Stability of CO₂ Gas Sensor Based on Hollow Waveguide Fiber

When a laser is disturbed by low-frequency factors ( e. g. , thermal fluctuations and mechanical vibrations), laser wavelength drift and light intensity fluctuations occur, which affect the measurement accuracy of the system. In this study, a 2.73-ttm distributed feedback (DFB) laser is used as the detection light source and a set of CO2 gas-sensing system based on hollow waveguide fiber is built. The proposed system uses the third harmonic absorption signal (3f ) of CO2 to achieve the frequency stabilization of the laser and then uses the harmonic signal to reverse the concentration of CO2. In the experiment, the standard CO2 gas is measured for a long time and the system is analyzed according to four measurement methods: second harmonic (2f), stabilized second harmonic (2f-lock) , second harmonic ratio first harmonic (2f/1f), and stabilized second harmonic ratio first harmonic (2f/1f -lock) . The system measurement accuracy obtained under the 2f-lock condition is 0.001255, which is 2.4 times higher than that under the 2f condition. The system measurement accuracy obtained under 2f/1f-lock condition is 0.00138, which is 2.34 times higher than that under 2f/1f condition. It can be seen that the 2f/1f-lock method has the longest stable time of 210 s and the lowest detection limit of 4. 24 X 10(-5). Comprehensive analyses reveal that 2f/1f-lock is the most optimal among the four methods. Kalman filtering is performed on the measured data under the 2f/lf-lock condition, and the measurement accuracy of the filtered system is 0.0002786, which is 4.95 times higher than that before the filtering.