High-precision quasi-synchronous measurement of CH4 and H2O with large spacing based on temperature periodic variation of a single laser and concentration averaging method

Using a single distributed-feedback (DFB) laser with a central wavelength of 1.65 μm, periodic measurements of CH4 and H2O absorption lines separated by 6.5 cm-1 were achieved.

By demodulating the rising and falling edges of the modulated signals containing absorption information obtained from dual optical path channels of the dual-path multi-pass cell (DP-MPC), four R2f/1f harmonic signals were generated. These harmonic signals were then inverted and averaged to obtain an average concentration signal, improving the performance of the sensor. Allan variance analysis showed that averaging concentration signals from four edges improves CH4 measurement precision to 9.9 ppb and H2O precision to 35.9 ppm under a 1-s integration time, representing enhancements of at least 1.93 times and 1.81 times, respectively, compared to single-edge measurements.