Equivalent Absorption Coefficient Research Articles

A compact portable photoacoustic spectroscopy sensor for multiple trace gas detection

A compact, highly sensitive, and cost-effective photoacoustic spectroscopic sensor for multiple trace gas detection was reported with a self-designed compact distributed feedback laser array and a 2-channel dual phase field programmable gate array based digital lock-in amplifier, only twentieth in size and weight of the commercial instruments. The capability of the portable sensor was verified by measuring CH4 and C2H2 simultaneously. With an integrated nonlocal means algorithm denoising, noise-equivalent concentrations were achieved as 6.89 ppb for CH4 and 2.74 ppb for C2H2, respectively, corresponding to a normalized noise equivalent absorption coefficient of 1.1 × 10−10 cm−1 W/Hz−1/2, a 79 folds higher than the original data. The results demonstrate that the developed minimized sensor has the potential for sensitive and portable measurement of multiple trace gases.

Miniature 3D-printed resonant photoacoustic cell for flowing gas detection

For trace gas flow detection, we propose and demonstrate a highly sensitive miniature resonant photoacoustic cell (PAC) made by 3D print. It was an integrated structure consisting of a photoacoustic cylindrical, two diversion holes, and two mufflers at the inlet and outlet. The finite element method (FEM) was applied to simulate the frequency response, noise reduction effect, and flow rate in the PAC. The transmission loss of the expansion muffler reached 28.7 dB. By measuring, the resonant frequency of the PAC and the quality (Q) factor were obtained to be 3950 Hz and 14.6, which was highly coordinated with the simulation. The detection limit (1σ) of the PA system was 141 parts-per-billion (ppb) with a 1 s lock-in integration time. As a result, the normalized equivalent noise absorption coefficient (NNEA) was 2.27 × 10−9 W•cm−1•Hz−1/2. The experiment results show that the proposed PAC has the merit of anti-flow noise for portable gas detecting systems.

Elliptical-tube off-beam quartz-enhanced photoacoustic spectroscopy

We propose an elliptical-tube off-beam quartz-enhanced photoacoustic spectroscopy (EO-QEPAS) method in which an elliptical tube is employed as an acoustic resonator, instead of a circular resonator in QEPAS, to match the stripe-like beam emitted from a high-power multimode laser diode (MLD). A lower noise level than that of conventional QEPAS is achieved due to the optimal matching between the elliptical resonator and the beam profile, hence resulting in a ∼3 times higher signal-to-noise ratio gain factor compared with the circular resonator. The parameters of the elliptical resonator are optimized, and a 1σ normalized noise equivalent absorption coefficient of 3.4 × 10−8 cm−1 W/Hz1/2 is obtained for dry NO2 detection at normal atmospheric pressure. EO-QEPAS paves the way for developing compact, cost-effective, and highly sensitive gas sensors based on the combination of MLDs and QEPAS.

High-power near-infrared QEPAS sensor for ppb-level acetylene detection using a 28 kHz quartz tuning fork and 10 W EDFA.

A high-power near-infrared (NIR) quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for part per billion (ppb) level acetylene (C2H2) detection was reported. A 1536 nm distributed feedback (DFB) diode laser was used as the excitation light source. Cooperated with the laser, a C-band 10 W erbium-doped fiber amplifier (EDFA) was employed to boost the optical excitation power to improve QEPAS detection sensitivity. A pilot line manufactured quartz tuning fork (QTF) with a resonance frequency of 28 kHz was used as the photoacoustic transducer. In the case of high excitation power, gas flow effect and temperature effect were found and studied. Benefitting from the low QTF resonance frequency, high excitation power, and vibrational-translational (V-T) relaxation promoter, a detection limit of ∼7 ppb was achieved for C2H2 detection, corresponding to a normalized noise equivalent absorption coefficient of 4.4×10-8cm-1 · W · Hz-1/2.

High-Sensitivity Silicon Cantilever-Enhanced Photoacoustic Spectroscopy Analyzer with Low Gas Consumption.

A silicon cantilever-enhanced photoacoustic spectroscopy (PAS)-based trace gas analyzer with low gas consumption is presented. A silicon cantilever-based fiber-optic Fabry-Perot (F-P) interferometric acoustic sensor with a compact structure and high sensitivity is designed for photoacoustic signal detection. The non-resonant photoacoustic cell (PAC) is a cylindrical copper tube with a volume of 0.56 mL. A near-infrared laser with a center wavelength of 1532.83 nm amplified using an erbium-doped fiber application amplifier is used as the excitation light. The wavelength modulation spectroscopy (WMS) technique is employed in the present work for second-harmonic photoacoustic signal detection. The experimental results show that the minimum detection limit of C2H2 is 199.8 parts per trillion (ppt) with an average time of 60 s. The normalized noise equivalent absorption coefficient is calculated as 1.72 × 10-9 cm-1 W/Hz1/2. Furthermore, the proposed silicon cantilever-enhanced non-resonant PAS-based gas analyzer can not only analyze the gas concentration in a closed small-capacity PAC with low gas consumption but also detect target gas leakage in real time at a long distance.

Mid-infrared photothermal gas sensor enabled by core-cladding mode interference in a hollow-core fiber

Photothermal spectroscopy provides an attractive way of sensitive gas detection with a large dynamic range. Previous photothermal gas sensors normally use an optical interferometer for measuring the slight phase change caused by gas absorption. However, an active opto-mechanical stabilization system is required in the photothermal interferometry, making the sensor complex, bulky and hard for field applications. Here, we report a new mid-infrared photothermal gas sensor based on core-cladding mode interference in a tellurite hollow-core antiresonant fiber (HC-ARF). This method relies on the co-propagation of the HE₁₁ mode of a mid-infrared pump laser and the core-cladding modes of a probe laser in the tellurite HC-ARF. As a proof-of-principle demonstration, we detect nitric oxide in the 35 cm long fiber by using a quantum cascade laser (5.2 µm, 21.7 mW) as the pump laser. We achieve a noise equivalent concentration of 50 ppbv, corresponding to a normalized noise equivalent absorption coefficient of 8.6 × 10^-9 cm^-1WHz^-1/2. Our technique has long-term stability without using any active stabilization devices and holds promise for fast, sensitive and background-free gas detection with a simplified configuration.

Design and structural optimization of T-resonators for highly sensitive photoacoustic trace gas detection

Thanks to the available ultra-wide wavelength range compared with broadband laser sources, the use of blackbody radiators in photoacoustic spectroscopy features the simultaneous detection of multiple gas species in the presence of cross-interfering absorption lines. The major problem associated with broadband incoherent sources is less power and less stable intensity per wavenumber than lasers and leads to limited gas detection sensitivity. In this paper, the detectivity of a broadband double optical path differential photoacoustic system was enhanced with the development of dimension-optimized high-responsivity T-resonators for simultaneous multiple trace gas detection. Enhanced Q-factor and external noise suppression level constitute dual criteria for the optimization of T-resonators. Three digital signal processing algorithms were separately investigated which further improved gas dectectability. The capability of the multiple–trace-gas detection framework was verified by measuring CO2, C2H2 and H2O simultaneously. The spectral results processed by a wavelet denoising algorithm present the best performance in terms of background noise suppression and spectral feature fidelity. [Q2.2] With the absorption enhancement of the optimized T-cells and background suppression of the wavelet denoising algorithm, a broadband differential photoacoustic system was achieved with only a 30 mW globar source and normalized noise equivalent absorption coefficient value of 4.1 × 10−10 W·cm−1·Hz−1/2 which is two orders of magnitude improvement over the original T-cell-based photoacoustic configuration. The noise equivalent detection limits were found to be 223 ppbv for CO2, 625 ppbv for C2H2 and 865 ppbv for H2O, respectively.

Non-Local Patch Regression Algorithm-Enhanced Differential Photoacoustic Methodology for Highly Sensitive Trace Gas Detection

A non-local patch regression (NLPR) denoising-enhanced differential broadband photoacoustic (PA) sensor was developed for the high-sensitive detection of multiple trace gases. Using the edge preservation index (EPI) and signal-to-noise ratio (SNR) as a dual-criterion, the fluctuation was dramatically suppressed while the spectral absorption peaks were maintained by the introduction of a NLPR algorithm. The feasibility of the broadband framework was verified by measuring the C2H2 in the background of ambient air. A normalized noise equivalent absorption (NNEA) coefficient of 6.13 × 10−11 cm−1·W·Hz−1/2 was obtained with a 30-mW globar source and a SNR improvement factor of 23. Furthermore, the simultaneous multiple-trace-gas detection capability was determined by measuring C2H2, H2O, and CO2. Following the guidance of single-component processing, the NLPR processed results showed higher EPI and SNR compared to the spectra denoised by the wavelet method and the non-local means algorithm. The experimentally determined SNRs of the C2H2, H2O, and CO2 spectra were improved by a factor of 20. The NNEA coefficient reached a value of 7.02 × 10−11 cm−1·W·Hz−1/2 for C2H2. The NLPR algorithm presented good performance in noise suppression and absorption peak fidelity, which offered a higher dynamic range and was demonstrated to be an effective approach for trace gas analysis.

An all-optical off-beam quartz-enhanced photoacoustic spectroscopy employing double-pass acoustic microresonators

An all-optical off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor employing double-pass acoustic microresonators was proposed and experimentally investigated. An optical fiber Fabry–Perot interferometric sensor with quadrature point self-stabilization was employed for the detection of quartz prong vibration. A commercial quartz tuning fork with a resonant frequency of 30.72 kHz was employed as an acoustic transducer. Water vapor as the catalyst was added for improving the vibrational energy transfer. The minimum detection limit of ∼2.41 ppmv was obtained with a 1 s averaging time, which corresponded to a normalized noise equivalent absorption coefficient of 7.76×10−8 cm−1 W Hz−1/2 at the methane absorption line of 6046.95 cm −1 with an optimized modulation depth of 0.172 cm −1. The detection sensitivity was enhanced by a factor of ∼1.48 in comparison to the off-beam QEPAS employing a single-pass microresonator. An Allan deviation analysis indicated that the ultimate detection limit for the QEPAS-based sensor was ∼0.288 ppmv at the optimum averaging time of ∼125 s.

High-sensitivity acetylene detection system using ellipsoid multi-pass cell (EMPC) based on the opposite dual optical source

A multi-pass cell with ellipsoid structure used dual optical source of opposite side was proposed firstly in this paper. The EMPC was composed of double concave spherical mirrors and an ellipsoidal resonator. The radius of curvature of the two symmetric concave spherical mirrors was 200 mm and the length of the cavity was 120 mm. The size of the resonator of the photoacoustic (PA) cell in the experiment is Φ28mm×100mm and the size of the buffer cell at both ends of the PA cell is Φ70mm×10mm. The two laser beams were injected from the two sides of the concave spherical mirrors by the optical fiber. The absorption spectrum line with the acetylene wavelength of 1531.58 nm was selected. The simulation results show that the light beam can be reflected 37 times in EMPC and the optical path length can reach 8.64 m. In the experiment, the 2f signal peak value fitting curve obtained at different concentrations with good linearity. Allan deviation analysis of experimental results shows the detection limit of EMPC was 60 ppb when the average time was 2 s. When the average time was 2 s, the normalized noise equivalent absorption (NNEA) coefficient of the system was9.84×10-8cm-1·W·Hz-1/2. The whole system has high detection sensitivity and stability.

Highly sensitive multi-pass enhanced photoacoustic cell based on three spot-ring structure (TSR-MPC)

A highly sensitive photoacoustic cell with three spot-ring (TSR) based on the cylindrical structure is proposed for trace gas detection of acetylene (C2H2). In the resonant cavity, the acoustic characteristic of the three spot-ring multi-pass cell (TSR-MPC), such as the resonance frequency, the distribution of the acoustic pressure and the acoustic pressure level, are simulated. The internal optical path of the TSR-MPC is given in the paper. The optical path length of the TSR-MPC is about three times of the traditional multi-pass cell, which is about 10.2 m with 51 reflections. Combined with a distributed feedback (DFB) laser with the center wavelength was located at C2H2 absorption line of 1531.588 nm, the sensor system was established to verify the feasibility. Experimental results show the normalized noise equivalent absorption (NNEA) coefficient of the traditional MPC was calculated as 2.33×10-9cm-1·W·Hz-1/2. The NNEA coefficient of the TSR-MPC was calculated as 1.47×10-9cm-1·W·Hz-1/2.

High-temperature dual-species (CO/NH3) detection using calibration-free scanned-wavelength-modulation spectroscopy at 2.3 μm

A dual-species sensor based on scanned-wavelength-modulation spectroscopy (scanned-WMS) with the 2f/1f detection is developed for calibration-free CO and NH3 detection in high-temperature environments. A 2.3 μm distributed feedback (DFB) diode laser was used to exploit the CO and NH3 lines centred at 4300.7 cm−1 and 4301.13 cm−1, respectively. The DFB laser was scanned across the target absorption lines at 100 Hz with fast sinusoidal modulation at 5 kHz. The spectral fitting strategy was employed to least-square fit the simulated and measured WMS-2f/1f spectra. The integrated absorbance of target transitions inferred from the scanned-WMS-2f/1f spectral fitting was used to determine the species concentration. The performance of the developed sensor was validated by simultaneously measuring species concentrations under various temperatures (300–700 K) and at 1 atm in a high-temperature gas cell. The noise equivalent absorption (NEA) coefficient of 3.5 × 10-6 cm−1 for CO and 4.9 × 10-7 cm−1 for NH3 was achieved at 700 K with an absorption path length of 20 cm. The developed dual-species sensor has the potential to be applied in the exhaust of practical combustion systems such as NH3 monitoring for NOx abatement in selective catalytic reduction (SCR) aftertreatment and CO sensing for optimizing combustor efficiency.

Investigation and Optimization of a Line-Locked Quartz Enhanced Spectrophone for Rapid Carbon Dioxide Measurement.

We have developed a rapid quartz enhanced spectrophone for carbon dioxide (CO2) measurement, in which the laser wavelength was tightly locked to a CO2 absorption line and a custom quartz tuning fork (QTF) operating at 12.5 kHz was employed. The intrinsic QTF oscillation-limited response time, as well as the optimal feedback interval, was experimentally investigated. By tightly locking the laser to the R(16) transition of CO2, we obtained a stable laser operation with its center wavelength variation kept within 0.0002 cm−1, merely three times the laser linewidth. The reported CO2 sensor achieved a detection limit of 7 ppm, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 4.7 × 10−9 W·cm−1·Hz−1/2, at a response time of 0.5 s. The detection limit can be further improved to 0.45 ppm at an integration time of 270 s, illustrating a good system stability. This spectrophone enables the realization of compact and fast-response gas sensors for many scenarios, where CO2 concentration from sub-ppm to hundreds of thousands of ppm is expected.

Heterodyne interferometric photothermal spectroscopy for gas detection in a hollow-core fiber

Photothermal spectroscopy with the optical pump-probe configuration has been used for sensitive gas detection in hollow-core fibers. In this study, we demonstrate a new method of heterodyne interferometric photothermal gas sensing with a simplified sensor design and enhanced sensor stability. As a proof-of-principle, we used an interband cascade pump laser at 3.6 μm for nitrous oxide (N2O) detection in a hollow-core anti-resonant fiber (HC-ARF). A Mach-Zehnder interferometer (MZI) with a 1.5 μm near-infrared probe laser was implemented to measure the photothermal-induced refractive index change inside the HC-ARF. The probe laser was split into two beams, one transmitted through the HC-ARF and one frequency-shifted by using an acoustic-optic modulator. Both beams were recombined to generate a beat note signal, which was then demodulated by two cascaded lock-in amplifiers to extract the photothermal signal. Based on the first harmonic detection of wavelength modulation, we achieved a normalized noise equivalent absorption (NNEA) coefficient of 7.7×10−9 cm-1WHz-1/2. Compared to conventional photothermal gas sensors with the MZI configuration, our new detection scheme eliminates the sophisticated opto-mechanical stabilization system and exhibits the intrinsic immunity to the power fluctuation of the probe laser.

Modeling of a cylindrical resonant photoacoustic cell for high sensitive gas detection

AbstractPhotoacoustic spectroscopy (PAS) is a sensitive spectroscopic technique for trace gas sensing. As a key component of a PA sensor system, the cell constant of the cylindrical PA cell has been simulated for optimizing geometric design, and the acoustic pressure distribution and frequency response of the first longitudinal and radial modes are analyzed by using a self‐established algorithm model. According to the theoretical simulation, a resonant PAS sensor system was finally designed and evaluated at the first longitudinal mode for trace water vapor detection. Highly sensitive wavelength modulation spectroscopy and second harmonic detection technique was combined with PAS detection method for enhancing sensitivity. Allan deviation analysis indicates that a detection limit of 2 ppm for H2O could be achieved at an optimal integration time of 200 s, corresponding to a normalized noise equivalent absorption coefficient of .

Light-induced off-axis cavity-enhanced thermoelastic spectroscopy in the near-infrared for trace gas sensing.

A trace gas sensing technique of light-induced off-axis cavity-enhanced thermoelastic spectroscopy (OA-CETES) in the near-infrared was demonstrated by combing a high-finesse off-axis integrated cavity and a high Q-factor resonant quartz tuning fork (QTF). Sensor parameters of the cavity and QTF were optimized numerically and experimentally. As a proof-of-principle, we employed the OA-CETES for water vapor (H2O) detection using a QTF (Q-factor ∼12000 in atmospheric pressure) and a 10cm-long Fabry-Perot cavity (finesse ∼ 482). By probing a H2O line at 7306.75 cm-1, the developed OA-CETES sensor achieved a minimum detection limit (MDL) of 8.7 parts per million (ppm) for a 300 ms integration time and a normalized noise equivalent absorption (NNEA) coefficient of 4.12 × 10-9cm-1 WHz-1/2. Continuous monitoring of indoor and outdoor atmospheric H2O concentration levels was performed for verifying the sensing applicability. The realization of the proposed OA-CETES technique with compact QTF and long effective path cavity allows a class of optical sensors with low cost, high sensitivity and potential for long-distance and multi-point sensing.

A Cathodic Electrochromic Material Based on Thick Perylene Bisimide Film with High Optical Contrast and High Stability

A Cathodic Electrochromic Material Based on Thick Perylene Bisimide Film with High Optical Contrast and High Stability

Palm-sized methane TDLAS sensor based on a mini-multi-pass cell and a quartz tuning fork as a thermal detector.

A palm-sized methane (CH4) tunable diode laser absorption spectroscopy (TDLAS) sensor is reported, in which a quartz tuning fork (QTF) is used as a thermal detector, working together with a mini-multi-pass cell (mini-MPC) to compose a gas detection module (GDM) with a compact dimension of 78 mm × 40 mm × 40 mm. A 1.65 µm near-infrared distributed feedback (DFB) laser is installed in the sensor for CH4 detection. A minimum detection limit (MDL) of 52 ppb is achieved at an integration time of 300 ms, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 2.1×10-8 cm-1W/Hz1/2. A seven-day continuous monitoring of atmospheric CH4 concentration is implemented to verify the sensor's long-term stability.

Polymer-coated quartz tuning fork for enhancing the sensitivity of laser-induced thermoelastic spectroscopy.

A novel laser-induced thermoelastic spectroscopy (LITES) sensor based on a polymer-coated quartz tuning fork (QTF) is reported. Two types of polymer films with different thicknesses are deposited on commercially available QTF to improve the conversion efficiency of laser energy deposition into vibration. CO2 was selected as the target analyte for validation measurements. The experimental results indicate that by introducing a polymer coating, a maximum gain factor of 3.46 and 3.21 is attained for the signal amplitude and signal-to-noise ratio (SNR), respectively, when compared to traditional LITES that using only a bare QTF. A minimum detectable concentration of 0.181% can be obtained, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 1.74×10-11 cm-1·W·Hz-1/2, and the measurement precision is approximately 0.06% with an averaging time of 200 s. Here, we show what we believe is the first demonstration of polymer coated QTF for LITES sensing, compared with custom QTF, the design has the virtues of lower cost, simple and easy-to-operate, is a promising new strategy for sensitive trace gas analysis.

Isotopic gas analysis by means of mid-infrared photoacoustic spectroscopy targeting human exhaled air.

There is a great need for cost-efficient non-invasive medical diagnostic tools for analyzing humanly exhaled air. Compared to present day methods, photoacoustic spectroscopy (PAS) can provide a compact and portable (bedside), sensitive and inexpensive solution. We demonstrate a novel portable photoacoustic spectroscopic platform for isotopic measurements of methane (CH4). We identify and discriminate the 12CH4- and 13CH4 isotopologues and determine their mixing ratio. An Allan deviation analysis shows that the noise equivalent concentration for CH4 is 200 ppt (pmol/mol) at 100 s of integration time, corresponding to a normalized noise equivalent absorption coefficient of 5.1×10-9Wcm-1Hz-1/2, potentially making the PAS sensor a truly disruptive instrument for bedside monitoring using isotope tracers by providing real-time metabolism data to clinical personnel.