Compact TDLAS based optical sensor for ppb-level ethane detection by use of a 3.34 μm room-temperature CW interband cascade laser

A mid-infrared ethane (C2H6) sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34 μm and a dense multi-pass gas cell (MPGC, 17 × 6.5 × 5.5 cm3) with a 54.6 m optical path length. A compact optical sensor system with a physical size of 35.5 × 18 × 12.5 cm3 was designed and constructed. An ICL was employed for targeting a strong C2H6 line at 2996.88 cm−1 at <100 Torr gas pressure in the fundamental absorption band of C2H6. The sensor performance, including the minimum detection limit (MDL) and the stability were improved by reducing the effect of laser power drift by means of the 2f/1f-WMS technique. A MDL of ∼1.2 parts per billion (ppbv) for 2f-WMS and ∼1.0 ppbv for 2f/1f-WMS were achieved, respectively, with a measurement time of 4 s. The MDL was further improved from 299 pptv (@108 s for 2f-WMS) to 239 pptv (@208 s for 2f/1f-WMS), based on an Allan deviation analysis. The rise time (@0 → 100 ppbv) and fall time (@100 → 0 ppbv) were determined to be ∼64 s and ∼48 s, respectively, at a gas pressure of <100 Torr for the C2H6 sensor operation.