Palm-sized methane TDLAS sensor based on a mini-multi-pass cell and a quartz tuning fork as a thermal detector

A palm-sized methane (CH₄) tunable diode laser absorption spectroscopy (TDLAS) sensor is reported, in which a quartz tuning fork (QTF) is used as a thermal detector, working together with a mini-multi-pass cell (mini-MPC) to compose a gas detection module (GDM) with a compact dimension of 78 mm × 40 mm × 40 mm. A 1.65 µm near-infrared distributed feedback (DFB) laser is installed in the sensor for CH₄ detection. A minimum detection limit (MDL) of 52 ppb is achieved at an integration time of 300 ms, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 2.1×10^-8 cm^-1W/Hz^1/2. A seven-day continuous monitoring of atmospheric CH₄ concentration is implemented to verify the sensor's long-term stability.