Standoff LITES Sensor Based on a Frequency‐Stabilized Encased Quartz Tuning Fork

ABSTRACT We propose and demonstrate a standoff light‐induced thermoelastic spectroscopy (standoff‐LITES) sensor for the first time. An encased quartz tuning fork (QTF) was employed in the standoff‐LITES sensor to improve the frequency stability. The QTF operating temperature was optimized to be ∼0°C. A distributed feedback (DFB) interband cascade laser (ICL) emitting at 3.27 µm was employed, whose emission wavenumber range of 3056.8–3058.1 cm −1 covers the absorption lines of water (H 2 O) and methane (CH 4 ). For the standoff gas sensing concept demonstration, the sensor employed an off‐axis parabolic mirror and a cooperative target positioned 5 m away from the mirror, creating a 10‐meter absorption path. The developed standoff‐LITES sensor system achieved minimum detection limits (MDLs) of 8.2 ppm for H 2 O and 1.6 ppb for CH 4 , corresponding to the NNEA of 6.7 × 10 −9 W·cm −1 Hz −1/2 and 6.3 × 10 −9 W·cm −1 Hz −1/2 , respectively. During a one‐week open‐field monitoring experiment, this sensor continuously measured the ambient H 2 O and CH 4 concentrations, and the acquired data were basically consistent with the observation data from the local meteorological station and atmospheric background station in terms of variation trends. These results verify that the standoff‐LITES sensor presented here possesses considerable application potential for long‐term standoff monitoring of atmospheric trace gases.