Development of a laser Raman spectrometer for deep-ocean science

We have extensively modified and successfully used a laser Raman spectrometer (DORISS, deep-ocean Raman in situ spectrometer) for geochemical studies in the deep ocean. The initial instrument, from Kaiser Optical, was separated into three components: an optical head, a laser-power supply telemetry unit, and the spectrometer. These components were modified to fit into custom designed pressure housings, and connected by deep-sea cables and optical penetrators designed to minimize signal loss. The instrument ensemble has been field deployed on remotely operated vehicles (ROVs) for a variety of experiments and observations, with successful operation at 1.6°C, 3600 m depth. Power supply, instrument control, and signal telemetry are provided through the ROV tether, which contains copper conductors and single mode optical fibers. The optical head is deployable by the ROV robotic arm for sample analysis; the remaining components are fixed within the vehicle tool-sled. Challenges of system calibration at depth, temperature and pressure artifacts, and system control through over 4 km of cable were successfully overcome. We present exemplary spectra obtained in situ of gas, liquid, and solid specimens, and of the ubiquitous signal of sea water itself. Future challenges include weight and size reduction, and advances in precise beam positioning on mineral targets on the sea floor.