Weilin Ye

Mid-infrared dual-gas sensor for simultaneous detection of methane and ethane using a single continuous-wave interband cascade laser

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.

Infrared Dual-Gas CH4/C2H6Sensor Using Two Continuous-Wave Interband Cascade Lasers

An infrared dual-gas sensor system for the simultaneous detection and monitoring of methane (CH₄) and ethane (C₂H₆) at parts-per-billion by volume (ppbv) concentration levels was developed using two room temperature, distributed feedback interband cascade lasers, and two miniature multipass cells with an effective absorption length of 54.6 m. Laser direct absorption spectroscopy was used to detect CH₄ utilizing the 3038.5-cm^-1 absorption line, and second-harmonic wavelength modulation spectroscopy method was used to detect C₂H₆ using the 2996.88-cm^-1 absorption line. The 1σ CH₄ detection limit is ~2.7 ppbv with a 1-s averaging time and exhibits a minimum value of ~1.7 ppbv for a 9-s averaging time; the 1σ C₂H₆ detection limit is ~2.6 ppbv with a 3.4-s averaging time and shows an optimum averaging time of 65 s corresponding to a stability of ~0.36 ppbv. Using the dualgas sensor system, 24 h monitoring of the two atmospheric gases was performed in the Greater Houston area, TX, USA.

Compact photoacoustic module for methane detection incorporating interband cascade light emitting device

A photoacoustic module (PAM) for methane detection was developed by combining a novel 3.2 μm interband cascade light emitting device (ICLED) with a compact differential photoacoustic cell. The ICLED with a 22-stage interband cascade active core emitted a collimated power of ~700 μW. A concave Al-coat reflector was positioned adjacent to the photoacoustic cell to enhance the gas absorption length. Assembly of the ICLED and reflector with the photoacoustic cell resulted in a robust and portable PAM without any moving parts. The PAM performance was evaluated in terms of operating pressure, sensitivity and linearity. A 1σ detection limit of 3.6 ppmv was achieved with a 1-s integration time.

Interband cascade laser based mid-infrared methane sensor system using a novel electrical-domain self-adaptive direct laser absorption spectroscopy (SA-DLAS)

To suppress sensor noise with unknown statistical properties, a novel self-adaptive direct laser absorption spectroscopy (SA-DLAS) technique was proposed by incorporating a recursive, least square (RLS) self-adaptive denoising (SAD) algorithm and a 3291 nm interband cascade laser (ICL) for methane (CH4) detection. Background noise was suppressed by introducing an electrical-domain noise-channel and an expectation-known-based RLS SAD algorithm. Numerical simulations and measurements were carried out to validate the function of the SA-DLAS technique by imposing low-frequency, high-frequency, White-Gaussian and hybrid noise on the ICL scan signal. Sensor calibration, stability test and dynamic response measurement were performed for the SA-DLAS sensor using standard or diluted CH4 samples. With the intrinsic sensor noise considered only, an Allan deviation of ~43.9 ppbv with a ~6 s averaging time was obtained and it was further decreased to 6.3 ppbv with a ~240 s averaging time, through the use of self-adaptive filtering (SAF). The reported SA-DLAS technique shows enhanced sensitivity compared to a DLAS sensor using a traditional sensing architecture and filtering method. Indoor and outdoor atmospheric CH4 measurements were conducted to validate the normal operation of the reported SA-DLAS technique.

A near-infrared carbon dioxide sensor system using a compact folded optical alignment structure

A compact optical alignment structure and a novel beam-tracing method were proposed for tunable laser absorption spectroscopy (TLAS) based gas measurements, in order to minimize sensor size and ease beam alignment procedure. A near-infrared carbon dioxide (CO2) sensor system was developed based on the alignment structure. A distributed feedback (DFB) laser centered at 6361.3 cm-1 and a multi-pass gas cell (MPGC) with an effective optical path length of 29.8 m were employed. The sensor system was integrated as standalone equipment by customizing an aluminum baseplate for a stable field operation. A series of experiments were carried out to assess the performance of the sensor system. A limit of detection (LoD) of ~ 7.1 parts-per-million in volume (ppmv) at a 0.4 s averaging time was obtained, and the LoD was reduced to ~ 277 parts-per-billion in volume (ppbv) at an optimum averaging time of 153.6 s. Considering gas mixing times, the rise and fall time were measured to be ~ 290 s and ~ 200 s, respectively.

Repetitively Mode-Locked Cavity-Enhanced Absorption Spectroscopy (RML-CEAS) for Near-Infrared Gas Sensing

A Pound-Drever-Hall (PDH)-based mode-locked cavity-enhanced sensor system was developed using a distributed feedback diode laser centered at 1.53 µm as the laser source. Laser temperature scanning, bias control of the piezoelectric ceramic transducer (PZT) and proportional-integral-derivative (PID) feedback control of diode laser current were used to repetitively lock the laser modes to the cavity modes. A gas absorption spectrum was obtained by using a series of absorption data from the discrete mode-locked points. The 15 cm-long Fabry-Perot cavity was sealed using an enclosure with an inlet and outlet for gas pumping and a PZT for cavity length tuning. The performance of the sensor system was evaluated by conducting water vapor measurements. A linear relationship was observed between the measured absorption signal amplitude and the H2O concentration. A minimum detectable absorption coefficient of 1.5 × 10–8 cm–1 was achieved with an averaging time of 700 s. This technique can also be used for the detection of other trace gas species by targeting the corresponding gas absorption line.

A compact mid-infrared dual-gas CH4/C2H6 sensor using a single interband cascade laser and custom electronics

A compact mid-infrared (MIR) dual-gas sensor system was demonstrated for simultaneous detection of methane (CH4) and ethane (C2H6) using a single continuous-wave (CW) interband cascade laser (ICL) based on tunable laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS). Ultracompact custom electronics were developed, including a laser current driver, a temperature controller and a lock-in amplifier. These custom electronics reduce the size and weight of the sensor system as compared with a previous version based on commercial electronics. A multipass gas cell with an effective optical length of 54.6 m was employed to enhance the absorption signal. A 3337 nm ICL was capable of targeting a C2H6 absorption line at 2996.88 cm-1 and a CH4 line at 2999.06 cm-1. Dual-gas detection was realized by scanning both the CH4 and C2H6 absorption lines. Based on an Allan deviation analysis, the 1 σ minimum detection limit (MDL) was 17.4 ppbv for CH4 and 2.4 ppbv for C2H6 with an integration time of 4.3 s. TDLAS based sensor measurements for both indoor and outdoor mixing ratios of CH4 and C2H6 were conducted. The reported single ICL based dual-gas sensor system has the advantages of reduced size and cost without influencing the midinfrared sensor detection sensitivity, selectivity and reliability.

Trace gas spectroscopy using state-of-the- art mid-infrared semiconductor laser sources: progress, status, and applications

The recent development of interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) based trace gas sensors enables the targeting of strong fundamental rotational-vibrational transitions in the mid-infrared, which are one to two orders of magnitude more intense than transitions in the near-infrared. This has led to the development of mid-infrared compact, field deployable sensors based on two sensor system platforms, laser absorption and quartz enhanced spectroscopy. These sensor platforms are applicable for environmental monitoring, atmospheric chemistry and for use in the petrochemical industry. The spectroscopic detection and monitoring of three molecular species, methane (CH4), ethane (C2H6) [1], formaldehyde (H2CO) [2] and hydrogen sulphide (H2S) [3] will be described.