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In-situ spectral absorption profiler using optical fibers

Phytoplankton spectral absorption of light energy is one of the important factors to estimate primary production of the ocean. We have developed a system which measures in situ spectral absorption of suspended matter, mainly phytoplankton, by a modified opal glass technique. The system was designed to provide sub meter-scale profile of phytoplankton distribution without water sampling. The system consists of a halogen lamp as a white light source, a multi-channel spectrometer and submergible optical sensor unit. The collimating lens and diffuser constitute this unit. The optical fiber cables are employed to transmit the light between the sensor unit and the equipment on shipboard. The received light was measured by spectrometer which ranges from 400 nm to 800 nm with 1024-channel temperature stabilized linear photodiode array. Our preliminary experiment with the cultured phytoplankton (diatoms, flagellate and green algae) has shown that minimum detectable absorption coefficient is 0.01 m-1.