Lessons Learned while Optimizing Instrument
Sensitivity for Deep Ocean Raman Spectroscopy

Alana Sherman, Edward T. Peltzer, William Kirkwood, Rachel M. Dunk,
Peter M.Walz, Jim Scholfield, Cheri Everlove, and Peter G. Brewer

Monterey Bay Aquarium Research Institute
7700 Sandholdt Rd. Moss Landing, CA. 95039

Proceedings of the Marine Technology Society / Institute of Electrical
and Electronics Engineers Oceans '06 Conference, Boston, MA, USA.


ABSTRACT

Over the last several years, scientists and engineers at the Monterey Bay Aquarium Research Institute (MBARI) have successfully developed and deployed two generations of deep ocean laser Raman spectrometers. There are many advantages to this type of spectroscopy: it is rapid, non-destructive, and can be used to analyze solids, liquids and gases. Unfortunately, one of the disadvantages of using Raman spectroscopy is that the return signal measured is very weak. Thus, it is difficult to detect dilute chemical compounds in solution, such as bicarbonate and carbonate ions at natural concentrations. The challenge we face now is improving the sensitivity of the instrument. This paper discusses the efforts we have made to increase the sensitivity of our second generation Deep Ocean Raman In-Situ Spectrometer (DORISS2).

In developing this second generation instrument, we incorporated new components which improved system performance. These components include a new ruggedized U-shaped spectrometer and a back illuminated CCD camera which is much more sensitive than the original front illuminated CCD camera. The element which has had the most impact on system sensitivity is a set of new custom-made fiber optic cables. We had built several sets of custom fiber optic cables, but despite our efforts, their performance degraded substantially over time. Recently, we have produced a new oil-filled pressure compensated fiber optic cable which performs far better in pressure testing and is much more robust. At 6000 psig, this cable shows losses of only 2-3 dB versus previous cables where losses of 25-30 dB was common. This new fiber optic cable was field tested in May and showed markedly improved performance.

© 2006 by Marine Technology Society.


Acknowledgements

The DORISS2 development program is supported by the David & Lucile Packard Foundation. Significant contributions to the program were provided by: Sheri White of Woods Hole Oceanographic Institution; MBARI engineers and technicians; and the pilots and crews of the R/V Point Lobos with ROV Ventana, and R/V Western Flyer with ROV Tiburon.


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