Lessons Learned while Optimizing Instrument Sensitivity for Deep Ocean Raman Spectroscopy

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. 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 our 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 developed 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 our previous cables where losses of 25-30 dB were common. This new fiber optic cable was field tested in May and showed markedly improved performance