Theoretical and experimental investigations on measuring underwater temperature by the coherent Brillouin scattering method.

In this paper, a new method of measuring a water-stimulated Brillouin scattering (SBS) frequency shift by optical coherent detection is presented, in order to remote-sense the underwater temperature of the ocean. A single longitudinal mode, passively Q-switched pulsed Nd:YAG laser is used as the light source, 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 the water temperature. To test and verify its practicability, the heterodyne waveforms at different water temperatures are recorded in the laboratory with 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 a water temperature measurement system based on coherent Brillouin scattering.