Two compact TDLAS sensor systems based on different structural optical cores were developed. The two optical cores combine two recent developments, gallium antimonide (GaSb)-based ICL and a compact multipass gas cell (MPGC) with the goal to create compact TDLAS based sensors for the mid-IR gas detection with high detection sensitivity and low power consumption. The sensors achieved minimum detection limits of ~5 ppbv and ~8 ppbv, respectively, for CH₄ and C₂H₆ concentration measurements with a 3.7-W power consumption.

A mid-infrared ethane (C 2 H 6) 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 cm 3) with a 54.6 m optical path length. A compact optical sensor system with a physical size of 35.5×18×12.5 cm 3 was designed and constructed. An ICL was employed for targeting a strong C 2 H 6 line at 2996.88 cm-1 at < 100 Torr gas pressure in the fundamental absorption band of C 2 H 6. 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 C 2 H 6 sensor operation.

A multi-gas sensor system was developed that uses a single broadband light source and multiple carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4) pyroelectric detectors by use of the time division multiplexing (TDM) technique. A stepper motor-based rotating system and a single-reflection spherical optical mirror were designed and adopted to realize and enhance multi-gas detection. Detailed measurements under static detection mode (without rotation) and dynamic mode (with rotation) were performed to study the performance of the sensor system for the three gas species. Effects of the motor rotating period on sensor performances were also investigated and a rotation speed of 0.4π rad/s was required to obtain a stable sensing performance, corresponding to a detection period of ~10 s to realize one round of detection. Based on an Allan deviation analysis, the 1σ detection limits under static operation are 2.96, 4.54 and 2.84 parts per million in volume (ppmv) for CO, CO2 and CH4, respectively and the 1σ detection limits under dynamic operations are 8.83, 8.69 and 10.29 ppmv for the three gas species, respectively. The reported sensor has potential applications in various fields requiring CO, CO2 and CH4 detection such as in coal mines.

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A mid-infrared methane (CH 4 ) sensor without pressure control was developed using a continuous-wave (CW) interband cascade laser (ICL) for targeting a CH 4 absorption line located at 3038.5 cm -1 . A multi-pass gas cell with an absorption path length of 54.6 m was utilized for enhancing gas absorption.The pressure inside the MPGC was measured using direct Lorentzian absorption fitting for the compensation of CH 4 concentration changes resulting from pressure variations. Laboratory pressure calibration was conducted in the range of 25 − 800 Torr using 1.3-, 1.5-, 1.7-and 2.1-ppmv CH 4 samples.A pressure precision of ~ 1.65 Torr with a ~ 2.5-s averaging time was achieved based on the measurement of a 2.1-ppmv CH 4 sample at 700-Torr. Concentration level measurements of a 2.1-ppmv CH 4 sample at a 700-Torr pressure yielded an Allan deviation of 2.25 ppbv for an averaging time of 2.5 s.The sensor functioned normally with CH 4 samples at 1.0, 1.2, 1.4, 1.6 and 2.1 ppmv concentration levels as the pressure changes from 25 to 800 Torr. Indoor/outdoor CH 4 concentration measurements on the Rice University campus and a field campaign in the Greater Houston Area (GHA) were conducted to evaluate the performance of the sensor system.

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A continuous-wave (CW) interband cascade laser (ICL) based mid-infrared sensor system was demonstrated for simultaneous detection of atmospheric methane (CH₄) and ethane (C₂H₆). A 3.337 µm CW ICL with an emitting wavenumber range of 2996.0-3001.5 cm^-1 was used to simultaneously target two absorption lines, C₂H₆ at 2996.88 cm^-1 and CH₄ at 2999.06 cm^-1, respectively. The sensor performance was first evaluated for single-gas detection by only targeting the absorption line of one gas species. Allan deviations of 11.2 parts per billion in volume (ppbv) for CH₄ and 1.86 ppbv for C₂H₆ with an averaging time of 3.4 s were achieved for the detection of these two gases. Dual-gas detection was realized by using a long-term scan signal to target both CH₄ and C₂H₆ lines. The Allan deviations increased slightly to 17.4 ppbv for CH₄ and 2.4 ppbv for C₂H₆ with an averaging time of 4.6 s due to laser temperature and power drift caused by long-term wavelength scanning. Measurements for both indoor and outdoor concentration changes of CH₄ and C₂H₆ were conducted. The reported single ICL based dual-gas sensor system has the advantages of reduced size and cost compared to two separate sensor systems.

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