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Deep-sea laser probes ocean floor

22 Nov 2002

Geophysicists take a Raman spectrometer 3600 m under the ocean.

Geophysicists studying the chemistry of the deep ocean have used a laser Raman spectrometer to gather data that could prove useful in debates over global warming.

Peter Brewer and colleagues at the US-based Monterey Bay Aquarium Research Institute (MBARI) adapted an instrument built by Kaiser Optical Systems, US. It is the first time that a Raman spectrometer has been used in situ for deep-sea work.

The geophysicists controlled the spectrometer from a ship on the surface connected to a remotely operated vehicle (ROV) 3600 m below them, just above the sea bed.

The surface ship and the spectrometer were linked via a 4 km-long fiber-optic cable, allowing the scientists to manipulate the beam focus remotely.

"The Raman technique allows us to measure solid phases, like rocks, bacteria and other particles. This has not been possible before without sample recovery," Brewer told Optics.org.

A frequency-doubled Nd:YAG laser operating at 532nm provides the Raman excitation, with a holographic grating used as a filter and a CCD array to detect the weak Raman signals. Brewer says that the sea water gives off almost no fluorescence to contaminate the Raman signal.

"We are still in the early stages of solving the problems of precise positioning and focus, so we measured large, transparent objects on the first dives," explained Brewer. He added that with a long path-length cell, the team should be able to detect species dissolved in carbon dioxide near to the sea bed.

Brewer said that the main difficulties in operating the instrument were the fragility of optical components and the size and weight of the Raman system, which makes precise positioning awkward.

Next year, the MBARI team plans to use the setup to measure the chemical signatures of hydrothermal gas vents 2000 m deep in the Gulf of California. Both methane and hydrogen sulfide, which are common gases found in such vents, are excellent Raman scatterers. The results of these experiments could give clues to the origins of life in the ocean. Brewer says that for this study, the team is designing a precise positioning system for measurement of solids.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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