A Dual-Laser Sensor Based on Off-Axis Integrated Cavity Output Spectroscopy and Time-Division Multiplexing Method

In this article, a compact dual-laser sensor based on an off-axis integrated-cavity output spectroscopy and time-division multiplexing method is reported. A complete dual-channel optical structure is developed and integrated on an optical cavity, which allows two distributed feedback (DFB) lasers operating at wavelengths of 1603 nm and 1651 nm to measure the concentration of CO₂ and CH₄, simultaneously. Performances of the dual-laser sensor are experimentally evaluated by using standard air (with a mixture of CO₂ and CH₄). The limit of detection (LoD) is 0.271 ppm and 1.743 ppb at a 20 s for CO₂ and CH₄, respectively, and the noise equivalent absorption sensitivities are 2.68 × 10^-10 cm^-1 Hz^-1/2 and 3.88 × 10^-10 cm^-1 Hz^-1/2, respectively. Together with a commercial instrument, the dual-laser sensor is used to measure CO₂ and CH₄ concentration over 120 h and verify the regular operation of the sensor for the detection of ambient air. Furthermore, a first-order exponential moving average algorithm is implemented as an effective digital filtering method to estimate the gas concentration.