基于微型多次反射腔的TDLAS二氧化碳测量系统

海气界面CO₂测量对于海洋科学研究具有重要意义，在目前的海洋CO₂测量仪器中，基于可调谐二极管激光吸收光谱技术(TDLAS)的设备因灵敏度高、环境适应性强等特点受到关注。TDLAS系统的体积和灵敏度通常受限于多次反射腔的大小和光程。针对海洋CO₂脱气量小且灵敏度高的测量需求，自主设计了一套微型多次反射腔，用于TDLAS系统的CO₂测量。该微型多次反射腔采用两片口径为25.4 mm、焦距为50 mm的球面反射镜，以38 mm的腔长实现了253次反射，获得了约10 m的光程，封装后的样品池体积仅有90 mL。基于该微型多次反射腔搭建了一套直接吸收TDLAS的CO₂气体浓度测量系统，通过标准气体对该系统进行了测试，检测限约为26×10-6，不同浓度气体线性相关度R2为99.986%。同时还将该系统与LGR公司生产的便携式温室气体分析仪(UGGA)进行了对比测量，结果表明二者在白天CO₂浓度波动较大和夜晚CO₂浓度变化较平稳两种情况下均表现出较好的一致性，R2大于97%。实验结果证明了系统性能，下一步将优化试验装置并进行现场应用。

Study on TDLAS System with a Miniature Multi-Pass Cavity for CO2 Measurements

The measurement of CO₂ at air-sea interface is significant to marine scientific research. There are many commercial instruments can be used for CO₂ measurements in marine environment, among which the instruments based on tunable diode laser absorption spectroscopy (TDLAS) play an important role due to the advantages of high sensitivity, good environmental adaptability, etc. As a core component of a TDLAS system, the volume and optical path of a multi-pass cavity limit the size and sensitivity of the system. For the application of underwater dissolved CO₂ measurements, due to the samll quantity of degassing gas and high sensitivity, both the small size and long optical path are required for a multi-pass cavity. In this work, a miniature multi-pass cavity was designed for the potential underwater dissolved CO₂ measurements. The miniature multi-pass cavity composed of two identical spherical mirrors (D=25.4 mm, f=50 mm)separated at a distance of 38 mm, realized 253 reflection times, providing an effective optical path length of 10 m and an inner volume of 90 mL. Based on the miniature multi-pass cavity, a direct absorption TDLAS system was developed for CO₂ measurement. The system was evaluated with a series of different concentrations of CO₂ standard gases. The obtained limit of detection (LOD) was about 26×10-6 (volume ratio), and the response was good linear with R2=99.986% over the whole range. A commercial ultraportable greenhouse gas analyzer (UGGA) from Los Gatos Research (LGR) was also used for comparison measurements, and the results showed a consistent trend with R2 of more than 97% under conditions of both high fluctuation at daytime and low changes at nighttime in laboratory. The experiments testified the performance of the developed TDLAS system for CO₂ measurements, which means that the improved system will be used in field experiments in the near future.