Off-beam Quartz-enhanced Photoacoustic Spectroscopy Research Articles

Mid-infrared swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork

We report the development of swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork for ultra-sensitive and high-resolution molecular spectroscopy. By using a quantum cascade laser (QCL) as the mid-infrared light source, a dual-feedback Pound–Drever–Hall locking method is proposed to lock the QCL frequency to a continuously swept optical cavity. By placing an off-beam quartz-enhanced photoacoustic spectroscopy module in a 48-mm Fabry–Pérot cavity, we are able to achieve ultra-sensitive gas detection based on the doubly resonant photoacoustic effect. As a proof-of-concept, we use a distributed feedback QCL to exploit the CO line at 2190.02 cm−1, where the cavity-locked QCL is scanned over a spectral range of 10 GHz with a spectral resolution of ∼3 MHz. With the incident laser power of 7.3 mW, the optical cavity (finesse 1931) builds up the intracavity power beyond 3 W. Our photoacoustic spectrometer achieves the minimum detection limit of 375 part-per-trillion (ppt) at the averaging time of 150 s and the normalized noise equivalent absorption coefficient of 1.27 × 10−9 Wcm−1 Hz−1/2.

Simultaneous Monitoring of Atmospheric CH4, N2O, and H2O Using a Single Gas Sensor Based on Mid-IR Quartz-Enhanced Photoacoustic Spectroscopy.

An optical sensor based on external-cavity quantum cascade laser (EC-QCL) was developed for simultaneous triple-species monitoring of CH4, N2O, and H2O vapor using off-beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS). The EC-QCL wavelength was scanned over three neighboring absorption lines of CH4 (1260.81 cm-1), N2O (1261.06 cm-1), and H2O vapor (1261.58 cm-1) by tuning the grating of the EC-QCL with a piezoelectric actuator. Molecular relaxation effects impacting the generation of the QEPAS signals resulting from light absorption by CH4 and N2O molecules were investigated in the mid-infrared region near 8 μm. A theoretical model was introduced for the mid-infrared region, including the beneficial influence of water vapor. An enhancement of the QEPAS signals by a factor of 3 for CH4 in air and of 20% for N2O in air was observed in humidified samples compared to that in dry samples. The QEPAS measurement was scaled by the calibrated reference spectrometers; detection limits of 98 ppbv for CH4, 12 ppbv for N2O, and 750 ppmv for H2O vapor were obtained with a 1σ signal-to-noise ratio (SNR = 1) in humidified gas mixtures. Real-time Kalman filtering was applied to improve the measurement precision by a factor of approximately 4 while keeping the same temporal resolution, leading to measurement precisions of 60 ppbv for CH4, 10 ppbv for N2O, and 0.07% for H2O in the measurements of 1.99 ppmv CH4 and 312 ppbv N2O humidified with 2.8% H2O vapor, with a 1 s lock-in amplifier time constant and an equivalent bandwidth of 0.1 Hz.

Infrared dual-gas CH4/C2H2 sensor system based on dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique

Highly sensitive and stable measurement of methane (CH4) and acetylene (C2H2) based on a novel dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique was realized by a compact 3D-printed gas cell with a size of 3 × 2 × 1 cm3. Two near-infrared distributed feedback diode lasers were employed to target the CH4 absorption line at 6046.9 cm−1 and the C2H2 absorption line at 6521.2 cm−1, respectively. Second-harmonic wavelength modulation spectroscopy method was used for photoacoustic signal recovery. A minimum detection level of ∼ 7.63 parts-per-million in volume (ppmv) for CH4 and a level of ∼ 17.47 ppmv for C2H2 were achieved with a 1 s lock-in integration time, leading to a normalized noise equivalent absorption (NNEA) coefficient of 7.24 × 10-8 cm−1·W·Hz−1 and 3.73 × 10-8 cm−1·W·Hz−1 for CH4 and C2H2, respectively. Allan-Werle deviation analysis was employed to evaluate the stability and the minimum detection limit (MDL) of the developed photoacoustic CH4/C2H2 dual-gas photoacoustic sensor. Owing to the high stability of the developed sensor system, an MDL of ∼ 0.73 ppmv and an MDL of ∼ 1.60 ppmv with a 100 s averaging time were achieved for CH4 and C2H2, respectively.

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.

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.

Research on Detecting CO2 With Off-Beam Quartz-Enhanced Photoacoustic Spectroscopy at 2.004 μm

Research on Detecting CO2 With Off-Beam Quartz-Enhanced Photoacoustic Spectroscopy at 2.004 μm

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

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

Quartz tuning fork embedded off-beam quartz-enhanced photoacoustic spectroscopy.

In order to achieve a high acoustic coupling strength and detection sensitivity and to simplify the assembly and alignment process in quartz-enhanced photoacoustic spectroscopy (QEPAS) technique, a novel quartz tuning fork (QTF) embedded off-beam QEPAS (E-OB-QEPAS) spectrophone was proposed. The structural parameters of the acoustic micro-resonator of the E-OB-QEPAS spectrophone were optimized for enhancing the signal-to-noise ratio gain based on experimental investigation. Compared with the on-beam configuration using a bare QTF, a detection sensitivity enhancement by a factor of ∼25 was achieved by embedding the QTF in one resonant tube. By using two resonant tubes simultaneously embedded with a QTF, dual-channel detection and a two-fold photoacoustic signal enhancement were realized and a detection sensitivity enhancement by a factor of ∼20 and ∼40 were achieved for the single-tube-enhanced and dual-tube-enhanced E-OB-QEPAS spectrophone, respectively.

Off-beam QEPAS sensor using an 11-μm DFB-QCL with an optimized acoustic resonator.

An off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor was designed for ethylene detection using a distributed-feedback quantum cascade laser (QCL) operating in the mid-infrared around 11 μm. The acoustic microresonator configuration was experimentally optimized using an original open-cell photoacoustic setup with a MEMS microphone. Correction factors based on theoretical acoustic models were introduced in order to accurately describe the response of millimeter-sized acoustic resonators. The optimized QEPAS sensor exhibited a limit of detection of 60 ppb for 60 s integration, giving a NNEA of 4.8 × 10-8 W·cm-1·Hz-0.5.

A Mid-Infrared Fiber-Coupled QEPAS Nitric Oxide Sensor for Real-Time Engine Exhaust Monitoring

We report the development and application of a compact and sensitive mid-infrared nitric oxide (NO) sensor using off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS). This QEPAS sensor exploits the strong absorption band of NO at $5.2~ \mu \text{m}$ using a distributed-feedback quantum cascade laser (QCL). The QCL was connected with the custom-designed photoacoustic gas cell with a mid-infrared multimode fiber to form a plug-and-play system. The selection of key sensor parameters, such as micro-resonator, laser modulation depth, and gas pressure, was discussed in detail about improving the detection sensitivity. With a minimum detection limit of 120 ppbv, our QEPAS NO sensor was deployed in real-time diesel-engine exhaust monitoring. We observed the influence of soot existing in the engine exhaust on the sensor performance, which was filtered by a high efficiency particulate arrestance material. Our sensor captured the varied NO concentrations between 126 and 187 ppmv in the exhaust at different engine rotational speeds.

NO2 trace gas monitoring in air using off-beam quartz enhanced photoacoustic spectroscopy (QEPAS) and interference studies towards CO2, H2O and acoustic noise

Abstract We present the development and characterization as well as comprehensive interference studies of a photoacoustic NO2 trace gas detection system. The system is based on an off-beam quartz enhanced photoacoustic scheme (off-beam QEPAS) and signal generation was initiated by amplitude modulating a low-cost diode laser emitting at 450 nm. The QEPAS sensor element features double-resonant amplification, still it is only ∼ 5 × 5 × 2.5 mm in size. The individual and combined resonance characteristics were investigated and specified to 52 dB amplification, adding up 15 dB acoustic- and 37 dB mechanical-resonance amplification. The linearity of the photoacoustic signal dependency on the analyte concentration was verified from 200 ppbV to 100 ppmV NO2 in synthetic air. The detection limit (3σ) was determined to 1.8 ppbV using a lock-in time constant of 10 s and an averaging time of 20 s. The normalized noise equivalent absorption coefficient was specified to 2.5·10−8 W cm−1 Hz−0.5. The stability of the signal was investigated over time and a slight drift by 1‰ was observed after 30 min without temperature stabilizing the photoacoustic cell (PAC). Noise analysis was performed by means of Allan deviation and the inverse dependency of response time and precision of the system on the lock-in time constant was outlined. We performed interference analyses towards N2, O2, CO2, H2O and acoustic noise, respectively. Although neither spectral interferences nor losses due to slow NO2 VT-relaxation were observed, O2 was identified to cause a 15% signal drop due to VVNO2-O2-relaxation. Changing H2O concentrations were found to cause acoustic detuning, which cannot be compensated by adjusting the frequency of modulation, because of the double-resonant feature of the PAC. However, alternative approaches of compensation were discussed. Finally, we carried out heavy traffic noise simulations and determined the QEPAS setup to be 46 times less susceptible towards ambient noise compared to standard microphone-based photoacoustic setups.

Off-beam quartz-enhanced photoacoustic spectroscopy-based sensor for hydrogen sulfide trace gas detection using a mode-hop-free external cavity quantum cascade laser

Hydrogen sulfide (H2S) trace gas detection based on off-beam quartz-enhanced photoacoustic spectroscopy using a continuous wave (CW), mode-hop-free external cavity (EC) quantum cascade laser tunable from 1310 to 1210 cm−1 was performed. A 1σ minimum detection limit of 492 parts per billion by volume (ppbv) using a 1 s lock-in time constant was obtained by targeting the line centered at 1234.58 cm−1. This value corresponds to a normalized noise equivalent absorption coefficient for H2S of 3.05 × 10−9 W cm−1 Hz−1/2.

Mid-infrared multimode fiber-coupled quantum cascade laser for off-beam quartz-enhanced photoacoustic detection.

A mid-infrared quartz-enhanced photoacoustic sensor was developed using a multimode fiber (MMF)-coupled quantum cascade laser (QCL) and demonstrated for sensitive nitric oxide (NO) detection at a wavelength near 5.26μm. The QCL radiation was readily coupled into a solid-core InF3 MMF (100μm core) with 97% coupling efficiency using an aspheric lens. Despite the 25.5% transmission loss for the 1m long MMF, the Gaussian beam-like fiber output of 5.72° divergence was almost completely focused through the microresonator tube (length, 8.0mm; ID, 600μm) designed for off-beam quartz-enhanced photoacoustic spectroscopy. The sensor exploiting the R6.5 (Π21/2) doublet of NO at 1900.08 cm-1 demonstrated a minimum detection limit of 24 parts per billion by volume at an averaging time of 130s. The sensor was found to be insensitive to the fiber bending noise for a bending radius >5 cm.

Enhanced near-infrared QEPAS sensor for sub-ppm level H2S detection by means of a fiber amplified 1582 nm DFB laser

A power-boosted quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor is developed for sub-ppm H2S trace-gas detection in the near-infrared spectral region. The sensor is based on off-beam QEPAS with an erbium-doped fiber amplified 1582nm distributed feedback (DFB) laser. The offset of the sensor floor noise caused by stray light and gas flow can be removed by an electrical modulation cancellation method, which lowers the noise to the theoretical thermal noise level. The sensor was optimized in terms of gas pressure and current modulation depth for H2S detection at 6320.6cm−1. The linearity of the sensor response to the laser power and H2S concentration confirms that saturation does not occur. With ∼1.4W optical excitation power and 67s averaging time, a H2S detection sensitivity of 142ppbv (parts per billion by volume) is achieved at atmospheric pressure and room temperature, which is the best value, reported in the literature so far for H2S QEPAS sensors. A side-by-side sensitivity comparison for different sensor systems is also reported.

Design and optimization of off-beam NO2 QEPAS sensor by use of E-MOCAM with a high power blue laser diode

A highly sensitive NO2 optical sensor has been designed by means of combining the electrical modulation cancellation method (E-MOCAM) and off-beam quartz enhanced photoacoustic spectroscopy (QEPAS). A high power multimode blue laser diode emitting at around 450 nm is used as the excitation light source of the photoacoustic signal. In the E-MOCAM, the balance signal is generated from a dual-channel function generator and introduced to the pin of the quartz tuning fork (QTF) to balance out the huge background noise. The principle of the E-MOCAM is explained in detail from the perspective of equivalent circuit of QTF, and the background noise of the high power LD-based QEPAS sensor is analyzed. Results show that stray light noises coming from the LD beam and blocked by the resonator and the photoacoustic cell are dominated in all the noises. Gas flow noise of QEPAS sensor is also estimated, and excessive noise could be introduced by the gas flow even at a rate below 200 sccm. The gas flow noise is measured at different gas flow rate, from 60 to 200 sccm. Compared with the QEPAS sensor based on wavelength modulation, the sensor based on amplitude modulation, especially in the case of high power light source, is more sensitive to the gas flow. The ultimate background noise of the off-beam QEPAS sensor can be reduced by 269 times after the E-MOCAM is applied. The performance of the NO2 QEPAS sensor is evaluated in the NO2/N2 mixtures of different concentrations, ranging from ppb to ppm levels. In the case of the 2.85 ppm NO2 measurement, the SNR of 630 is achieved. A linear fitting is implemented to evaluate the response of the sensor, resulting in an R square value of 0.999. Allan plot is used to investigate the long term stability of the sensor. The original background noise produced from the off-beam QEPAS configuration is less than that from the on-beam QEPAS configuration, thus the combination of off-beam QEPAS configuration and E-MOCAM shows a better stability. A detection limit of 0.34 ppb (1, 46 s integration time) for NO2 in N2 at atmospheric pressure can be achieved, which corresponds to a normalized noise equivalent absorption coefficient of 2.210-8 cm-1W/Hz1/2.

Trace-Gas Detection with Off-Beam Quartz Enhanced Photoacoustic Spectroscopy

Trace-gas sensors have a wide range of potential applications such as environmental monitoring, climate research, agriculture, workplace safety, medical diagnostics, and industrial process control. A recently introduced technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) is described. QEPAS use a quartz tuning fork as an acoustic transducer for a photoacoustic signal induced in an absorbing gas by modulated optical radiation. Advantages of the QEPAS compared to conventional photoacoustic spectroscopy include immunity to environmental acoustic noise and ultra-small sample volume. Trace gases of \(\hbox {CH}_{4}, \hbox {H}_{2}\hbox {O}\), and \(\hbox {NO}_{2}\) were monitored with a novel off-beam QEPAS approach and are described in detail.

Off-beam quartz-enhanced photoacoustic spectroscopy with LEDs

A photoacoustic ozone sensor based on quartz-enhanced photoacoustic spectroscopy is presented. Instead of a laser, a UV-LED at 285 nm is utilized as light source. Using an acoustical resonator in off-beam configuration, it is feasible to align the light of the LED through the resonator tube. The sensor is integrated in a miniaturized flow-through gas cell which allows fast gas exchange. The sensor performance and the influence of the speed of sound on the measurement signal are investigated. For the detection of ozone in the Hartley band, a detection limit of S = 1.27 ± 0.08 ppmv (1σ) and a noise equivalent absorption sensitivity of D = (3.02 × 10−8 ± 1 × 10−10) cm−1W(Hz)−1/2 have been achieved.

Theoretical analysis of off beam quartz-enhanced photoacoustic spectroscopy sensor

Off beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) sensors are based on a recently developed approach to off-beam photoacoustic (PA) detection which employs a quartz tuning fork (QTF) as an acoustic transducer. A microresonator (mR) with a side slit in the middle is used to enhance PA signal. This paper describes a theoretical model of an OB-QEPAS-based sensor. By deriving the acoustic impedances of the mR at two ends and the side slit in the middle in the model, we obtain a formula for numerically calculating the optimal mRs' parameters of OB-QEPAS-based sensor. We use the model to calculate the optimal mRs' lengths with respect to the resonant frequency of the QTF, acoustic velocities inside mRs, inner diameters of mRs, and acoustic conductivities of the mRs' side slits, and found out that the calculated results closely match experimental data. We also investigated the relationship between the mR selected in “on beam” QEPAS, OB-QEPAS, and an acoustic resonator (AR) excited in its first longitudinal mode used in conventional photoacoustic spectroscopy (PAS).

T-shape microresonator-based high sensitivity quartz-enhanced photoacoustic spectroscopy sensor

A novel spectrophone sensor prototype consisting of a T-shaped acoustic microresonator (T-mR) in off-beam quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS) is introduced for the first time. Its performance was evaluated and optimized through an acoustic model and experimental investigation via detection of water vapor in the atmosphere. The present work shows that the use of T-mR in QEPAS based sensor can improve the detection sensitivity by a factor of up to ~30, compared with that using only a bare QTF. This value is as high as that obtained in a conventional "on-beam" QEPAS, while keeping the advantages of "off-beam" QEPAS configuration: it is no longer necessary to couple excitation light beam through the narrow gap between the QTF prongs. In addition, the T-mR is really suitable for mass production with high precision.

Application of a broadband blue laser diode to trace NO_2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450 nm. A detection limit of 18 ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7 mW at a 1 s integration time, which corresponds to a 1 σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1) W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.