Information about subsurface water temperature is valuable to the defence, oceanography and environmental science sectors, with particular applications relating to underwater sound propagation, environmental monitoring and predicting algal bloom events. The mainstream technology for measuring subsurface water temperature is to deploy strings of thermocouples from ships or buoys, and accordingly there exists an opportunity for a new technology that can be used to systematically map subsurface water temperature on a local or regional scale.
Raman scattering is an inelastic scattering process, and in the case of water, the spectral content and the polarisation properties of the Raman signal both vary in a systematic manner with temperature. The use of Raman spectroscopy to predict water temperature was first proposed in the 1970s, and the potential for extending this to determine depthresolved temperature profiles using LIDAR methods has been investigated by ourselves and others. When used in combination with LIDAR methods and fast, sensitive detection by photomultipliers, there are strong prospects for developing a relatively-simple optical system to make depth-resolved measurements of water temperature, and potentially other parameters such as salinity, turbidity and chlorophyll concentration.
I will present the design principles and operation of a custom-built LIDAR-compatible, four-channel Raman spectrometer integrated to a 532nm pulsed laser. The multichannel design allowed for simultaneous collection of polarised Raman photons at two spectral regions identified as highly sensitive to changes in water temperature. Four independent temperature “markers” could be calculated from the Raman signals, and each of these was evaluated with regards to the accuracy with which water temperature could be predicted. When multiple linear regression models were constructed using a linear combination of the simultaneously acquired temperature markers, accuracies of ±0.3°C to ±0.7°C were achieved for a range of natural water samples from Sydney Harbour. I will also outline our current plans to transition our methods from the lab to the field.
Professor Helen Pask has wide experience in laser physics, optics, and spectroscopy. Her specialist expertise is the development of crystalline Raman and Terahertz lasers for particular applications, and the application of Raman spectroscopy to remote sensing of the water column. She received her PhD in Physics from Macquarie University in 1992, worked for two years at Southampton University in the UK on optical fibres and amplifiers, and has been at Macquarie University since 1995. Her current position involves a combination of research and undergraduate teaching. She has published over 100 journal and conference papers and contributed to numerous patents which have been licensed to companies.
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