High precision and sensitivity detection of gas measurement by laser wavelength modulation spectroscopy implementing an optical fringe noise suppression method

In this work, a novel method to suppress optical fringe noise in the tunable diode laser absorption spectroscopy is proposed and applied to the CH4 gas sensor perturbed by optical fringes for higher detection precision and sensitivity. For the 2nd harmonic(2f) signal of 20 ppm CH4 spoiled by optical fringes and random noise, by the novel method, the signal-to-noise ratio (SNR) of the 2f signal is improved about 6.5 times from 17 to 182 with an optimal averaging spectral range Δλ. A ∼1.5 times improvement in the measurement precision of CH4 is achieved compared to unprocessed raw signal. The corresponding minimum detectable concentration can be improved from 3 ppb down to 0.78 ppb. The corresponding noise equivalent absorption sensitivity (NNEA) and the noise equivalent concentration (NEC) of the system is 6.13×10−11 cm−1 W Hz−1/2 and 0.181 ppm, respectively.