Research On Optical Fiber Fluorescent System For Measuring Concentration Of Chlorophyll-a In Alga

Optical-fiber sensor is one of important technique widely used in the field of ocean measuring. Algae is the main ocean phytoplankton and chlorophyll-a is the main pigment of algae. Chlorophyll-a is one of the important parameters of water ecological system. Its content also is an important observation item which reflects phytoplankton's biomass or stock on hand in the sea, and also the basis of calculating the primary productive forces. Basing on old consistency measuring skill, the system applies optical-fiber as transmitting device with fluorescent technique to realize high efficiency on-line measuring of alga concentration. The main contributions of this dissertation are as follows:The basic principle and characteristic of fluorescence measurement method are discussed. The fluorescence specific properties of alga chlorophyll-a in the sea are analyzed. Therefore theoretical basis is established for the reasonable selection of excitation light source , optical fiber and optical detector.The total scheme of fluorescent optical-fiber measuring system is designed. According to the structure and characteristic of optical-fiber probe, the mathematical model of the fluorescent measurement and transfer is established in order to provide theoretical foundation of system calibration.The system adopts weak signal measuring circuit with He-Ne laser as light house, low-loss silicon optical-fiber bundle as transmission channel of light signal, high sensitivity and low-noise PMT as fluorescent signal detector and photo counter as main detecting device to realize the excitation, collection, transmission, detection and processing of weak fluorescence signal.To eliminate the signal variation due to undulation of excited light source, the detecting method of double light routes and double channels at the same time are used. In order to effectively separate the fluorescence produced by algae chlorophyll-a and the scatting light from the excitation light, the interference filter with high block coefficient and narrow band is used as spectral filter.Qualitative analysis and research of the measuring system are processed in the lab to educe the results of experiments, including the absorbing and excited fluorescence spectra character of chlorophyll-a experiments. The linear curve between the variation of fluorescence density and variation of algae concentration is educed and validates the feasibility of the system.