A Prototype of ppbv-Level Midinfrared CO2 Sensor for Potential Application in Deep-Sea Natural Gas Hydrate Exploration

For deep-sea natural gas hydrate (NGH) exploration, the highly sensitive detection of dissolved gas in seawater near the seabed is important. A prototype of a midinfrared (MIR) carbon dioxide (CO ₂ ) sensing system based on a wavelength modulation spectroscopy (WMS) technique is developed using a thermoelectrically cooled continuous-wave interband cascade laser at 4319.2 nm and an optical multipass cell (20 × 7.6 × 10.5 cm ³ ) with a 24-m effective optical path. A MIR HgCdTe detector and a lock-in amplifier are used to detect and demodulate light source signals. The reported sensing system utilizes a linear optical structure to achieve a highly precision detection performance. The minimum detection limit (MDL) of 48.2 parts per billion (ppb) (at 74 s) at a gas pressure of 20 Torr for the 2f-WMS technique was achieved. The MDL was improved to 38.6 ppbv (at 89 s) at a gas pressure of 20 Torr using the 2f/1f-WMS technique, which reduces the effects of laser power drift. The CO ₂ sensor was deployed with a gas-liquid separator for CO ₂ detection in the gas extracted from water. Results validated the reported sensor system's potential application to deep-sea NGH exploration.