Near-infrared laser photoacoustic gas sensor for simultaneous detection of CO and H2S

High-sensitivity trace gas detection based on differential Helmholtz photoacoustic cell with dense spot pattern

A high-sensitivity photoacoustic spectroscopy (PAS) sensor based on differential Helmholtz photoacoustic cell (DHPAC) with dense spot pattern is reported in this paper for the first time. A multi-pass cell based on two concave mirrors was designed to achieve a dense spot pattern, which realized 212 times excitation of incident laser. A finite element analysis was utilized to simulate the sound field distribution and frequency response of the designed DHPAC. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power of the laser to achieve strong excitation. In order to assess the designed sensor's performance, an acetylene (C2H2) detection system was established using a near infrared diode laser with a central wavelength 1530.3 nm. According to experimental results, the differential characteristics of DHPAC was verified. Compared to the sensor without dense spot pattern, the photoacoustic signal with dense spot pattern had a 44.73 times improvement. The minimum detection limit (MDL) of the designed C2H2-PAS sensor can be improved to 5 ppb when the average time of the sensor system is 200 s.

Compact gas cell for simultaneous detection of atmospheric aerosol optical properties based on photoacoustic spectroscopy and integrating sphere scattering enhancement

Atmospheric aerosols play a pivotal role in the earth-atmospheric system. Analyzing their optical properties, specifically absorption and scattering coefficients, is essential for comprehending the impact of aerosols on climate. When different optical properties of aerosols are individually measured using multiple devices, cumulative errors in the detection results inevitably occur. To address this challenge, based on photoacoustic spectroscopy (PAS) and integrating sphere (IS) scattering enhancement, a compact gas cell (PASIS-Cell) was developed. The PASIS-Cell comprises a dual-T-type photoacoustic cell (DTPAC) and an IS. IS is coupled with DTPAC through a transparent quartz tube, thereby enhancing the scattering signal without compromising the acoustic characteristics of DTPAC. Concurrently, DTPAC can realize high-performance photoacoustic detection of absorption signal. Experimental results demonstrate that PASIS-Cell can simultaneously invert atmospheric aerosol absorption and scattering coefficients, with a minimum detection limit of less than 1 Mm-1, showcasing its potential in the analysis of aerosol optical properties.

Construction of response surface model for photoacoustic-based H2S measurement system and significance analysis of multiple influential factors

Herein we first introduce the relationship between the photoacoustic (PA) signals' intensity of hydrogen sulfide (H2S) versus multiple parameters of optical path conditions, following by the construction of response surface method (RSM)-based models of the PA signals' intensity versus the distance l1 from the laser head to the convex lens, the distance l2 from the convex lens to the PA cell (PAC), and the distance l3 from the geometric center line of the light beam to the acoustic sensor. After that, we perform the significance analysis. The results show that the RSM model with a third-order configuration is relatively preferred. The distances l1, l2 and l3 all have significant influences on the PA signals' intensity. Additionally, we ameliorated the performance of the full third-order model by removing the non-significant terms.

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.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) and Beat Frequency-QEPAS techniques for air pollutants detection: A comparison in terms of sensitivity and acquisition time

In this work, a comparison between Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) and Beat Frequency-QEPAS (BF-QEPAS) techniques for environmental monitoring of pollutants is reported. A spectrophone composed of a T-shaped Quartz Tuning Fork (QTF) coupled with resonator tubes was employed as a detection module. An interband cascade laser has been used as an exciting source, allowing the targeting of two NO absorption features, located at 1900.07 cm^-1 and 1900.52 cm^-1, and a water vapor absorption feature, located at 1901.76 cm^-1. Minimum detection limits of 90 ppb and 180 ppb were achieved with QEPAS and BF-QEPAS techniques, respectively, for NO detection. The capability to detect multiple components in the same gas mixture using BF-QEPAS was also demonstrated.

Near-infrared sensitive differential Helmholtz-based hydrogen sulfide photoacoustic sensors

A near-infrared (NIR) sub-ppm level photoacoustic sensor for hydrogen sulfide (H₂S) using a differential Helmholtz resonator (DHR) as the photoacoustic cell (PAC) was presented. The core detection system was composed of a NIR diode laser with a center wavelength of 1578.13 nm, an Erbium-doped optical fiber amplifier (EDFA) with an output power of ∼120 mW, and a DHR. Finite element simulation software was used to analyze the influence of the DHR parameters on the resonant frequency and acoustic pressure distribution of the system. Through simulation and comparison, the volume of the DHR was 1/16 that of the conventional H-type PAC for a similar resonant frequency. The performance of the photoacoustic sensor was evaluated after optimizing the DHR structure and modulation frequency. The experimental results showed that the sensor had an excellent linear response to the gas concentration and the minimum detection limit (MDL) for H₂S detection in differential mode can reach 460.8 ppb.

Highly sensitive photoacoustic acetylene detection based on differential photoacoustic cell with retro-reflection-cavity

In this paper, a highly sensitive photoacoustic spectroscopy (PAS) sensor based on retro-reflection-cavity-enhanced differential photoacoustic cell (DPAC) is demonstrated for the first time. Acetylene (C₂H₂) was selected as the analyte. The DPAC was designed to effectively suppress noise and increase signal level. The retro-reflection-cavity consisted of two right-angle prisms was designed to reflect the incident light to realize four passes. The photoacoustic response of the DPAC was simulated and investigated based on the finite element method. Wavelength modulation and second harmonic demodulation technologies were applied for sensitive trace gas detection. The first-order resonant frequency of the DPAC was found to be 1310 Hz. The differential characteristics were investigated and the 2f signal amplitude for this C₂H₂-PAS sensor based on retro-reflection-cavity-enhanced DPAC had a 3.55 times improvement compared to the system without the retro-reflection-cavity. An Allan deviation analysis was performed to investigate the long-term stability of the system. The minimum detection limit (MDL) was measured to be 15.81 ppb with an integration time of 100 s.

Signal-to-Noise Ratio Analysis for the Voltage-Mode Read-Out of Quartz Tuning Forks in QEPAS Applications

Quartz tuning forks (QTFs) are employed as sensitive elements for gas sensing applications implementing quartz-enhanced photoacoustic spectroscopy. Therefore, proper design of the QTF read-out electronics is required to optimize the signal-to-noise ratio (SNR), and in turn, the minimum detection limit of the gas concentration. In this work, we present a theoretical study of the SNR trend in a voltage-mode read-out of QTFs, mainly focusing on the effects of (i) the noise contributions of both the QTF-equivalent resistor and the input bias resistor R_L of the preamplifier, (ii) the operating frequency, and (iii) the bandwidth (BW) of the lock-in amplifier low-pass filter. A MATLAB model for the main noise contributions was retrieved and then validated by means of SPICE simulations. When the bandwidth of the lock-in filter is sufficiently narrow (BW = 0.5 Hz), the SNR values do not strongly depend on both the operating frequency and R_L values. On the other hand, when a wider low-pass filter bandwidth is employed (BW = 5 Hz), a sharp SNR peak close to the QTF parallel-resonant frequency is found for large values of R_L (R_L > 2 MΩ), whereas for small values of R_L (R_L < 2 MΩ), the SNR exhibits a peak around the QTF series-resonant frequency.

Resonant photoacoustic spectrometer enhanced by multipass absorption for detecting atmospheric CH4 at the ppb-level

A resonant photoacoustic spectrometer (PAS) was developed for detecting trace atmospheric CH₄. The sensitivity of the PAS was significantly increased via a Herriott-type multipass cell with a beam pattern concentrated in the cavity. The effective optical pathlength of the PAS can be optimized to 6.8 m with 34 reflections and a diameter of 6 mm. A distributed feedback diode laser at 1,653 nm was employed as the light source, and wavelength modulation spectroscopy was used for the 2nd harmonic signal to reduce the noise of the system. The resonant cell of PA and optimal modulation frequency were obtained by varying the measurements. In comparison with a single path, the sensitivity of the multipass strategy was improved 13 times. To evaluate the long-term stability and minimum detection limit (MDL) of the system, an Allan variance analysis was performed, and the analysis illustrated that the MDL accomplished 116 ppb at an average time of 84 s. The system was utilized for 2 days test campaign to validate the feasibility and robustness of the sensor. The system provides a promising technique for online monitoring of greenhouse gasses.

Frequency-Division-Multiplexed Multicomponent Gas Sensing with Photothermal Spectroscopy and a Single NIR/MIR Fiber-Optic Gas Cell

We report a multicomponent gas sensor based on hollow-core fiber (HCF) photothermal spectroscopy with frequency-division multiplexing (FDM). A single antiresonant HCF (AR-HCF) is used as the gas cell, which supports broadband transmission from near-infrared (NIR) to mid-infrared (MIR), covering the absorption lines of water vapor (H₂O) at 1.39 μm, carbon dioxide (CO₂) at 2.00 μm, and carbon monoxide (CO) at 4.60 μm. The NIR and MIR pump lasers at the above wavelengths are coupled into the AR-HCF from the opposite ends and modulated at 7.5, 8.0, and 8.5 kHz, respectively, to produce photothermal phase modulations at different frequencies. A common probe Fabry-Perot interferometer at 1.55 μm is adopted to detect the phase modulations, which are demodulated simultaneously using three lock-in amplifiers at the respective second harmonic frequencies. With a 13-cm-long AR-HCF, simultaneous detections of H₂O, CO₂, and CO are demonstrated with the limits of detection (LODs) of 2.7 ppm, 25 ppb, and 9 ppb for 1 s lock-in time constant, respectively. The LODs go down to 222, 1.5, and 0.6 ppb, respectively, for 1000 s averaging time. The photothermal signals of CO and CO₂, which are humidity-level dependent, are well calibrated by use of the measured H₂O signal. The multicomponent gas sensor is compact in configuration and shows good stability with signal fluctuation less than 1.7% over 2 h.

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.

Design of a high-sensitivity differential Helmholtz photoacoustic cell and its application in methane detection

A high-sensitivity differential Helmholtz photoacoustic cell based on multiple reflection was reported, and its performance parameters and gas replacement time were optimized by finite element simulation. To realize the long absorption path of the measured gas, the collimated excitation light was reflected multiple times on the gold-plated wall of the absorption cavity, and the wavelength modulation technology was used to reduce the multiple reflection noise. Additionally, the differential could suppress external co-phase noise and double the photoacoustic signal. When a laser with a central wavelength of 1653 nm was employed as the excitation light source, the minimum detection limit of 177 ppb (signal-to-noise ratio, SNR = 1) for methane was achieved within a detection time of 1 s, and the corresponding normalized noise equivalent absorption coefficient was 4.1×10^-10 cm^-1WHZ^-1/2.

Ultra-sensitive ppb-level methane detection based on NIR all-optical photoacoustic spectroscopy by using differential fiber-optic microphones with gold-chromium composite nanomembrane

In this paper, we propose and experimentally demonstrate an ultra-sensitive all-optical PAS gas sensor, incorporating with a near-infrared (NIR) diode laser, fiber-optic microphones (FOMs) and a double channel differential T-type photoacoustic cell. The FOM is realized by Fabry-Perot interferometry and novel gold-chromium (Au-Cr) composite nanomembranes. To meet the demand of high sensitivity and flat frequency response for the FOMs, the Au-Cr composite diaphragm is deliberately designed and fabricated by E-beam evaporation deposition with 330 nm in thickness and 6.35 mm in radius. Experimental results show that the FOM has a sensitivity of about 30 V/Pa and a flat frequency response from 300 to 900 Hz with fluctuation below 1 dB. Moreover, a double channel differential T-type photoacoustic cell is designed and employed in the all-optical PAS gas sensor, with the first-order resonant frequency of 610 Hz. The all-optical gas sensor is established and verified for CH₄ detection and the normalized noise equivalent absorption (NNEA) is 4.42 × 10^-10 W∙cm^-1∙Hz^-1/2. The minimum detection limit (MDL) of 36.45 ppb is achieved with a 1 s integration time. The MDL could be further enhanced to 4.87 ppb with an integration time of 81 s, allowing ultra-sensitive trace gas detection.

Compact quartz-enhanced photoacoustic sensor for ppb-level ambient NO2 detection by use of a high-power laser diode and a grooved tuning fork

A compact quartz-enhanced photoacoustic sensor for ppb-level ambient NO₂ detection is demonstrated, in which a high-power blue laser diode module with a small divergence angle was employed to take advantages of the directly proportional relationship between sensitivity and power, hence improving the detection sensitivity. In order to extend the stability time, a custom grooved quartz tuning fork with 800-μm prong spacing is employed to avoid complex signal balance and/or optical spatial filter components. The sensor performance is optimized and assessed in terms of optical coupling, power, gas flow rate, pressure, signal linearity and stability. A minimum detectable concentration (1σ) of 7.3 ppb with an averaging time of 1 s is achieved, which can be further improved to be 0.31 ppb with an averaging time of 590 s. Continuous measurements covering a five-day period are performed to demonstrate the stability and robustness of the reported NO₂ sensor system.

Compact QEPAS humidity sensor in SF6 buffer gas for high-voltage gas power systems

In SF₆ insulated high-voltage gas power systems, H₂O is the most problematic impurity which not only decreases insulation performance but also creates an acidic atmosphere that promotes corrosion. Corrosion damages electrical equipment and leads to leaks, which pose serious safety hazards to people and the environment. A QEPAS-based sensor system for the sub-ppm level H₂O detection in SF₆ buffer gas was developed by use of a near-infrared commercial DFB diode laser. Since the specific physical constants of SF₆ are strongly different from that of N₂ or air, the resonant frequency and Q-factor of the bare quartz tuning fork (QTF) had changed to 32,763 Hz and 4173, respectively. The optimal vertical detection position was 1.2 mm far from the QTF opening. After the experimental optimization of acoustic micro-resonator (AmR) parameters, gas pressures, and modulation depths, a detection limit of 0.49 ppm was achieved for an averaging time of 1 s, which provided a powerful prevention tool for the safety monitoring in power systems.

Sensitivity enhanced NIR photoacoustic CO detection with SF6 promoting vibrational to translational relaxation process

A challenge for slowly relaxing carbon monoxide (CO) molecules detection using photoacoustic spectroscopy (PAS) is to promote the vibration-translation (V-T) relaxation process. Addressing this challenge, a sensitivity enhanced photoacoustic CO sensor with sulfur hexafluoride (SF₆) as the promotor is investigated and demonstrated. A 1568 nm near-infrared (NIR) laser diode and a customized optical amplifier are used as the excitation source to generate the photoacoustic signal. A differential photoacoustic cell is simulated and designed to obtain identical laminar flow distribution in the resonant cell to suppress the flow noise. The modulation frequency and added SF₆ volume ratio are optimized experimentally to achieve optimal sensitivity. Feasibility and performance of the CO sensor with a small amount of SF₆ as promotor is discussed and evaluated, obtaining a ~ 2 times improvement of signal value compared to the one with pure N₂ background and resulting in a minimum detection limit of 467.5 ppb for CO detection.

Experimental Research on Measuring the Concentration of CO2 in Gas-Liquid Solution Based on PZT Piezoelectric-Photoacoustic Spectroscopy

The feasibility of a scheme in which the concentration of CO2 in gas-liquid solution is directly measured based on PZT piezoelectric-photoacoustic spectroscopy was evaluated. The existing device used for the measurement of gas concentration in gas-liquid solution has several limitations, including prolonged duration, loss of gas, and high cost due to the degassing component. In this study, we developed a measuring device in order to solve the problems mentioned above. Using this device, how the intensity of the photoacoustic signal changes with the concentration of CO2 was demonstrated through experiment. The impact that variation of the laser modulation frequency has on the photoacoustic signal was also studied. Furthermore, the experimental data generated from measuring the concentration of CO2 in gas-liquid solution was verified for a wide range of concentrations. It was found that, not only can the error rate of the device be less than 7%, but the time of measurement can be within 60 s. To sum up, the scheme is highly feasible according to the experimental results, which makes measurement of the concentration of a gas in gas-liquid solution in the future more straightforward.