Quartz enhanced photoacoustic spectroscopy based trace gas sensors using different quartz tuning forks

Acoustic microresonator based in-plane quartz-enhanced photoacoustic spectroscopy sensor with a line interaction mode

An acoustic microresonator (AmR) based in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) sensor with a line interaction mode is proposed for what is believed to be the first time. The interaction area for the acoustic wave of the proposed AmR, with a slotted sidewall, is not limited to a point of the quartz tuning fork (QTF) prongs, but extends along the whole plane of the QTF prongs. Sixteen types of AmRs are designed to identify the best parameters. Water vapor (H₂O) is chosen as the analyte to verify the reported method. The results indicate that this AmR for IP-QEPAS with a line interaction mode not only provides a high signal level, but also reduces the thermal noise caused by the laser directly illuminating the QTF. Compared with standard IP-QEPAS without an AmR, the minimum detection limit (MDL) is improved by 4.11 times with the use of the technique proposed in this study.

Ppb-level gas detection using on-beam quartz-enhanced photoacoustic spectroscopy based on a 28 kHz tuning fork

In this paper, an on-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a custom quartz tuning fork (QTF) acting as a photoacoustic transducer, was realized and tested. The QTF is characterized by a resonance frequency of 28 kHz, ~15% lower than that of a commercially available 32.7 kHz standard QTF. One-dimensional acoustic micro resonator (AmR) was designed and optimized by using stainless-steel capillaries. The 28 kHz QTF and AmRs are assembled in on-beam QEPAS configuration. The AmR geometrical parameters have been optimized in terms of length and internal diameter. The laser beam focus position and the AmR coupling distance were also adjusted to maximize the coupling efficiency. For comparison, QEPAS on-beam configurations based on a standard QTF and on the 28 kHz QTF were compared in terms of H2O and CO2 detection sensitivity. In order to better characterize the performance of the system, H2O, C2H2 and CO2 were detected for a long time and the long-term stability was analyzed by an Allan variance analysis. With the integration time of 1 s, the detection limits for H2O, C2H2 and CO2 are 1.2 ppm, 28.8 ppb and 2.4 ppm, respectively. The detection limits for H2O, C2H2 and CO2 can be further improved to 325 ppb, 10.3 ppb and 318 ppb by increasing the integration time to 521 s, 183 s and 116 s.

A Review of Antiresonant Hollow-Core Fiber-Assisted Spectroscopy of Gases

Antiresonant Hollow-Core Fibers (ARHCFs), thanks to the excellent capability of guiding light in an air core with low loss over a very broad spectral range, have attracted significant attention of researchers worldwide who especially focus their work on laser-based spectroscopy of gaseous substances. It was shown that the ARHCFs can be used as low-volume, non-complex, and versatile gas absorption cells forming the sensing path length in the sensor, thus serving as a promising alternative to commonly used bulk optics-based configurations. The ARHCF-aided sensors proved to deliver high sensitivity and long-term stability, which justifies their suitability for this particular application. In this review, the recent progress in laser-based gas sensors aided with ARHCFs combined with various laser-based spectroscopy techniques is discussed and summarized.

Ultra-Highly Sensitive Ammonia Detection Based on Light-Induced Thermoelastic Spectroscopy

This invited paper demonstrated an ultra-highly sensitive ammonia (NH₃) sensor based on the light-induced thermoelastic spectroscopy (LITES) technique for the first time. A quartz tuning fork (QTF) with a resonance frequency of 32.768 kHz was employed as a detector. A fiber-coupled, continuous wave (CW), distributed feedback (DFB) diode laser emitting at 1530.33 nm was chosen as the excitation source. Wavelength modulation spectroscopy (WMS) and second-harmonic (2f) detection techniques were applied to reduce the background noise. In a one scan period, a 2f signal of the two absorption lines located at 6534.6 cm⁻¹ and 6533.4 cm⁻¹ were acquired simultaneously. The 2f signal amplitude at the two absorption lines was proved to be proportional to the concentration, respectively, by changing the concentration of NH₃ in the analyte. The calculated R-square values of the linear fit are equal to ~0.99. The wavelength modulation depth was optimized to be 13.38 mA, and a minimum detection limit (MDL) of ~5.85 ppm was achieved for the reported NH₃ sensor.

InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor

Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs-InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 μm in the low-index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer-Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs-InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device's footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy.

Quartz-enhanced photoacoustic spectroscopy employing pilot line manufactured custom tuning forks

Pilot line manufactured custom quartz tuning forks (QTFs) with a resonance frequency of 28 kHz and a Q value of >30, 000 in a vacuum and ∼ 7500 in the air, were designed and produced for trace gas sensing based on quartz enhanced photoacoustic spectroscopy (QEPAS). The pilot line was able to produce hundreds of low-frequency custom QTFs with small frequency shift < 10 ppm, benefiting the detecting of molecules with slow vibrational-translational (V-T) relaxation rates. An Au film with a thickness of 600 nm were deposited on both sides of QTF to enhance the piezoelectric charge collection efficiency and reduce the environmental electromagnetic noise. The laser focus position and modulation depth were optimized. With an integration time of 84 s, a normalized noise equivalent absorption (NNEA) coefficient of 1.7 × 10-8 cm-1∙W∙Hz-1/2 was achieved which is ∼10 times higher than a commercially available QTF with a resonance frequency of 32 kHz.

Quartz-tuning-fork enhanced photothermal spectroscopy for ultra-high sensitive trace gas detection

A gas sensing method based on quartz-tuning-fork enhanced photothermal spectroscopy (QEPTS) is reported in this paper. Unlike usually used thermally sensitive elements, a sharply resonant quartz-tuning-fork with the capability of enhanced mechanical resonance was used to amplify the photothermal signal level. Acetylene (C₂H₂) detection was used to verify the QEPTS sensor performance. The measured results indicate a minimum detection limit (MDL) of 718 ppb and a normalized noise equivalent absorption coefficient (NNEA) of 7.63 × 10^-9 cm^-1W/√Hz. This performance demonstrates that QEPTS can be an ultra-high sensitive technique for gas detection and shows superiority when compared to usually used methods of tunable diode laser absorption spectroscopy (TDLAS) and quartz-enhanced photoacoustic spectroscopy (QEPAS). Furthermore, when compared to an optical detector, especially a costly mercury cadmium telluride (MCT) detector with cryogenic cooling used in TDLAS, a quartz-tuning-fork is much cheap and tiny. Besides, compared to the QEPAS technique, QEPTS is a non-contact measurement technique and therefore can be used for standoff and remote trace gas detection.

Review of Recent Advances in QEPAS-Based Trace Gas Sensing

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is an improvement of the conventional microphone-based photoacoustic spectroscopy. In the QEPAS technique, a commercially available millimeter-sized piezoelectric element quartz tuning fork (QTF) is used as an acoustic wave transducer. With the merits of high sensitivity and selectivity, low cost, compactness, and a large dynamic range, QEPAS sensors have been applied widely in gas detection. In this review, recent developments in state-of-the-art QEPAS-based trace gas sensing technique over the past five years are summarized and discussed. The prospect of QEPAS-based gas sensing is also presented.

Quartz-Enhanced Photoacoustic Spectroscopy Sensor with a Small-Gap Quartz Tuning Fork

A highly sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a custom quartz tuning fork (QTF) with a small-gap of 200 μm was demonstrated. With the help of the finite element modeling (FEM) simulation software COMSOL, the change tendency of the QEPAS signal under the influence of the laser beam vertical position and the length of the micro-resonator (mR) were calculated theoretically. Water vapor (H₂O) was selected as the target analyte. The experimental results agreed well with those of the simulation, which verified the correctness of the theoretical model. An 11-fold signal enhancement was achieved with the addition of an mR with an optimal length of 5 mm in comparison to the bare QTF. Finally, the H₂O-QEPAS sensor, which was based on a small-gap QTF, achieved a minimum detection limit (MDL) of 1.3 ppm, indicating an improvement of the sensor performance when compared to the standard QTF that has a gap of 300 μm.

HCN ppt-level detection based on a QEPAS sensor with amplified laser and a miniaturized 3D-printed photoacoustic detection channel

Ultra-high sensitive and stable detection of hydrogen cyanide (HCN) based on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor was realized using an erbium-doped fiber amplifier (EDFA) as well as a miniaturized 3D-printed photoacoustic detection channel (PADC) for the first time. A HCN molecule absorption line located at 6536.46 cm^-1 was selected which was in the range of the EDFA emission spectrum. The detection sensitivity of the reported EDFA-QEPAS sensor was enhanced significantly due to the high available EDFA excitation laser power. A 3D printing technique was used to develop the compact PADC, resulting in a size of 29 × 15 × 8 mm³ and a mass of ~5 g in order to improve the sensor stability and implement sensor applications requiring a compact size and light weight. At atmospheric pressure, room temperature and a 1 s acquisition time, a minimum detection limit (MDL) of 29 parts per billion (ppb) was achieved, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.08 × 10^-8 cm^-1W/Hz^-1/2. The long-term performance and the stability of the HCN EDFA-QEPAS sensor system were investigated using an Allan deviation analysis. It indicated that the MDL can be improved to 220 parts per trillion (ppt) with an integration time of 300 s, which demonstrated this compact, integrated and miniaturized 3D-printed PADC based sensor had an excellent stability.

A Miniaturized QEPAS Trace Gas Sensor with a 3D-Printed Acoustic Detection Module

A 3D printing technique was introduced to a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and is reported for the first time. The acoustic detection module (ADM) was designed and fabricated using the 3D printing technique and the ADM volume was compressed significantly. Furthermore, a small grin lens was used for laser focusing and facilitated the beam adjustment in the 3D-printed ADM. A quartz tuning fork (QTF) with a low resonance frequency of 30.72 kHz was used as the acoustic wave transducer and acetylene (C₂H₂) was chosen as the analyte. The reported miniaturized QEPAS trace gas sensor is useful in actual sensor applications.

A Portable Laser Photoacoustic Methane Sensor Based on FPGA

A portable laser photoacoustic sensor for methane (CH₄) detection based on a field-programmable gate array (FPGA) is reported. A tunable distributed feedback (DFB) diode laser in the 1654 nm wavelength range is used as an excitation source. The photoacoustic signal processing was implemented by a FPGA device. A small resonant photoacoustic cell is designed. The minimum detection limit (1σ) of 10 ppm for methane is demonstrated.

Improved Tuning Fork for Terahertz Quartz-Enhanced Photoacoustic Spectroscopy

We report on a quartz-enhanced photoacoustic (QEPAS) sensor for methanol (CH₃OH) detection employing a novel quartz tuning fork (QTF), specifically designed to enhance the QEPAS sensing performance in the terahertz (THz) spectral range. A discussion of the QTF properties in terms of resonance frequency, quality factor and acousto-electric transduction efficiency as a function of prong sizes and spacing between the QTF prongs is presented. The QTF was employed in a QEPAS sensor system using a 3.93 THz quantum cascade laser as the excitation source in resonance with a CH₃OH rotational absorption line located at 131.054 cm⁻¹. A minimum detection limit of 160 ppb in 30 s integration time, corresponding to a normalized noise equivalent absorption NNEA = 3.75 × 10⁻¹¹ cm⁻¹W/Hz^½, was achieved, representing a nearly one-order-of-magnitude improvement with respect to previous reports.

Compact quantum cascade laser based quartz-enhanced photoacoustic spectroscopy sensor system for detection of carbon disulfide

A compact gas sensor system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a continuous wave (CW) distributed feedback quantum cascade laser (DFB-QCL) operating at 4.59 µm was developed for detection of carbon disulfide (CS₂) in air at trace concentration. The influence of water vapor on monitored QEPAS signal was investigated to enable compensation of this dependence by independent moisture sensing. A 1 σ limit of detection of 28 parts per billion by volume (ppbv) for a 1 s lock-in amplifier time constant was obtained for the CS₂ line centered at 2178.69 cm^-1 when the gas sample was moisturized with 2.3 vol% H₂O. The work reports the suitability of the system for monitoring CS₂ with high selectivity and sensitivity, as well as low sample gas volume requirements and fast sensor response for applications such as workplace air and process monitoring at industry.

Analysis of overtone flexural modes operation in quartz-enhanced photoacoustic spectroscopy

A detailed investigation of a set of custom quartz tuning forks (QTFs), operating in the fundamental and first overtone flexural modes is reported. Support losses are the dominant energy dissipation processes when the QTFs vibrate at the first overtone mode. These losses can be decreased by increasing the ratio between the prong length and its thickness. The QTFs were implemented in a quartz enhanced photoacoustic spectroscopy (QEPAS) based sensor operating in the near-IR spectral range and water vapor was selected as the gas target. QTF flexural modes having the highest quality factor exhibit the largest QEPAS signal, demonstrating that, by optimizing the QTF prongs sizes, overtone modes can provide a higher QEPAS sensor performance with respect to using the fundamental mode.

Fiber-amplifier-enhanced QEPAS sensor for simultaneous trace gas detection of NH₃ and H₂S

A selective and sensitive quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplifier (EDFA), and a distributed feedback (DFB) laser operating at 1582 nm was demonstrated for simultaneous detection of ammonia (NH₃) and hydrogen sulfide (H₂S). Two interference-free absorption lines located at 6322.45 cm⁻¹ and 6328.88 cm⁻¹ for NH₃ and H₂S detection, respectively, were identified. The sensor was optimized in terms of current modulation depth for both of the two target gases. An electrical modulation cancellation unit was equipped to suppress the background noise caused by the stray light. An Allan-Werle variance analysis was performed to investigate the long-term performance of the fiber-amplifier-enhanced QEPAS sensor. Benefitting from the high power boosted by the EDFA, a detection sensitivity (1σ) of 52 parts per billion by volume (ppbv) and 17 ppbv for NH₃ and H₂S, respectively, were achieved with a 132 s data acquisition time at atmospheric pressure and room temperature.