High Precision Light Detecting Method Research For Marine In-situ Sensors And Sensors Development

Marine in-situ sensor, which is an important part of undersea observatory network, has great significance for human to observe and understand the ocean. As the optical sensors are not easily contaminated and have the advantage of high precision and stability, in-situ optical sensors have become the mainstream of instrumentations for marine survey. In natural harsh environment, the performance of the sensor is not only determined by the sensitivity, but also by the anti-interference ability of the system. This thesis discussed on how to seek the optimal balance between the sensitivity and anti-interference ability of the system, the two contradictions which complement each other and restrict each other. This thesis had put forward an ultra-weak optical signal detecting model and developed in-situ chlorophyll-a sensor, in-situ turbidity sensor and in-situ CDOM (Colored Dissolved Organic Matter, CDOM) sensor.The followings are innovations and main parts of the thesis:1. Ultra-weak optical signal detecting model of in-situ optical sensor was set up; this model unified in-situ chlorophyll-a detection、in-situ turbidity detection and in-situ CDOM detection from the perspective of electronic measurement. Based on this model, the thesis studied the correlation among noise、interference and modulation sequence and designed the light detecting light path, mechanical structure and high precision electrical circuits of the optical sensor. In situ optical sensor with single frequency modulation method was developed, and realized the ultra-weak light signal detecting with sunlight interference existed. Experiments showed that the in-situ chlorophyll-a sensor with single frequency modulation method had a sensitivity of0.0081μg/L, with R-square0.9985. This result was better than the similar products from Seapoints Inc. and Wetlabs Inc.2. Ultra-weak optical signal detecting method based on pseudo-random sequence with constrains of band-pass signal pass was proposed to improve the anti-interference ability of the system. Minimum loss power spectrum index eN was developed to optimize the length of m sequence to get high precision detecting with constrains of band-pass signal pass. Experiments showed that the m sequence modulation with length of15bits and31bits had the minimum system noise, and the anti-interference of31bits modulation system was2.3times better than single frequency modulation system.3. The m sequence with length of31bits and chip time1/4096s is the most suitable modulation sequence method in this study. Based on this modulation method, the in-situ chlorophyll-a sensor, in-situ turbidity sensor and in-situ CDOM sensor were developed. The sensitivity of these sensors are0.0103μg/L(chlorophyll-a calibrated),0.0078FTU(chlorophyll-a calibrated、0.137μg/L(Quinine calibrated)。4. In-situ chlorophyll-a sensor and in-situ turbidity sensor developed in this work had been applied in long-term buoy observation experiments in the Southern China Sea successfully. In situ turbidity sensor has accomplished the’JiaoLong’deep-sea diving experiment with good performance. These indicate that sensors developed in this work have high precision, high anti-interference ability and reliability.In conclusion, this thesis solved the ultra-weak light detecting under sunlight and other electrical interference, and proposed a novel ultra-weak light detecting method. This method improved the anti-interference ability of the sensor without increasing the complexity of the system’s hardware. This study has important scientific significance and application value.