Design and Performance Validation of a Low-Loss Optical System with Light Source Fluctuation Compensation for In Situ Deep-Sea Nitrate Sensing

In situ measurement of deep-sea nitrate concentration is of significant importance for marine environmental monitoring and management. Nitrate sensors based on ultraviolet (UV) spectrophotometry, featuring simple, rapid, and reagent-free detection, are highly suitable for long-term autonomous monitoring and have attracted increasing research interest. However, existing systems still suffer from increased measurement errors and reduced detection ranges due to light source attenuation and optical transmission losses. To address these limitations, this study presents an optical system for a deep-sea nitrate sensor designed for deployment on mobile platforms. A co-aperture coupling design integrating the measurement and reference paths is proposed to achieve low optical loss while accurately compensating for the impact of light source attenuation on measurement accuracy. Furthermore, full-UV optical component optimization is performed to significantly improve optical energy utilization. Light-source calibration experiments demonstrate stable absorbance responses and effectively compensate for measurement deviations caused by light source attenuation. Performance evaluation of the system demonstrated a detection limit of 0.0056 mg/L-N(0.4 µmol/L, 10 mm optical path), with high measurement accuracy and precision across samples with varying nitrate concentrations. Sea trial results confirmed that the proposed system satisfies the requirements for stability and reliability in deep-sea in situ nitrate concentration monitoring, highlighting its potential for practical marine applications.