Study On Fluorescence Measurement For Algae Chlorophyll-a Concentration With Optical Fiber

Fluorescence optical-fiber sensor is one of the hot-spot problems in the field of modern optical fiber sensor technology, ft 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. Algae is the main ocean phytoplankton whose content is usually shown with chlorophyll concentration. Chlorophyll-a is one of the important parameters of water ecological system. Its content 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. This paper presents a new type of fluorescence optical-fiber measurement system for measuring chlorophyll-a concentration in the sea in situ by means of the fluorescence methods, optical-fiber sensing techniques and weak light signal detection techniques. The system can realize the on-line measurement of algae concentration. 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 mam contributions of this dissertation are as follows:The basic principle and characteristic of fluorescence measurement method are discussed. The fluorescence specific properties of the algae chlorophyll-a in the sea are analyzed. Therefore theoretical basis is established for the reasonable selection of excitation light source, optical filter and optical detector.The total scheme of fluorescence optical-fiber measurement system is designed. According to the structure and characteristic of optical-fiber probe, the mathematical model of the fluorescence measurement and transfer is established in order to provide theoretical foundation of system calibration. The principle and characteristic of excited light source, interference optical filter, transmission optical fiber and optical detector are discussed. The coupling between optical fiber with light source and optical detector are simply analyzed in order to provide foundation for the reasonable selection of excitation and receiving optical fiber.The fluorescence signal produced by chlorophyll-a in the sea is very weak and influenced by background light noises. Therefore, some steps are adopted as follows in order to realize the excitation, collection, transmission, detection and processing of weakfluorescence signal. The system uses the pulse xenon lamp as the excited light source which has the characteristics of low power consumption, high intensity and narrow pulse width. The transmission channel of light signal is the low-loss silicon optical-fiber bundle. The photomultiplier (PMT) with high sensitivity and low noise is used as fluorescence signal detector. The weak signal detecting circuit is based on Boxcar integrator.In order to effectively separate the fluorescence produced by algae chlorophyll-a and the scattering light from the excitation light, the interference filter with high block coefficient and narrow band is used as spectral filter. The center wavelength of the excitation filter (broad width) is 420nm and the bandwidth is lOOnm. The center wavelength of fluorescent emittance filter (narrow width) is 685nm and the bandwidth is lOnm. In this way, the light flux received by PMT is the fluorescence produced by chlorophyll-a.The detecting methods of double light routes and double channels at the same time are used to eliminate the signal variation due to undulation of the excited light source. The excitation light is divided into two paths with optical-fiber divider, one as reference light received by PIN through optical-fiber bundle, the other as excitation light reaching to the probe through optical-fiber bundle to excite chlorophyll-a in the sea emitting fluorescence which is collected by receiving optical-fiber bundle and transferred to PMT. The chlorophyll-a concentration is...