The Marine In-situ Chlorophyll Fluorescence Measurement System Based On Pseudo-Random Sequences Modulation

In-situ chlorophyll fluorescence detecting instrument has been widely used in the marine scientific research, environmental monitoring, and other fields. However, the market of similar high-end devices has been occupied by imported products for a long time. The most important function of in-situ fluorescence instrument is to detect weak fluorescence signal in nature environment. It is difficult to significantly improve the measuring accuracy by using existing design scheme due to the complex background noise and interference produced in measurement process, as well as the limitation of the volume and power of devices. Aiming at this situation and starting with modulation theory, this dissertation puts forward a new exciting light modulation method to improve instrument performance.Pseudo-random signal processing theory has also been widely applied in project fields. Combining the pseudo-random signal, optical modulation technology and the achievements of submersible organic matter in-situ measurement researches in recent years, this dissertation proposed an exciting light modulation method for fluorescence measurement system by using pseudo-random sequences, based on its correlation and spread-spectrum characteristic.The main tasks and results of this dissertation are summarized as following:1. Starting from the characteristics of in situ fluorescence measurements instrument, the necessity and methods of improving measurement accuracy were discussed, and the effect of excitation light modulation on fluorescence system was analyzed.2. Through establishing mathematical model, the quantitative analysis of noise and interference performance in this system has been given in this dissertation, as well as the relationship between sequence parameters and the system SNR.3. The simulation was carried out with Matlab/simulink to to verify the correctness of theoretical analysis. Comparing with the square-wave modulation system, the result indicated that the new modulation method was feasible.4. The hardware design of in-situ fluorescence detection system, including optical and electronic modules was given. The pseudo-random sequence modulation scheme was applied in fluorescence in-situ detection prototype. By using different modulation baseband and modulation sequence, an overall assessment to the system performance parameters was carried out, especially the improvement of signal to noise/interference ratio. The optimization selection of modulation sequence in narrow-band system was emphatically discussed.Experiment data showed that the in-situ fluorescence detection system based on pseudo-random sequence modulation, which was developed in this dissertation, can significantly improve the signal to noise/interference ratio, especially for the narrow-band interference inhibiting ability, which is much better than the system that used normal modulation. The accuracy and power consumption of measurement system is up to the highest level of similar products at home and abroad. The pseudo-random sequence modulation mode has wide practical prospects in various fluorescence analysis systems.