Fig. 1. (a) Vector diagram of Brillouin scattering in view of quantum physics with the process of incident photon releasing and absorbing a phonon. (b) Brillouin spectra of water varies as the temperature changes. (c) Principle of the Brillouin spectrum measurement using double edge technique. Two edge filters are set at the steep edge of the Brillouin Stokes spectrum BS(v_B, Γ_B ), where v_B is the Brillouin shift Γ_B and is the Brillouin linewidth. For BS, the energies after these two filters are I₁(v_B, Γ_B ) and I₂(v_B, Γ_B). v_B and Γ_B can be deduced from I₁ and I₂, and Brillouin spectrum BS can be reconstructed. When the Brillouin spectrum changes to BS′, new energies ${\text{I}}_{1^{\prime }}$ and ${\text{I}}_{2^{\prime }}$ will be acquired and the corresponding Brillouin shift and linewidth can be deduced and used for BS′ reconstruction.

Fig. 2. Sketch of experimental apparatus. Pulse light of wavelength 532.293 nm with pulse width of approximately 7.5 ns, repetition rate of 100 Hz, and pulse energy of 20 mJ goes through a half waveplate, a polarization beam splitter (PBS), and a quarter-wave plate successively.Then the light is spread by a telescope combined by two convex lenses, L₁ (f=0.5 m) and L₂ (f = 0.1 m), and yields into a water tank of 1.8 m length and 0.5 m width to produce scattering light. The 180º back scattering light is received by the telescope and reflected by the PBS to an iodine tube which can absorb the Rayleigh part. The scattering light after the iodine tube is split into two by M₂. One small fraction beam is detected by the first photomultiplier tube (PMT1) as reference energy. The other part enters to a double-edge filter which is made by two Fabry-Perot interferometers and the light beams after these two interferometers are accepted by PMT2 and PMT3 separately. D is diaphragm, M_i and L_i(i=1, 2..) are mirrors and lens, respectively.

Fig. 3. Spatially resolved Brillouin backscatter intensity from the water tank at room temperature recorded (a) by PMT1 (c) by PMT2 (e) by PMT3 with 200 successive shots (color lines) and their average (white dots) and the corresponding background noise measured by (b) by PMT1 (d) by PMT2 (f) by PMT3.

Fig. 4. Signals after subtracting background noise from the data obtained from three PMTs, I_g, I₁ and I₂, and the two transmission ratios are plotted (dark yellow). The gray region where the received signals are strong relatively shows the ratios that do not vary a lot.

Fig. 5. (a) Theoretical transmission ratios and the calibration of the transmission ratios in different temperatures. (b) Theoretical Brillouin shift and the shift deduced from calibration transmission ratios S₁ and S₂. (c) The theorical Brillouin linewidth and the linewidth deduced from calibration transmission ratios S₁ and S₂.

Fig. 6. (a) Reconstructed Brillouin spectrum with 200 successive shots (color lines) and their average (white dots). (b) Errors of Brillouin shift and linewidth between the retrieved value and theorical value when the recorded signal averaged from 1 to 500 times for water in 20.4 °C.

Fig. 7. Absolute deviation of temperature and salinity between retrieved values and real values at different average numbers for the water in five groups of experiment. Water temperatures of the five groups are 20.4 °C, 22.6 °C, 24.8 °C, 26.6 °C and 28.2 °C, respectively; all the water salinities are 0%. Color lines represent the deviation and the black dots as reference line is the square root of the average number.