Mid-infrared methane sensor system using self-adaptive interband cascade laser absorption spectroscopy.*

  In order to suppress sensor noises with unknown statistical property, a novel self-adaptive direct laser absorption spectroscopy (SA-DLAS) technique was proposed in a 3.291 µm continuous-wave (CW) interband cascade laser (ICL) based mid-infrared methane (CH₄) sensor system. Background noises can be well evaluated and suppressed by introducing an additional electrical-domain noise-channel and a modern expectation-known-based recursive least square (RLS) self-adaptive denoising (SAD) algorithm. Both numerical simulations and experiments were carried out to study the SA-DLAS sensor’s denoising and gas detection performances by imposing low-frequency/high-frequency/White-Gaussian/composite noises on the laser scan signal. Both indoor and outdoor atmospheric CH₄ measurements were conducted to evaluate the field sensor performance. Under an unknown noise environment, the reported SA-DLAS technique shows enhanced sensitivity and reliability compared to a DLAS sensor using classic sensing architecture and filtering method, which can also be extended to other infrared gas sensing applications.