Ppbv-Level Ethane Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a Continuous-Wave, Room Temperature Interband Cascade Laser

Roadmap on optical sensors

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

Small-volume highly-sensitive all-optical gas sensor using non-resonant photoacoustic spectroscopy with dual silicon cantilever optical microphones

A small-volume highly-sensitive photoacoustic spectroscopy (PAS) methane detection system based on differential silicon cantilever optical microphones (SCOMs) is proposed and experimentally demonstrated. The system contains a compact non-resonant photoacoustic cell with a small volume of 1.2 mL and symmetrically-located dual SCOMs, as well as a distributed feedback laser at 1650.96 nm. The two identical SCOMs utilize the Fabry-Perot interferometric fiber-optic structure, with the differential Q-point demodulation algorithm to suppress the external vibration noise. Experimental results show that the SCOM has a high displacement sensitivity about 7.1 µm/Pa at 150 Hz and within 2.5 dB fluctuation between 5 Hz and 250 Hz. In the PAS gas sensing experiment, the normalized noise equivalent absorption coefficient of the PAS system is estimated to be 1.2 × 10^-9 cm^-1·W·Hz^-1/2 and the minimum detection limit for methane is about 111.2 ppb with 1 s integration time. External disturbance is also applied to the dual SCOM system and results show excellent stability and noise resistance. The proposed PAS system exhibits superiorities of low gas consumption, high sensitivity and immunity to vibration and electromagnetic interference, which has an enormous potential in medicine, industry and environment.

High-concentration methane and ethane QEPAS detection employing partial least squares regression to filter out energy relaxation dependence on gas matrix composition

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor capable to detect high concentrations of methane (C1) and ethane (C2) is here reported. The hydrocarbons fingerprint region around 3 µm was exploited using an interband cascade laser (ICL). A standard quartz tuning fork (QTF) coupled with two resonator tubes was used to detect the photoacoustic signal generated by the target molecules. Employing dedicated electronic boards to both control the laser source and collect the QTF signal, a shoe-box sized QEPAS sensor was realized. All the generated mixtures were downstream humidified to remove the influence of water vapor on the target gases. Several natural gas-like samples were generated and subsequently diluted 1:10 in N₂. In the concentration ranges under investigation (1%-10% for C1 and 0.1%-1% for C2), both linear and nonlinear responses of the sensor were measured and signal variations due to matrix effects were observed. Partial least squares regression (PLSR) was employed as a multivariate statistical tool to accurately determine the concentrations of C1 and C2 in the mixtures, compensating the matrix relaxation effects. The achieved results extend the range of C1 and C2 concentrations detectable by QEPAS technique up to the percent scale.

Multicomponent gas detection technology of FDM and TDM based on photoacoustic spectroscopy

In this paper, a multicomponent gas detection system based on photoacoustic spectroscopy (PAS) is proposed with a combination of frequency division multiplexing (FDM) and time division multiplexing (TDM), combining a resonance photoacoustic cell and broadband microphone. A PAS gas cell with a wide frequency response bandwidth was used to achieve the FDM by selecting a specific modulation frequency of each component gas. The sawtooth wave driver current of each laser was output at a constant time interval for achieving the TDM. Compared with the laser channel control using a photoswitch, the driver current control was a simpler and more convenient means to implement TDM. The four gas components of methane (CH₄), water (H₂O) vapor, carbon dioxide (CO₂), and acetylene (C₂H₂) were selected as sample gases for testing the feasibility of the method. The experimental results showed that the gas detection limits of CH₄, H₂O vapor, CO₂, and C₂H₂ were 75.435, 2.502, 341.960, and 4.284 ppm, respectively. In addition, the linear fittings of gas concentration were 0.99386, 0.99772, 0.98995, and 0.98955, respectively.

Quartz-enhanced photoacoustic spectroscopic methane sensor system using a quartz tuning fork-embedded, double-pass and off-beam configuration

Development of a methane (CH₄) sensor system was reported based on a novel quartz-tuning-fork (QTF)-embedded, double-pass, off-beam quartz-enhanced photoacoustic spectroscopy (DP-OB-QEPAS). A simplified and accurate numerical model was presented to optimize the DP-OB-QEPAS spectrophone and to enhance the detection sensitivity. A compact and fiber-coupled acoustic detection module (ADM) with a volume of 3 × 2×1 cm³ and a weight of 9.7 g was fabricated. A continuous-wave distributed feedback diode laser was used to target the CH₄ absorption line at 6046.95 cm^-1. With the combination of wavelength modulation spectroscopy (WMS) and second harmonic (2f) detection technique, the CH₄ sensor system reveals a 1σ detection limit of 8.62 parts-per-million in volume (ppmv) for a 0.3 s averaging time with an optimized modulation depth of 0.26 cm^-1. The proposed CH₄ sensor shows a similar or even lower level in the normalized noise equivalent absorption coefficient (NNEA) (1.8 × 10^-8 cm^-1∙W/√Hz), compared to previously reported QEPAS-based CH₄ sensors.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.