A Sensitive Methane Sensor Of A Ppt Detection Level Using A Mid-Infrared Interband Cascade Laser And A Long-Path Multipass Cell

An ultra-high sensitivity sensor combining a mid-IR (MIR) interband cascade laser (ICL) at center wavelength of 3.37 ?m and a multi-pass cell (MPC) with a path-length of 580 m was designed and implemented for methane detection. The ultra-high sensitivity is achieved due to the higher absorbance in the MIR than in the near infrared (NIR), a long interaction path length, and the implementation of a wavelet denoising processing algorithm. The performance of the sensor was evaluated with direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) in a series of measurements. For the sensor working in the DAS measurement mode, the optimal set of sub-wavelet functions was chosen and utilized for denoising. A calibration gas with methane concentration of 2.0 ppm was measured with a relative error of ? 1%. The minimum of the Allan deviation of the sensor of 3 ppb was achieved with an averaging time of 200 s. For the WMS technique with the Allan deviation analysis the sensor detection limit for methane of 560 ppt with 290 s averaging time was determined, which is a significant improvement compared to our DAS results and previous reports. The developed sensor has broad applicability in the high-sensitivity measurements of methane and other trace gases.