A LIDAR-Compatible, Multichannel Raman Spectrometer for Remote Sensing of Water Temperature

Abstract

The design and operation of a custom-built LIDAR-compatible, four-channel Raman spectrometer integrated to a 532 nm pulsed laser is presented. The multichannel design allowed for simultaneous collection of Raman photons at two spectral regions identified as highly sensitive to changes in water temperature. For each of these spectral bands, the signals having polarization parallel to (∥) and perpendicular to (⟂), the excitation polarization were collected. Four independent temperature markers were calculated from the Raman signals: two-colour(∥), two-colour(⟂), depolarization(A) and depolarization(B). A total of sixteen datasets were analysed for one ultrapure (Milli-Q) and three samples of natural water. Temperature accuracies of ±0.4 °C–±0.8 °C were achieved using the two-colour(∥) marker. When multiple linear regression models were constructed (linear combination) utilizing all simultaneously acquired temperature markers, improved accuracies of ±0.3 °C–±0.7 °C were achieved.

Remote Sensing of Natural Waters Using a Multichannel, Lidar-Compatible Raman Spectrometer and Blue Excitation

“…When all four markers were incorporated in the linear combination model, enhanced RMSTEs up to ±0.2 • C were achieved. Those RMSTEs were similar to values reported in previous studies for green excitation (de Lima Ribeiro et al, 2019b).…”

“…In keeping with previous studies Pask, 2015, 2017;de Lima Ribeiro et al, 2019b), the relationships between temperature markers and reference temperatures are found to be linear, allowing for the use of linear regression models with coefficients gradient and intercept. These coefficients were then rearranged in order to calculate a new set of temperatures dependent on the markers, hereafter called "predicted temperatures" (Equation 7).…”

“…The design of our multichannel LIDAR-compatible RS using blue excitation is conceptually similar to the RS reported in de Lima Ribeiro et al (2019b), which used a green excitation laser. In practice, the two excitation lasers differed, most notably in pulse energy, and the band pass filters defining the spectral channels also differed in regard to their width and their positions relative to the Raman band.…”

Impact of fluorescence on Raman remote sensing of temperature in natural water samples

“…The work presented here provides proof of concept and future work will test the viability of this promising method. To this end, we have recently reported [19] a 4-channel Raman spectrometer that uses pulsed excitation and fast detection by photomultipliers. It has been used, in the laboratory, to predict water temperature of 3 natural water samples with accuracies in the range ± 0.4 °C to ± 0.8 °C.…”

Challenges and Opportunities in Lidar Remote Sensing

“…Ocean-optimized space lidars synergized with other ocean measurements (Hostetler et al, 2018;Chen et al, 2019;Jamet et al, 2019) would transform our ocean measurement capability. Further advancing lidar measurements with Brillouin scattering, Raman scattering, and fluorescence signals could provide ocean mixing layer temperatures, salinity, other chemical component information simultaneously, at least from airborne platforms (Hoge et al, 2005;de Lima Ribeiro et al, 2019).…”