Doubly resonant sub-ppt photoacoustic gas detection with eight decades dynamic range

Multicomponent Gas Measurement Method Based on Miniaturized and Scalable Multiresonator Photoacoustic Spectroscopy

This paper presents and demonstrates a multicomponent gas measurement method based on miniaturized and expandable multiresonator photoacoustic spectroscopy (MR-PAS). Existing multiresonant photoacoustic cells (MR-PAC) with parallel-placed H-type structures are large in size and difficult to expand the number of resonators due to the need for separate collimation. In this paper, a new type of MR-PAC with acoustic resonators uniformly distributed in the vertical section of the absorption cavity was designed based on T-type structure. The shared absorption cavity allows multiple laser beams to be combined and then collimated, while the buffer cavity is not required, which significantly reduces the volume. The circumferential distribution of the acoustic resonators solves the problem of expanding the number of detected gases. The cavity volume of the MR-PAC with four acoustic resonators designed in this paper was only 154.34 μL. The feasibility and performance of MR-PAC were verified by the simultaneous detection of CH4, C2H2, C2H6, and C2H4. Within an integration time of 1000 s, the minimum detectable concentrations were as low as 86.1, 4.3, 188.1, and 127.9 ppb, respectively. The MR-PAC designed in this paper has the advantages of miniaturization and scalability, which has high potential for application in the field of multicomponent gas sensing.

Improved T-shaped quartz tuning fork with isosceles-trapezoidal grooves optimized for quartz-enhanced photoacoustic spectroscopy

The quartz tuning fork (QTF) being the acoustic-electrical conversion element for quartz-enhanced photoacoustic spectroscopy (QEPAS) system directly affects the detection sensitivity. However, the low electromechanical conversion efficiency characteristic of standard QTF limits the further enhancement of the system. Therefore, the optimized design for QTF is becoming an important approach to improve the performance of QEPAS. In this work, 9 kHz T-shaped QTFs with isosceles-trapezoidal grooves are firstly applied to gas sensing experiments. Four types of 9 kHz QTFs are fabricated and applied to gas detection experiments. Simulation results reveal QTFs with isosceles-trapezoidal grooves are conducive to optimizing the stress distribution and enhancing electromechanical conversion efficiency. The results of the gas sensing experiment (acetylene C2H2) indicate that the signal peak and signal-to-noise ratio values of T-shaped QTF with positive isosceles-trapezoidal grooves can reach 1.44 and 1.85 times greater than the normal QTF with rectangular cross-section prongs.

Methane, Ethane, and Propane Detection Using a Quartz-Enhanced Photoacoustic Sensor for Natural Gas Composition Analysis

A compact and portable gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) for the detection of methane (C1), ethane (C2), and propane (C3) in natural gas (NG)-like mixtures is reported. An interband cascade laser (ICL) emitting at 3367 nm is employed to target absorption features of the three alkanes, and partial least-squares regression analysis is employed to filter out spectral interferences and matrix effects characterizing the examined gas mixtures. Spectra of methane, ethane, and propane mixtures diluted in nitrogen are employed to train and test the regression algorithm, achieving a prediction accuracy of ∼98%, ∼96%, and ∼93% on C1, C2, and C3, respectively. With respect to previously reported QEPAS sensors for natural gas analysis, the high prediction accuracy as well as the capability to discriminate and detect C3 within natural gas-like complex mixtures provided by the employment of partial least-squares regression mark significant improvements. Furthermore, these results enable an improved performance of the sensor for in situ, real-time, and online natural gas composition analysis.

Calibration-free infrared absorption spectroscopy using cantilever-enhanced photoacoustic detection of the optical power

We report on sensitive tunable laser absorption spectroscopy using a multipass gas cell and a solid-state photoacoustic optical power detector. Unlike photoacoustic spectroscopy (PAS), this method readily allows a low gas pressure for high spectral selectivity and a free gas flow for continuous measurements. Our photoacoustic optical power detector has a large linear dynamic range and can be used at almost any optical wavelength, including the middle infrared and THz regions that are challenging to cover with traditional optical detectors. Furthermore, our approach allows for compensation of laser power drifts with a single detector. As a proof of concept, we have measured very weak CO2 absorption lines at 9.2 µm wavelength and achieved a normalized noise equivalent absorption (NNEA) of 2.35·10-9 Wcm-1Hz-1/2 with a low-power quantum cascade laser. The absolute value of the gas absorption coefficient is obtained directly from the Beer-Lambert law, making the technique calibration-free.

Kinetic cooling in mid-infrared methane photoacoustic spectroscopy: A quantitative analysis via digital twin verification

This study presents a detailed quantitative analysis of kinetic cooling in methane photoacoustic spectroscopy, leveraging the capabilities of a digital twin model. Using a quantum cascade laser tuned to 1210.01 cm⁻¹, we investigated the effects of varying nitrogen-oxygen matrix compositions on the photoacoustic signals of 15 ppmV methane. Notably, the photoacoustic signal amplitude decreased with increasing oxygen concentration, even falling below the background signal at oxygen levels higher than approximately 6 %V. This phenomenon was attributed to kinetic cooling, where thermal energy is extracted from the surrounding gas molecules rather than added, as validated by complex vector analysis using a previously published digital twin model. The model accurately reproduced complex signal patterns through simulations, providing insights into the underlying molecular mechanisms by quantifying individual collision contributions. These findings underscore the importance of digital twins in understanding the fundamentals of photoacoustic signal generation at the molecular level.

A Review on Photoacoustic Spectroscopy Techniques for Gas Sensing

The rapid growth of industry and the global drive for modernization have led to an increase in gas emissions, which present significant environmental and health risks. As a result, there is a growing need for precise and sensitive gas-monitoring technologies. This review delves into the progress made regarding photoacoustic gas sensors, with a specific focus on the vital components of acoustic cells and acoustic detectors. This review highlights photoacoustic spectroscopy (PAS) as an optical detection technique, lauding its high sensitivity, selectivity, and capability to detect a wide range of gaseous species. The principles of photoacoustic gas sensors are outlined, emphasizing the use of modulated light absorption to generate heat and subsequently detect gas pressure as acoustic pressure. Additionally, this review provides an overview of recent advancements in photoacoustic gas sensor components while also discussing the applications, challenges, and limitations of these sensors. It also includes a comparative analysis of photoacoustic gas sensors and other types of gas sensors, along with potential future research directions and opportunities. The main aim of this review is to advance the understanding and development of photoacoustic gas detection technology.

In Situ High-Precision Measurement of Deep-Sea Dissolved Methane by Quartz-Enhanced Photoacoustic and Light-Induced Thermoelastic Spectroscopy

Rapid and accurate realization of in situ analysis of deep-sea dissolved gases imperative to the study of ecological geology, oil and gas resource exploration, and global climate change. Herein, we report for the first time the deep-sea dissolved methane (CH4) in situ sensor based on quartz-enhanced photoacoustic and light-induced thermoelastic spectroscopy. The developed sensor system has a volume of φ120 mm × 430 mm and a power consumption of 7.6 W. The sensor, in the manner of frequency division multiplexing, is able to simultaneously measure the photoacoustic signals and light-induced thermoelastic signals, which can accurately correct laser-intensity induced influence on concentration. The spectral response of CH4 concentration varying from 0.01 to 5% is calibrated in detail based on the pressure and temperature in the application environment. The trend of the photoacoustic signal of CH4 at different water molecule (H2O) concentrations is investigated. An Allan variance analysis of several hours demonstrates a minimum detection limit of 0.21 ppm for the CH4 spectrometer. The sensor combined with the gas-liquid separation and enrichment unit is integrated into a compact marine standalone system. Since the specifically designed photoacoustic cell has a volume of only 1.2 mL, the time response for dissolved CH4 detection is reduced to 4 min. Furthermore, the sensor is successfully deployed in the vicinity of the "HaiMa" cold seeps at 1380 m underwater in the South China Sea, completing three consecutive days of measurements of dissolved CH4.

New silicon-based micro-electro-mechanical systems for photo-acoustic trace-gas detection

The achievable sensitivity level of photo-acoustic trace-gas sensors essentially depends on the performances of the acoustic transducer. In this work, the mechanical response of different silicon-based micro-electro-mechanical systems (MEMS) is characterized, aiming at investigating both their mechanical properties, namely the resonance frequency and the quality factor, and the minimum detection limit (MDL) achievable when they are exploited as an acoustic-to-voltage transducer in a trace-gas photoacoustic setup. For this purpose, a 4.56 µm Continuous-Wave (CW) quantum cascade laser (QCL) is used to excite a strong N2O roto-vibrational transition with a line strength of 2.14 × 10-19 cm/molecule, and the detection of MEMS oscillations is performed via an interferometric readout. As a general trend, the minimum detection limit decreases when the resonance frequency investigated increases, achieving a value of 15 parts per billion with a 3 dB cut-off lock-in bandwidth equal to 100 mHz, around 10 kHz.

Harmonic phase-sensitive detection for quartz-enhanced photoacoustic-thermoelastic spectroscopy

Quartz tuning fork (QTF)-based techniques of photoacoustic spectroscopy and thermoelastic spectroscopy play a significant role in trace gas sensing due to unique high sensitivity and compactness. However, the stability of both techniques remains plagued by the inevitable and unpredictable laser power variation and demodulation phase variation. Herein, we investigate the phase change of a QTF when integrating both techniques for enhanced gas sensing. By demonstrating harmonic phase-sensitive methane detection as an example, we achieve stable gas measurement at varying laser power (2.4-9.4 mW) and varying demodulation phase (-90-90°). Besides, this method shows more tolerance to resonant frequency drift, contributing to a small signal fluctuation of ≤ 6.4 % over a wide modulation range (>10 times of the QTF bandwidth). The realization of harmonic-phase detection allows strengthening the stability of QTF-based sensors in a simple manner, especially when stable parameters, such as laser power, demodulation phase, even resonant frequency, cannot always be maintained.

Sub-ppt level detection of carbon monoxide and nitrous oxide enabled by mid-infrared doubly resonant photoacoustic spectroscopy

We report highly sensitive detection of carbon monoxide (CO) and nitrous oxide (N2O) using doubly resonant photoacoustic spectroscopy paired with a quantum cascade laser (QCL) at 4.57 μm. The butterfly-packaged QCL is used to exploit the CO absorption line at 2190.02 cm-1 and the N2O absorption line at 2191.42 cm-1 by scanning the injection current. Leveraging the simultaneous acoustic and optical resonances and adopting a lower photoacoustic detection frequency, we achieve a minimum detection limit of 0.85 part-per-trillion (ppt) for CO over the 500 s averaging time, and 0.7 ppt for N2O over the 200 s averaging time. Our approach demonstrates record sensitivity for CO and N2O detection compared to state-of-the-art optical gas sensors.

Fiber-Optic Photoacoustic Gas Microsensor Dual Enhanced by Helmholtz Resonator and Interferometric Cantilever

A high-sensitivity fiber-optic photoacoustic (PA) gas microsensor is demonstrated with dual enhancement based on acoustics and detection. Due to the characteristic of small size, a Helmholtz resonator is integrated into a miniature PA sensor. The acoustically amplified PA signal is detected by a high-sensitivity fiber Fabry-Perot (F-P) interferometric cantilever. The first-order resonant frequencies of the interferometric cantilever and Helmholtz resonator are matched by subtle adjustments. The weak PA signal is significantly enhanced in a volume of only 0.35 mL, which breaks the volume limitation of the resonance modes in traditional PA sensing systems. To improve the resolution of the microsensor, a white light interferometry (WLI)-based spectral demodulation algorithm is utilized. The experimental results indicate that the minimum detection limit of acetylene (C2H2) drops to about 15 ppb with an averaging time of 100 s, corresponding to the normalized noise equivalent absorption (NNEA) coefficient of 2.7 × 10-9 W·cm-1·Hz-1/2. The dual resonance enhanced fiber-optic PA gas microsensor has the merits of high sensitivity, intrinsic safety, and compact structure.

Ultra-highly sensitive dual gases detection based on photoacoustic spectroscopy by exploiting a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser

Photoacoustic spectroscopy (PAS) as a highly sensitive and selective trace gas detection technique has extremely broad application in many fields. However, the laser sources currently used in PAS limit the sensing performance. Compared to diode laser and quantum cascade laser, the solid-state laser has the merits of high optical power, excellent beam quality, and wide tuning range. Here we present a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser used as light source in a PAS sensor for trace gas detection. The self-built solid-state laser had an emission wavelength of ~2 μm with Tm:YAP crystal as the gain material, with an excellent wavelength and optical power stability as well as a high beam quality. The wide wavelength tuning range of 9.44 nm covers the absorption spectra of water and ammonia, with a maximum optical power of ~130 mW, allowing dual gas detection with a single laser source. The solid-state laser was used as light source in three different photoacoustic detection techniques: standard PAS with microphone, and external- and intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS), proving that solid-state laser is an attractive excitation source in photoacoustic spectroscopy.

Differential Cantilever Enhanced Fiber-Optic Photoacoustic Sensor for Diffusion Gas Detection

Aiming at the problem of the fiber-optic photoacoustic (PA) sensor being easily disturbed by external vibration and noise, a differential cantilever enhanced fiber-optic PA sensor is proposed for diffusion gas detection. The sensor comprises two PA tubes with the same structure and a pair of differential interferometric cantilevers. The two PA tubes are symmetrically distributed. The laser is incident on the PA tube as the signal channel to excite the PA pressure wave. Another tube without incident laser is used as the reference channel to suppress external disturbance. The external interference signals and PA signals superimposed with disturbance are detected by the differential cantilevers from the two channels. The signals are simultaneously restored by a single white-light interferometry demodulator, which multiplexed the spectral frequency domain of the superimposed interference spectrum. The experimental results show that the suppression effect of the differential cantilever enhanced PA sensor on ambient noise is improved by 80%, compared to the traditional single-cantilever sensor. The external cofrequency disturbance is suppressed by 20.9 dB. The minimum detection limit to acetylene (C2H2) downs to about 60 ppb with an integration time of 100 s. The sensor has excellent antivibration and electromagnetic interference ability.

Dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy

A novel dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy (DFH-QEPAS) was demonstrated for what we believe to be the first time in this study. In traditional H-QEPAS, the frequency of modulated sinusoidal wave has a frequency difference (Δf) with the resonance frequency (f0) of a quartz tuning fork (QTF). Owing to the resonance characteristic of QTF, it cannot excite QTF to the strongest response. To achieve a stronger response, a sinusoidal wave with a frequency of f0 was added to the modulation wave to compose a dual-frequency modulation. Acetylene (C2H2) was chosen as the target gas to verify the sensor performance. The proposed DFH-QEPAS improved 4.05 times of signal-to-noise ratio (SNR) compared with the traditional H-QEPAS in the same environmental conditions.

Trace gas sensor based on a multi-pass-retro-reflection-enhanced differential Helmholtz photoacoustic cell and a power amplified diode laser

A high-sensitive photoacoustic spectroscopy (PAS) sensor, which is based on a multi-pass-retro-reflection-enhanced differential Helmholtz photoacoustic cell (DHPAC) and a high power diode laser amplified by erbium-doped fiber amplifier (EDFA), is presented in this work for the first time. In order to improve the interaction length between the light and target gas, the incident light was reflected four times through a multi-pass-retro-reflection-cell constructed by two right-angle prisms. A 1.53 µm distributed feedback (DFB) diode laser was selected to excite photoacoustic signal. Moreover, its power was amplified by an EDFA to 1000 mW to improve the amplitude of photoacoustic signal. Acetylene (C2H2) was chosen as the target analysis to verify the reported sensor performance. Compared to double channel without multiple reflections, the 2f signal of double channel with four reflections was improved by 3.71 times. In addition, when the output optical power of EDFA was 1000 mW, the 2f signal has a 70.57-fold improvement compared with the multi-pass-retro-reflection-cell without EDFA. An Allan deviation analysis was carried out to evaluate the long-term stability of such PAS sensor. When the averaging time was 400 s, the minimum detection limit (MDL) of such PAS sensor was 14 ppb.

PAS-based analysis of natural gas samples

Photoacoustic spectroscopy (PAS) is well known for the detection of short-chain hydrocarbons, such as methane, ethane and propane, in the ppm (parts per million) or ppb (parts per billion) range. However, in the production process of natural gas and its combustion in gas-fired devices the composition, especially the concentrations of the main alkanes, plays a decisive role. Gas chromatography (GC) is considered the gold standard for natural gas analysis. We present a method to analyze natural gas samples by PAS. Furthermore, we describe a method to prepare storage gas samples, which are usually under atmospheric pressure, for PAS analysis. All measurements are validated by means of GC. The investigation allows conclusions to be drawn to what extent PAS is suitable for the investigation of natural gas samples.

Fiber-Optic Photoacoustic Gas Sensor with Multiplexed Fabry-Pérot Interferometric Cantilevers

A fiber-optic photoacoustic (PA) gas sensor with multiplexed Fabry-Pérot (F-P) interferometric cantilevers is demonstrated. A compact cylindrical nonresonant PA tube with a volume of only 0.45 mL is designed. The PA signal is measured by two symmetrically installed fiber-optic interferometric cantilever microphones (FOICMs) to improve the signal-to-noise ratio (SNR). For multiplexing the two cantilevers by a single demodulation system, a dual cavity length synchronous measurement method based on total-phase demodulation algorithm with ultrahigh resolution is developed. The PA signal detection is realized by the second-harmonic wavelength modulation spectroscopy (2f-WMS) technique. The sensor performance is verified by conducting the detection of trace acetylene (C2H2). The normalized noise equivalent absorption (NNEA) coefficient is 2.5 × 10-9 cm-1·W·Hz-1/2, and the minimum detection limit (MDL) downs to about 0.2 ppm with an averaging time of 1 s. The fiber-optic PA gas sensor has characteristics of high resolution and immunity to electromagnetic and vibration interference. Furthermore, the technical scheme of the multiplexed cantilever demodulation shows great potential for remote multipoint monitoring of gases in harsh environments.

Direct readout of mirror reflectivity for cavity-enhanced gas sensing using Pound-Drever-Hall signals

The operation of cavity-enhanced techniques usually requires independent pre-calibration of the mirror reflectivity to precisely quantify the absorption. Here we show how to directly calibrate the effective mirror reflectivity without using any gas samples of known concentration or high-speed optical/electrical devices. Leveraging a phase modulator to generate sidebands, we are able to record Pound-Drever-Hall error signals shaped by cavity modes that can reveal the effective reflectivity after waveform analysis. As an example, we demonstrated the reflectivity calibration of a pair of near-infrared mirrors over 80 nm with a free spectral range-limited resolution, illustrating a reflectivity uncertainty of 2 × 10-5 in the center part of the refection wavelength range of the mirrors and larger at the edges. With an effective reflectivity of 0.9982 (finesse ∼1746) inferred at 1531.6 nm, a short ∼ 8-cm Fabry-Pérot cavity achieved a minimum detectable absorption coefficient of 9.1 × 10-9 cm-1 for trace C2H2 detection. This method, by providing convenient calibration in an almost real-time manner, would enable more practical cavity-enhanced gas measurement even with potential mirror reflectivity degradation.

Design of flexible hollow core fiber based photoacoustic gas sensor with high cell constant and compact size

Here we designed, optimized, and proposed a flexible low frequency resonant photoacoustic (PA) gas sensor by using a large core leaky hollow core fiber (L-HCF). The influences from the dimensions, the transmission loss and the bending loss on the performance of the flexible PA gas sensor were systematically investigated. In this work, the optimized inner diameter and length of the L-HCF were 1.7 mm and 300 mm, respectively. The L-HCF based PA cell constant was calculated to be 12115 Pa/(W·cm-1). The minimum detectable limit (MDL) for trace C2H2 detection achieved 23.0 ppb when the lock-in integration time was 200 s by using a near-infrared distributed feedback (DFB) laser source and a low-cost electrical micro-electro-mechanical system (MEMS) microphone. Besides, the amplitude decay ratio of the of the PA signal was only 11.3% when the bending radius of the L-HCF was 100 mm. The normalized noise equivalent absorption (NNEA) coefficient is calculated to be 6.6 × 10-9 W•cm-1•Hz-1/2. The L-HCF based PA cell was proved to own merits of compact size, high cell constant, small gas volume and low cost.

Ppbv-level mid-infrared photoacoustic sensor for mouth alcohol test after consuming lychee fruits

A ppbv-level mid-infrared photoacoustic spectroscopy sensor was developed for mouth alcohol tests. A compact CO2 laser with a sealed waveguide and integrated radio frequency (RF) power supply was used. The emission wavelength is ∼9.3 µm with a power of 10 W. A detection limit of ∼18 ppbv (1σ) for ethanol gas with an integration of 1 s was achieved. The sensor performed a linear dynamic range with an R square value of ∼0.999. A breath measurement experiment after consuming lychees was conducted. The photoacoustic signal amplitude decreased with the quality of lychee consumed, confirming the existence of residual alcohol in the mouth. During continuous measurement, the photoacoustic signal decreased in < 10 min when consuming 30 g lychee fruits, proving that the alcohol detected in exhaled breath originated from the oral cavity rather than the bloodstream. This work provided valuable information on the distinction of alcoholism and crime.

Ppb-level NH3 photoacoustic sensor combining a hammer-shaped tuning fork and a 9.55 µm quantum cascade laser

We present a quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensor designed for precise monitoring of ammonia (NH3) at ppb-level concentrations. The sensor is based on a novel custom quartz tuning fork (QTF) with a mid-infrared quantum cascade laser emitting at 9.55 µm. The custom QTF with a hammer-shaped prong geometry which is also modified by surface grooves is designed as the acoustic transducer, providing a low resonance frequency of 9.5 kHz and a high-quality factor of 10263 at atmospheric pressure. In addition, a temperature of 50 °C and a large gas flow rate of 260 standard cubic centimeters per minute (sccm) are applied to mitigate the adsorption and desorption effect arising from the polarized molecular of NH3. With 80-mW optical power and 300-ms lock-in integration time, the detection limit is achieved to be 2.2 ppb which is the best value reported in the literature so far for NH3 QEPAS sensors, corresponding to a normalized noise equivalent absorption coefficient of 1.4 × 10-8 W cm-1 Hz-1/2. A five-day continuous monitoring for atmospheric NH3 is performed, verifying the stability and robustness of the presented QEPAS-based NH3 sensor.

Low-frequency Resonant Photoacoustic Gas Sensor by Employing Hollow Core Fiber-Based O-Shaped Multipass Cells

A low-frequency flexible resonant photoacoustic (PA) gas sensor using an O-shaped multipass cell is demonstrated. The PA sensor employed a flexible gradually tapered leaky hollow core fiber (LHCF). The LHCF was bent to be an end-to-end structure to make full use of the incident light. Additionally, the two ends of the LHCF were put inside a single buffer chamber, yielding an equivalent H-type acoustic resonator. The geometric size was reduced thanks to the bending structure. The geometric length of the LHCF was 500 mm. A micro-electro-mechanical-systems electrical microphone was installed at the center of the resonant tube to detect the PA signal. The proposed PA gas sensor exhibited a first-order longitudinal resonance frequency of 408 Hz. Trace acetylene (C2H2) was used as the target gas. The minimum detectable limit was calculated to be 25.8 parts-per-billion (ppb) with an average time of 400 s, which was 1.93 times higher than that of a single-pass PA gas sensor. The normalized noise-equivalent absorption coefficient and the PA cell constant were calculated to be 9.6 × 10-9 W·cm-1·Hz-1/2 and 8295 Pa/W·cm-1, respectively. The PA gas sensor owns a low resonance frequency and can be used for detection of most of the polar gaseous molecules, especially suitable for gas molecules with a long V-T relation time, such as carbon monoxide and carbon dioxide.

Dual-comb optomechanical spectroscopy

Optical cavities are essential for enhancing the sensitivity of molecular absorption spectroscopy, which finds widespread high-sensitivity gas sensing applications. However, the use of high-finesse cavities confines the wavelength range of operation and prevents broader applications. Here, we take a different approach to ultrasensitive molecular spectroscopy, namely dual-comb optomechanical spectroscopy (DCOS), by integrating the high-resolution multiplexing capabilities of dual-comb spectroscopy with cavity optomechanics through photoacoustic coupling. By exciting the molecules photoacoustically with dual-frequency combs and sensing the molecular-vibration-induced ultrasound waves with a cavity-coupled mechanical resonator, we measure high-resolution broadband ( > 2 THz) overtone spectra for acetylene gas and obtain a normalized noise equivalent absorption coefficient of 1.71 × 10-11 cm-1·W·Hz-1/2 with 30 GHz simultaneous spectral bandwidth. Importantly, the optomechanical resonator allows broadband dual-comb excitation. Our approach not only enriches the practical applications of the emerging cavity optomechanics technology but also offers intriguing possibilities for multi-species trace gas detection.

T-type cell mediated photoacoustic spectroscopy for simultaneous detection of multi-component gases based on triple resonance modality

Enhancing multi-gas detectability using photoacoustic spectroscopy capable of simultaneous detection, highly selectivity and less cross-interference is essential for dissolved gas sensing application. A T-type photoacoustic cell was designed and verified to be an appropriate sensor, due to the resonant frequencies of which are determined jointly by absorption and resonant cylinders. The three designated resonance modes were investigated from both simulation and experiments to present the comparable amplitude responses by introducing excitation beam position optimization. The capability of multi-gas detection was demonstrated by measuring CO, CH4 and C2H2 simultaneously using QCL, ICL and DFB lasers as excitation sources respectively. The influence of potential cross-sensitivity towards humidity have been examined in terms of multi-gas detection. The experimentally determined minimum detection limits of CO, CH4 and C2H2 were 89ppb, 80ppb and 664ppb respectively, corresponding to the normalized noise equivalent absorption coefficients of 5.75 × 10-7 cm-1 W Hz-1/2, 1.97 × 10-8 cm-1 W Hz-1/2 and 4.23 × 10-8 cm-1 W Hz-1/2.

Ultra-Stable Temperature Controller-Based Laser Wavelength Locking for Improvement in WMS Methane Detection

In the wavelength modulation spectroscopy (WMS) gas detection system, the laser diode is usually stabilized at a constant temperature and driven by current injection. So, a high-precision temperature controller is indispensable in every WMS system. To eliminate wavelength drift influence and improve detection sensitivity and response speed, laser wavelength sometimes needs to be locked at the gas absorption center. In this study, we develop a temperature controller to an ultra-high stability level of 0.0005 °C, based on which a new laser wavelength locking strategy is proposed to successfully lock the laser wavelength at a CH4 absorption center of 1653.72 nm with a fluctuation of fewer than 19.7 MHz. For 500 ppm CH4 sample detection, the 1σ SNR is increased from 71.2 dB to 80.5 dB and the peak-to-peak uncertainty is improved from 1.95 ppm down to 0.17 ppm with the help of a locked laser wavelength. In addition, the wavelength-locked WMS also has the absolute advantage of fast response over a conventional wavelength-scanned WMS system.

A Miniaturized 3D-Printed Quartz-Enhanced Photoacoustic Spectroscopy Sensor for Methane Detection with a High-Power Diode Laser

In this invited paper, a highly sensitive methane (CH₄) trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) technique using a high-power diode laser and a miniaturized 3D-printed acoustic detection unit (ADU) is demonstrated for the first time. A high-power diode laser emitting at 6057.10 cm^-1 (1650.96 nm), with the optical power up to 38 mW, was selected as the excitation source to provide a strong excitation. A 3D-printed ADU, including the optical and photoacoustic detection elements, had a dimension of 42 mm, 27 mm, and 8 mm in length, width, and height, respectively. The total weight of this 3D-printed ADU, including all elements, was 6 g. A quartz tuning fork (QTF) with a resonant frequency and Q factor of 32.749 kHz and 10,598, respectively, was used as an acoustic transducer. The performance of the high-power diode laser-based CH₄-QEPAS sensor, with 3D-printed ADU, was investigated in detail. The optimum laser wavelength modulation depth was found to be 0.302 cm^-1. The concentration response of this CH₄-QEPAS sensor was researched when the CH₄ gas sample, with different concentration samples, was adopted. The obtained results showed that this CH₄-QEPAS sensor had an outstanding linear concentration response. The minimum detection limit (MDL) was found to be 14.93 ppm. The normalized noise equivalent absorption (NNEA) coefficient was obtained as 2.20 × 10^-7 cm^-1W/Hz^-1/2. A highly sensitive CH₄-QEPAS sensor, with a small volume and light weight of ADU, is advantageous for the real applications. It can be portable and carried on some platforms, such as an unmanned aerial vehicle (UAV) and a balloon.

Helmholtz-resonator quartz-enhanced photoacoustic spectroscopy

In this work, Helmholtz-resonator quartz-enhanced photoacoustic spectroscopy (HR-QEPAS) was developed for trace gas sensing. A pair of Helmholtz resonators with high-order resonance frequency was designed and coupled with a quartz tuning fork (QTF). Detailed theoretical analysis and experimental research were carried out to optimize the HR-QEPAS performance. As a proof-of-concept experiment, the water vapor in the ambient air was detected using a 1.39 µm near-infrared laser diode. Benefiting from the acoustic filtering of the Helmholtz resonance, the noise level of QEPAS was reduced by >30%, making the QEPAS sensor immune to environmental noise. In addition, the photoacoustic signal amplitude was improved significantly by >1 order of magnitude. As a result, the detection signal-to-noise ratio was enhanced by >20 times, compared with a bare QTF.

Parts-per-billion-level detection of hydrogen sulfide based on doubly resonant photoacoustic spectroscopy with line-locking

We report on the development of a highly sensitive hydrogen sulfide (H₂S) gas sensor exploiting the doubly resonant photoacoustic spectroscopy technique and using a near-infrared laser emitting at 1578.128 nm. By targeting the R(4) transition of H₂S, we achieved a minimum detection limit of 10 part per billion in concentration and a normalized noise equivalent absorption coefficient of 8.9 × 10^-12 W cm^-1 Hz^-1/2. A laser-cavity-molecule locking strategy is proposed to enhance the sensor stability for fast measurement when dealing with external disturbances. A comparison among the state-of-the-art H₂S sensors using various spectroscopic techniques confirmed the record sensitivity achieved in this work.

NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets

Faraday rotation spectroscopy (FRS) exploits the magneto-optical effect to achieve highly selective and sensitive detection of paramagnetic molecules. Usually, a solenoid coil is used to provide a longitudinal magnetic field to produce the magneto-optical effect. However, such a method has the disadvantages of excessive power consumption and susceptibility to electromagnetic interference. In the present work, a novel FRS approach based on a combination of a neodymium iron boron permanent magnet ring array and a Herriott multipass absorption cell is proposed. A longitudinal magnetic field was generated by using 14 identical neodymium iron boron permanent magnet rings combined in a non-equidistant form according to their magnetic field's spatial distribution characteristics. The average magnetic field strength within a length of 380 mm was 346 gauss. A quantum cascade laser was used to target the optimum 4₄₁ ← 4₄₀ Q-branch nitrogen dioxide transition at 1613.25 cm^-1 (6.2 μm) with an optical power of 40 mW. Coupling to a Herriott multipass absorption cell, a minimum detection limit of 0.4 ppb was achieved with an integration time of 70 s. The low-power FRS nitrogen dioxide sensor proposed in this work is expected to be developed into a robust field-deployable environment monitoring system.

Micro-nano fiber-assisted active photoacoustic spectroscopy for gas sensing

We report on the development of all-fiber active photoacoustic spectroscopy, where active photoacoustic effect is generated by embedding a micro-nano fiber inside a fiber laser resonator to exploit the evanescent field of the high intracavity power. Acetylene detection at 1530.37 nm was selected for gas sensing demonstration. With a small diameter of 1.1 µm, the tapped fiber exploited ∼20% intracavity power for the evanescent-wave photoacoustic excitation, while only introduced a low intrinsic cavity loss of 0.08 dB. Our sensor achieved a minimum detection limit of 1 ppm at an integration time of 10 s, which can be improved to 73 ppb at 1000 s benefited from the high system stability. The sensing dynamic range was determined to be more than five orders. This spectroscopic technique combines fiber laser, photoacoustic spectroscopy, and fiber evanescent-wave absorption to achieve gas sensing with high flexibility, low optical noise, and easy optical alignment. Current limitations were discussed in detail to explore feasible ways to improve the performance in response time, dynamic range and sensitivity.

Super tiny quartz-tuning-fork-based light-induced thermoelastic spectroscopy sensing

In this Letter, a sensitive light-induced thermoelastic spectroscopy (LITES)-based trace gas sensor by exploiting a super tiny quartz tuning fork (QTF) was demonstrated. The prong length and width of this QTF are 3500 µm and 90 µm, respectively, which determines a resonant frequency of 6.5 kHz. The low resonant frequency is beneficial to increase the energy accumulation time in a LITES sensor. The geometric dimension of QTF on the micrometer scale is advantageous to obtain a great thermal expansion and thus can produce a strong piezoelectric signal. The temperature gradient distribution of the super tiny QTF was simulated based on the finite element analysis and is higher than that of the commercial QTF with 32.768 kHz. Acetylene (C₂H₂) was used as the analyte. Under the same conditions, the use of the super tiny QTF achieved a 1.64-times signal improvement compared with the commercial QTF. The system shows excellent long-term stability according to the Allan deviation analysis, and a minimum detection limit (MDL) would reach 190 ppb with an integration time of 220 s.

Dual-comb quartz-enhanced photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) using two optical combs is a new-born technique, offering appealing features, including broad optical bandwidths, high resolutions, fast acquisition speeds, and wavelength-independent photoacoustic detection, for chemical sensing. However, its further application to, e.g., trace detection, is jeopardized due to the fundamentally and technically limited sensitivity and specificity. Here, we take a different route to comb-enabled PAS with acoustically enhanced sensitivity and nonlinear spectral hole-burning defined resolution. We demonstrate dual-comb quartz-enhanced PAS with two near-infrared electro-optic combs and a quartz tuning fork. Comb-line-resolved multiplexed spectra are acquired for acetylene with a single-pass detection limit at the parts-per-billion level. The technique is further extended to the mid-infrared (for methane), enabling improved sensitivity. More importantly, we measure nonlinear dual-comb photoacoustic spectra for the 12C2H2 ν1 + ν3 band P(17) transition with sub-Doppler pressure-broadening dominated homogeneous linewidths (e.g., 45.8 MHz), hence opening up new opportunities for Doppler-free photoacoustic gas sensing.

Highly sensitive HF detection based on absorption enhanced light-induced thermoelastic spectroscopy with a quartz tuning fork of receive and shallow neural network fitting

Due to its advantages of non-contact measurement and high sensitivity, light-induced thermoelastic spectroscopy (LITES) is one of the most promising methods for corrosive gas detection. In this manuscript, a highly sensitive hydrogen fluoride (HF) sensor based on LITES technique is reported for the first time. With simple structure and strong robustness, a shallow neural network (SNN) fitting algorithm is introduced into the field of spectroscopy data processing to achieve denoising. This algorithm provides an end-to-end approach that takes in the raw input data without any pre-processing and extracts features automatically. A continuous wave (CW) distributed feedback diode (DFB) laser with an emission wavelength of 1.27 µm was used as the excitation source. A Herriott multi-pass cell (MPC) with an optical length of 10.1 m was selected to enhance the laser absorption. A quartz tuning fork (QTF) with resonance frequency of 32,767.52 Hz was adopted as the thermoelastic detector. An Allan variance analysis was performed to demonstrate the system stability. When the integration time was 110 s, the minimum detection limit (MDL) was found to be 71 ppb. After the SNN fitting algorithm was used, the signal-to-noise ratio (SNR) of the HF-LITES sensor was improved by a factor of 2.0, which verified the effectiveness of this fitting algorithm for spectroscopy data processing.

All-Fiber Photoacoustic Gas Sensing with Interferometric Location

Photoacoustic spectroscopy (PAS) is a promising gas detection technique with high sensitivity, fast response, and good stability. Frequency-modulated continuous-wave (FMCW) interferometry offers precise distance detection with high spatial resolution. The combination of PAS and FMCW may lead to an optical technique for the simultaneous extraction of gas concentration and location information. Herein, we demonstrate this technique in an all-fiber sensing system by blending a fiber-pigtailed PAS sensor with an FMCW interferometer. As an example, we have measured the methane concentration and location by employing time-division multiplexing, showing a minimum detection limit of 28 ppm and a spatial resolution of 3.87 mm over a distance of ~4.9 m. This study enables the realization of a versatile technique for multiparameter gas sensing in gas leakage detection and gas emission monitoring.