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

Abstract

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 3.34μm and a dense multi-pass gas cell (MPGC, 17×6.5×5.5cm3) with a 54.6m optical path length. A compact optical sensor system with a physical size of 35.5×18×12.5cm3 was designed and constructed. An ICL was employed for targeting a strong C2H6 line at 2996.88cm−1 at <100Torr 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 ∼1.0ppbv for 2f/1f-WMS were achieved, respectively, with a measurement time of 4s. The MDL was further improved from 299pptv (@108s for 2f-WMS) to 239pptv (@208s for 2f/1f-WMS), based on an Allan deviation analysis. The rise time (@0→100ppbv) and fall time (@100→0ppbv) were determined to be ∼64s and ∼48s, respectively, at a gas pressure of <100Torr for the C2H6 sensor operation.