Range-resolved water temperature measurement using Raman LiDAR with multiplexed excitation

We present the remote measurement of temperature vs length profiles in up to 50 m of clear (swimming pool) water. A pair of blue gallium-nitride laser diodes, with wavelengths separated by 4 nm, was multiplexed in time as the excitation source. Elastic and Raman returns were collected using two detector channels. The ratio of the Raman measurements can be calibrated to retrieve temperature as a function of depth, while the ratio of the elastic measurements can be used to correct for differential attenuation between the wavelengths. The temperature accuracy decreases with range and is typically better than 0.5 °C at 10 m and 1 °C at 50 m with depth resolution of 0.5-4 m, respectively, using 5 ns excitation pulses.