Normalized Noise Equivalent Absorption Research Articles

Quartz Enhanced Photoacoustic Detection Based on an Elliptical Laser Beam

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor system based on an elliptical laser beam for trace gas detection was demonstrated. A Powell lens was exploited to shape the circular laser beam into an elliptical laser beam for the full utilization of the quartz tuning fork (QTF) prong spacing. Based on the finite element modeling (FEM) simulation software COMSOL, the distribution of acoustic pressure on QTF prongs with different beam shapes was simulated theoretically. The experimental results showed that the QEPAS signal based on the elliptical laser beam had a 1.4-fold improvement compared with the circular laser beam, resulting in a minimum detection limit of 418.6 ppmv and the normalized noise equivalent absorption (NNEA) of 1.51 × 10−6 cm−1 W/√Hz at atmospheric pressure.

A Calibration-Free Methodology for Resonantly Enhanced Photoacoustic Spectroscopy Using Quantum Cascade Lasers

Photoacoustic spectroscopy (PAS) is a highly sensitive technique for trace gas sensing, which requires frequent re-calibration for changing environmental influences and input light power fluctuation. This is a major drawback against its deployment for on-site, long-term remote applications. To address this drawback here we present the theory and application of a Calibration-Free Wavelength Modulation Photoacoustic Spectroscopy (CF-WM-PAS) technique. It is applied for measurements of CH4 gas concentration in the mid-infrared at $8.6~\mu {m}$ wavelength using a Quantum Cascade Laser (QCL). The method normalizes the second harmonic ( ${R}_{2{f}}$ ) component, dominated by laser-gas interaction and optical intensity, by the first harmonic ( ${R}_{1{f}}$ ) component dominated by Residual Amplitude Modulation (RAM) DC offset, to isolate the output from changes in the gas matrix, optical intensity and electrical gain. This normalization technique removes influences from changes in the resonant frequency, gas concentration and incident optical power. It is confirmed using a ±1600 Hz change in modulation frequency around the resonance, a 1% to 10% change in gas concentration and an up to 78.3% attenuation in input light intensity with a custom built miniaturized 3D-printed sensor. A Normalized Noise Equivalent Absorption (NNEA) of $4.85\times 10^{-9}$ Wcm−1Hz−1/2 for calibration-free ${R}_{2{f}}/{R}_{1{f}}$ measurements is demonstrated.

Ultra-high sensitive trace gas detection based on light-induced thermoelastic spectroscopy and a custom quartz tuning fork

A highly sensitive trace gas sensor based on light-induced thermoelastic spectroscopy (LITES) and a custom quartz tuning fork (QTF) is reported. The QTF has a T-shaped prong geometry and grooves carved on the prongs' surface, allowing a reduction of both the resonance frequency and the electrical resistance but retaining a high resonance quality factor. The base of the QTF prongs is the area maximizing the light-induced thermoelastic effect. The front surface of this area was left uncoated to allow laser transmission through the quartz, while on the back side of the QTF, a gold film was coated to back-reflect the laser beam and further enhance the light absorption inside the crystal. Acetylene (C2H2) was chosen as the target gas to test and validate the LITES sensor. We demonstrated that the sensor response scales linearly with the laser power incident on the prong base, and the optimum signal to noise ratio was obtained at an optical power of 4 mW. A minimum detection limit of ∼325 ppb was achieved at an integration time of 1 s, corresponding to a normalized noise equivalent absorption coefficient of 9.16 × 10−10 cm−1W/√Hz, nearly one order of magnitude better with respect to the value obtained with a standard 32.768 kHz QTF-based LITES sensor under the same experimental conditions.

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

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

Near-Infrared Quartz-Enhanced Photoacoustic Sensor for H2S Detection in Biogas

A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for H2S detection operating in near-infrared spectral range is reported. The optical source is an erbium-doped fiber amplified laser with watt-level optical power. The QEPAS spectrophone is composed of a quartz tuning fork with a resonance frequency of 7.2 kHz, a quality factor of 8500, and a distance between prongs of 800 µm, and two tubes with a radius of 1.3 mm and a length of 23 mm acting as an organ pipe resonator. With this spectrophone geometry, the photothermal noise contribution of the spectrophone was removed and the theoretical thermal noise level was achieved. The position of both tubes with respect to custom quartz tuning fork has been investigated as a function of signal amplitude, Q-factor, and noise of the QEPAS sensor when a high-power laser was used. Benefit from the linearity of the QEPAS signal to the excitation laser power, a detection sensitivity of 330 ppb for H2S detection was achieved at atmospheric pressure and room temperature, when the laser power was 1.6 W and the signal integration time was set to 300 ms, corresponding to a normalized noise equivalent absorption of 3.15 × 10−9 W cm−1/(Hz)1/2. The QEPAS sensor was then validated by measuring H2S in a biogas sample.

Application of Micro Quartz Tuning Fork in Trace Gas Sensing by Use of Quartz-Enhanced Photoacoustic Spectroscopy.

A novel quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a micro quartz tuning fork (QTF) is reported. As a photoacoustic transducer, a novel micro QTF was 3.7 times smaller than the usually used standard QTF, resulting in a gas sampling volume of ~0.1 mm3. As a proof of concept, water vapor in the air was detected by using 1.39 μm distributed feedback (DFB) laser. A detailed analysis of the performance of a QEPAS sensor based on the micro QTF was performed by detecting atmosphere H2O. The laser focus position and the laser modulation depth were optimized to improve the QEPAS excitation efficiency. A pair of acoustic micro resonators (AmRs) was assembled with the micro QTF in an on-beam configuration to enhance the photoacoustic signal. The AmRs geometry was optimized to amplify the acoustic resonance. With a 1 s integration time, a normalized noise equivalent absorption coefficient (NNEA) of 1.97 × 10−8 W·cm−1·Hz−1/2 was achieved when detecting H2O at less than 1 atm.

Piezo-enhanced acoustic detection module for mid-infrared trace gas sensing using a grooved quartz tuning fork.

A grooved quartz tuning fork (QTF) with a prong spacing of 800 µm for QEPAS application is reported. The prongs spacing is large enough to facilitate optical alignments when a degraded laser beam is used for QEPAS-based trace gas sensors. The grooved QTF has a resonance frequency of 15.2 kHz at atmospheric pressure and is characterized by four rectangular grooves carved on the QTF prong surfaces. With a grooved-prong, the electrical resistance R of the QTF is reduced resulting in an enhanced piezoelectric signal, while the Q factor is not affected, remaining as high as 15000 at atmospheric pressure. The geometric parameters of the acoustic micro resonators (AmRs) for on-beam QEPAS were optimized to match the grooved QTF, and a signal-to-noise gain factor of ∼ 30 was obtained with an optimum configuration. The performance of the QEPAS-based sensor was demonstrated exploiting an interband cascade laser (ICL) for CH4 detection and a 1σ normalized noise equivalent absorption (NNEA) coefficient of 4.1×10-9 cm-1 W/√Hz was obtained at atmospheric pressure.

Tube-cantilever double resonance enhanced fiber-optic photoacoustic spectrometer

An ultra-high sensitive trace gas detection method based on tube-cantilever double resonance enhanced fiber-optic photoacoustic spectroscopy (PAS) is proposed. The first-order resonant frequencies of the acoustic resonant tube and the fiber-optic cantilever microphone were both equal to the frequency of the photoacoustic pressure signal. This method combines the amplitude amplification of the photoacoustic pressure wave in an acoustic resonant tube with the response enhancement of the photoacoustic signal by the cantilever, making the gas detection extremely sensitive. An experimental double resonance enhanced photoacoustic spectrometer was built for trace acetylene detection at the wavelength of 1532.83 nm. A noise equivalent detection limit (1σ) was achieved to be 27 ppt with a 200-s averaging time, which is the best value reported so far. In addition, the normalized noise equivalent absorption (NNEA) coefficient reached 4.2 × 10−10 cm−1 W Hz−1/2.

Quartz-Enhanced Photothermal-Acoustic Spectroscopy for Trace Gas Analysis

A crystal quartz tuning fork (QTF) was used as a detector to collect and amplify laser-induced photoacoustic and photothermal waves simultaneously for trace chemical analysis. A wavelength modulation technique was applied to the proposed quartz-enhanced photothermal-acoustic spectroscopy (QEPTAS) to improve the detection signal-to-noise ratio. The QTF detector was exposed to the illumination of a near-infrared distributed feedback laser at distances of 1 m and 2 m to evaluate the QEPTAS sensor performance. The QEPTAS sensor performance was determined by detecting water vapor in ambient air using a near-infrared distributed feedback laser with a power of ~10 mW and a wavelength of 1.39 μm. With an optimized modulation depth of 0.47 cm−1, the normalized noise equivalent absorption (NNEA) coefficients of 8.4 × 10−7 W·cm−1·Hz−1/2 and 3.7 × 10−6 W·cm−1·Hz−1/2 were achieved for a distance of 1 m and 2 m, respectively. The developed QEPTAS technique reduces the requirements for laser beam quality, resulting in a simple but robust sensor structure and demonstrates the ability of remote sensing of gas concentrations.

Quartz tuning fork enhanced photothermal spectroscopy gas detection system with a novel QTF-self-difference technique

A novel self-difference technique based on quartz tuning fork (QTF) is proposed in this manuscript. C2H2 is selected as the target analyte to verify the effectiveness of this technique. The measurement technique consists of two laser beams, one laser beam passes through the gas cell as the probe laser beam and the other beam is used as the reference laser beam. The vibration direction of the QTF caused by the two laser beams is opposite by adjusting the position where the two laser beams illuminate QTF. The two quartz tuning fork enhanced photothermal spectroscopy (QEPTS) signals are subtracted by utilizing the mechanical vibration characteristics of the QTF. As a result, the SNR increases 5.1 times in QTF-self-difference based system compared to a traditional differential gas detection system. Finally, the system achieved a minimum detection limit (MDL) of 723 ppbv, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 7.85 × 10−10 cm−1 W/Hz 1/2.

Photothermal CO detection in a hollow-core negative curvature fiber.

We demonstrate the first, to the best of our knowledge, photothermal carbon monoxide (CO) sensor using a hollow-core negative curvature fiber. The hollow-core fiber features a typical structure of one ring cladding containing eight nontouching capillaries to form a negative curvature core-surround. The photothermal effect in a 40-μm hollow core is induced by CO absorption at 2327nm and detected by a Mach-Zehnder interferometer operating at 1533nm. By using wavelength modulation spectroscopy, we achieve a normalized noise equivalent absorption coefficient of 4.4×10-8 cm-1 WHz-1/2. As CO has a very slow vibrational-translational relaxation process, we enhance the photothermal signal by enhancing the relaxation with the water vapor additive.

Scanned-wavelength intra-cavity QEPAS sensor with injection seeding technique for C2H2 detection

A high sensitivity scanned-wavelength intra-cavity quartz enhanced photoacoustic spectroscopy (QEPAS) system, based on injection seeding technique and Q-switched technique, was proposed in this paper. In this system, the gas cell was placed in the acousto-optic Q-switched fiber laser cavity, and the high power Q-switched pulses were used to acoustic wave excitation. A distributed feedback (DFB) diode laser with a wavelength scanning range from 1532.63 nm to 1533.03 nm was used as the seed laser to select the laser wavelength. The laser power emitted by the seed laser operating at an injection current of 56 mA (the gas absorption peak) was 3 mW. A custom-made broadband fiber Bragg grating (FBG) with a bandwidth of 500 pm (picometer) was used to stabilize the cavity operation and suppress the amplified spontaneous emission (ASE) spectrum. With an intra-cavity peak pulse power of 0.88 W, the system achieved a minimum detection limit (MDL) of 507 ppbv at atmospheric pressure and room temperature, corresponding to a normalized noise-equivalent absorption (NNEA) of 1.616 × 10−8 W cm−1 Hz−1/2.

Trace Gas Detection System Based on All-Optical Quartz-Enhanced Photoacoustic Spectroscopy.

An all-optical quartz-enhanced photoacoustic spectroscopy system (QEPAS) with quadrature point stabilization for trace gas detection was reported. The extrinsic interferometry-based optical fiber Fabry-Perot sensor with quadrature point self-stabilization for detection of quartz prong vibration was used to replace the conventional one. The optimal coefficient of the modulation depth was ∼2.2 theoretically and experimentally, corresponding to the modulation depth of ∼0.1795 cm-1 at an acetylene (C2H2) absorption line of 6534.36 cm-1. Furthermore, the enhancement of QEPAS signal was obtained by using different microresonators. The minimum detectable limit of ∼580 parts per billion by volume (ppbv) was obtained. A normalized noise equivalent absorption coefficient for C2H2 of 2.95 × 10-7 cm-1·W·Hz-1/2 was obtained. The detection sensitivity was enhanced by a factor of ∼2.1 in comparison to the conventional QEPAS system. The linear correlation coefficient of the QEPAS signal response to the C2H2 concentration was 0.998 within the range from 10 parts per million by volume (ppmv) to 500 ppmv. Finally, the long-term stability of the QEPAS system was evaluated using Allan deviation analysis, and the ultimate detection limit of ∼130 ppbv was reached for an optimum averaging time of ∼108 s at atmospheric pressure and ambient temperature.

Intensity Modulated Photothermal Measurements of NO2 with a Compact Fiber-Coupled Fabry-Pérot Interferometer.

Sensors for the reliable measurement of nitrogen dioxide concentrations are of high interest due the adverse health effects of this pollutant. This work employs photothermal spectroscopy to measure nitrogen dioxide concentrations at the parts per billion level. Absorption induced temperature changes are detected by means of a fiber-coupled Fabry–Pérot interferometer. The small size of the interferometer enables small detection volumes, paving the way for miniaturized sensing concepts as well as fast response times, demonstrated down to 3 s. A normalized noise equivalent absorption of cm−1W/ is achieved. Additionally, due to the rigid structure of the interferometer, the sensitivity to mechanical vibrations is shown to be minor.

Highly sensitive acetylene detection based on multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy and a fiber amplified diode laser.

In this paper, a multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy (PAS) based gas sensor is reported for the first time. The multi-pass retro-reflection-cavity consisted of two right-angle prisms and was designed to reflect the laser beam to pass through the photoacoustic (PA) cell four times, which improved the acetylene (C2H2)-PAS sensor signal level significantly. The optical power of a near-infrared distributed feedback (DFB) diode laser emitting a continuous wave (CW) was amplified to 1000 mW with an erbium-doped fiber amplifier. The background noise was reduced with wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques. The linear optical power and concentration response of such a PAS sensor were investigated, and the experimental results showed excellent characteristics. When the integration the time of the sensor system was set to 1 s, the minimum detection limit (MDL) for C2H2 detection was 8.17 ppb, which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of 1.84 × 10-8 cm-1W/√Hz. The long-term stability of such a multi-pass retro-reflection-cavity-enhanced PAS based C2H2 sensor was evaluated by an Allan deviation analysis. It was demonstrated that the multi-pass retro-reflection-cavity-enhanced PAS sensor had an excellent stability. An MDL of 600 ppt was achieved when the integration time was set to ~1000 s. It was verified that the method of multi-pass retro-reflection-cavity-enhanced PAS with an amplified laser source improved the sensor performance significantly. If an appropriate cavity design with increasing reflection times is used, the MDL of such a PAS-based sensor can be further improved.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy.

An optical cavity can be utilized as an excellent transducer for highly sensitive gas detection with the application of photothermal spectroscopy, featuring the beneficial property of an ultra-low absorption volume within a rugged sensing element. We report the novel implementation of balanced detection in Fabry-Perot photothermal interferometry via two identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous cavity-based photothermal sensors was effectively rejected close to the fundamental limit of shot noise. A quantum cascade laser served as mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced variations. The metrological qualities of the sensor were investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm-1, a 5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 7.5 × 10-9 cm-1 W Hz-1/2. Additionally, the sensor showed excellent long-term stability, enabling integration times of a few thousand seconds.

Ppb-Level Quartz-Enhanced Photoacoustic Detection of Carbon Monoxide Exploiting a Surface Grooved Tuning Fork.

A compact and sensitive carbon monoxide (CO) sensor was demonstrated by using quartz enhanced photoacoustic spectroscopy (QEPAS) exploiting a novel 15.2 kHz quartz tuning fork (QTF) with grooved surfaces. The custom QTF was designed to provide a quality factor as high as 15 000 at atmospheric pressure, which offers a high detection sensitivity. A large QTF prong spacing of 800 μm was selected, allowing one to avoid the use of any spatial filters when employing a quantum cascade laser as the excitation source. Four rectangular grooves were carved on two prong surfaces of the QTF to decrease the electrical resistance and hence enhance the signal amplitude. With water vapor as the catalyst for vibrational energy transfer, the sensor system using the novel surface grooved QTF achieved a CO minimum detection limit of 7 ppb for a 300 ms averaging time, which corresponds to a normalized noise equivalent absorption coefficient of 8.74 × 10-9 cm-1W /√Hz. Continuous measurements covering a seven-day period for atmospheric CO were implemented to verify the reliability and validity of the developed CO sensor system.

Ultrasensitive photoacoustic detection in a high-finesse cavity with Pound-Drever-Hall locking.

We demonstrate an ultrasensitive photoacoustic sensor using a low laser power (4mW) and high-finesse (>9000) optical cavity. The Pound-Drever-Hall (PDH) method is adopted to lock the external cavity diode laser at 1531.58nm to the Fabry-Pérot cavity. By placing a photoacoustic cell inside the 130-mm-long optical cavity, we obtain an enhancement of more than 630 times in laser power for acetylene (C2H2) detection. The present photoacoustic spectroscopy (PAS) sensor achieves a normalized noise equivalent absorption coefficient of 1.1×10-11 cm-1 WHz-1/2, which is unprecedented sensitivity among all the current PAS sensors. Our results demonstrate the feasibility of merging PAS with a high-finesse cavity using PDH locking for ultrasensitive trace gas detection.

Quartz Enhanced Photoacoustic Spectroscopy Based on a Custom Quartz Tuning Fork.

We have designed and fabricated a custom quartz tuning fork (QTF) with a reduced fundamental frequency; a larger gap between the prongs; and the best quality factor in air at atmospheric conditions ever reported, to our knowledge. Acoustic microresonators have been added to the QTF in order to enhance the sensor sensitivity. We demonstrate a normalized noise equivalent absorption (NNEA) of 3.7 × 10−9 W.cm−1.Hz−1/2 for CO2 detection at atmospheric pressure. The influence of the inner diameter and length of the microresonators has been studied, as well as the penetration depth between the QTF’s prongs. We investigated the acoustic isolation of our system and measured the Allan deviation of the sensor.

Highly sensitive photoacoustic gas sensor based on multiple reflections on the cell wall

A highly sensitive and miniature photoacoustic (PA) sensor based on multiple reflections on the inner cell wall is presented for trace gas detection. The collimated laser light is obliquely incident from the side wall of the cylindrical PA cell. Multiple reflections occur on the inner surface of the cell wall to increase the length of the absorption path. The PA pressure signal can thus be enhanced. The PA signal is measured by the second harmonic wavelength modulation spectrum (2f-WMS) technology to eliminate interference from the fundamental frequency signal generated by the wavelength-independent absorption of the cell wall. The internal volume of the PA cell is only 848 μL. To further minimize the PA sensor, a micro-electro-mechanical system (MEMS) microphone is used as the acoustic detector. In order to evaluate the performance of the designed sensor, trace C2H2 is detected as the target gas at the wavelength of 1531.588 nm. Experimental results show that the amplitude of the PA pressure signal using the multi-pass cell is 6.4 times that of a single-pass cell. The detection limit is achieved to be 31 ppb for an averaging time of 400 s. Furthermore, the normalized noise equivalent absorption (NNEA) coefficient is calculated as 1.1×10-8 cm−1 W Hz−1/2.