Development of high sensitivity shore-based laser induced fluorescence radar and its application in high-precision online monitoring of chlorophyll concentration

Abstract

Monitoring aquatic chlorophyll is essential for environmental observation and the conservation of natural ecosystems, especially concerning the water quality in regions like lakefronts and coastlines, which are closely interrelated human activities. The development of shore-based devices capable of conducting high-frequency, high-precision, and automated chlorophyll detection is crucial. Optical sensing technology, distinguished by its exceptional specificity, sensitivity, and detection accuracy, is widely used in the field of water monitoring, involving the applications of reflectance and fluorescence spectral detection. Passive spectra detection can be affected by irregular variations in natural illumination, which needs to be corrected through calibration algorithms. Furthermore, water surface undulating and aquatic substances that can produce fluorescence besides chlorophyll can influence the original detection signal, increasing the complexity of the data processing. To overcome the above issues, this work presents a triple-mode shore-based sensing instrument. As the core of the system, the laser-induced fluorescence spectroscopy remote sensing system (LIFSRS) include active laser and imaging spectrometer with high sensitivity and excellent signal-to-noise ratio, along with an unsupervised processing algorithm. Its common path design ensures an accurate detection distance between detection systems and surface, while the compact transmissive diffraction configuration greatly minimize optical distortion due to the large detection aperture with in the imaging spectrometer. This setup effectively mitigates interference from extraneous fluorescence in the water without the need to subtract background fluorescence. The unsupervised algorithm demonstrates significant robustness and adaptability, capable of correcting the acquired fluorescence spectra based on the incident angle and detection distance. In addition, the instrument integrates a thermal imaging module and a millimeter wave radar module for bio-detection as well as for acquiring water surface temperature and altitude. Consequently, the system has been deployed at various locations across two cities, enabling the accurate acquisition of chlorophyll fluorescence concentrations in natural water bodies without the need for supervision. The detection time for a single measurement is 200 milliseconds, with the overall error percentage being minimal 15 %.