optical coherent detection method is presented to do real time measurement of water SBS frequency shift and temperature. The backward water stimulated Brillouin scattering light combine with the backscattering laser, the heterodyne is detected by a high-speed photodetector, and the heterodyne frequency is the Brillouin frequency shift. According to the relationship between Brillouin frequency shift and water temperature,the water temperature can be determined. To test and verify its practicability, the heterodyne waveforms at different water temperatures are recorded in laboratory by a wide-band, and the Brillouin frequency shifts are deduced by Fourier transform. The experimental results are consistent with theoretical analysis. Due to the Brillouin frequency shifts between the reference beam of SBS and measured SBS beam is smaller, it will reduce the requirements of the bandwidth of detector and oscilloscope, but the higher demands of pulse width and peak power of pulse laser is requied. Because if the laser pulse width is small, only a few signal cycles can be detected, it is difficult to improve the accuracy of frequency measurement. Consider the development of broadband detector, oscilloscope and laser technology, another measurement method based on one SBS coherent beam is further put forward. The water SBS beam is used as the signal beam, and a portion of the incident laser beam is used as the local oscillator. The heterodyne is detected by a high-speed photodetector, and the heterodyne frequency is the Brillouin frequency shift. Therefore, the underwater temperature can be determined according to the relationship between the Brillouin frequency shift and water temperature. To test and verify its practicability, the heterodyne waveforms at different water temperatures are recorded in the laboratory by a wide-band oscilloscope, and the Brillouin frequency shifts are deduced by a Fourier transform. The experimental results are consistent with the theoretical analysis. This work provides the foundation for the development of water temperature measurement system based on coherent Brillouin scattering. This method has the advantages of simple structure, high stability, high speed, and it is hopeful to develop a field-deployable Brillouin Lidar system for seawater temperature remote sensing.Then, the difficulties faced by Raman scattering measurement technology are analyzed, the influences of excitation wavelength on the peak position and width of water stretching vibration Raman spectrum are analyzed by theoretical and experimental study, as well as the effect of excitation wavelength on the Raman scattering coefficient and the water temperature measurement precision are analyzed, and the Raman scattering Lidar equation is established, the influences of excitation wavelength on the Lidar system detection depth is studied. Finally, a practically low costly Raman scattering real-time temperature remote sensing system based on grating spectrometer, intensifier and an area-array CCD detector is designed, a filtering algorithm based on frequency distribution is proposed, a real-time Raman spectrum acquisition, data processing and temperature inversion software system based on Labview is developed. Many laboratory and field test results show that this system can fast measure the high resolution Raman spectroscopy of water and obtain the water temperature in real-time. It has a high temperature measurement accuracy, and good stability for long time running.