Phytoplankton Fluorescence Sensing System For Plastic Optical Fiber

Fluorescence optical-fiber sensor is one of the hot-spot problems in the field of modern optical fiber sensor technology. It discriminates the category and concentration of measured materials by measuring the fluorescence spectral characteristic and has great application potentialities in the field of ocean environment monitoring. Phytoplankton in the sea is the main primary producer and energy transducer in ocean ecological system, and also the base of the primary productive forces. This paper presents a new type of plastic optical-fiber measurement system for measuring phytoplankton fluorescence in the sea by means of the fluorescence measuring methods, optical-fiber sensing techniques and weak light signal detection techniques. It provides a new kind of detection method in monitoring and governing ocean pollution and in the study of estimating phytoplankton's biomass in the sea.The paper discusses the basic principle and characteristic of fluorescence measurement method and the sensing theory of the POF, analyses the fluorescence characteristic of phytoplankton, and designs the whole project of the fluorescence optical-fiber measuring system. The fluorescence signal produced by phytoplankton in the sea is very weak and influenced by background light noise. Therefore, the photomultiplier (PMT) with high sensitivity and low noise is used as fluorescence signal detector. The weak signal detecting circuit is based on lock-in amplifier. Besides, a high-precision switch mode power of high voltage for PMT is designed. They realize the excitation, collection, transmission, detection and processing of weak fluorescence signal. The paper includes hardware circuit diagrams and parameters design. Because of using the lock-in amplifier in the circuit, the detecting circuit has lower noise coefficient and higher Common Mode Rejection Ratio (CMRR), and then enhances the gain.The experiments proved the feasibility of the POF fluorescence system for measuring phytoplankton concentration by researching the relationship between excitation light wavelengths and fluorescence intensity, the algae concentration and fluorescence intensity.