Mid‐infrared dissolved CO2 measurement system using the ultra‐compact multipass cell

To measure natural gas hydrate, a mid‐infrared dissolved CO2 measurement system is designed using the tunable diode laser spectroscopy with ultra‐compact multipass cell. The QCL laser of 4319 nm has a scanning range from 2315.05 to 2315.40 cm−1, which contains 12CO2 and 13CO2 for isotope measurement and wide range of concentration detection. The optimized 3f + 1f wavelength modulation method reduces the limit of detection to 7.1 ppbv. An ultra‐compact multi‐pass cell with effective optical path length of 6.3 m and small volume of 15 ml is designed to achieve the system response time of 40 s. The dissolved CO2 sensing system has been used in deep sea natural gas hydrate detection.