- Email: [email protected]
A portable gas sensor for sensitive CO detection based on quartz-enhanced photoacoustic spectroscopy
Optics and Laser Technology 115 (2019) 129–133
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A portable gas sensor for sensitive CO detection based on quartz-enhanced photoacoustic spectroscopy
T
Ying Hea, Yufei Maa, Yao Tonga, Xin Yua, Frank K. Tittelb a b
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
H I GH L IG H T S
low power consumption, rechargeable CO-QEPAS sensor was demonstrated. • AA compact, minimum detection limit shows an excellent sensitivity. • TheppbCOlevel concentration from combustion was measured for a practical application. •
A R T I C LE I N FO
A B S T R A C T
Keywords: QEPAS ppb-level detection Compact CO sensor
A compact, portable and rechargeable sensor platform based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was demonstrated for the sensitive detection of carbon monoxide (CO). The acoustic detection module and the circuit components were both integrated which formed a sensor system with a weight of 3.2 kg and dimensions of 250 × 250 × 50 mm3. Water vapor was added to the gas mixture in order to accelerate the vibration translation rate of CO molecules and improve the CO-QEPAS signal performance. An Allan deviation analysis was performed to assess the long-term performance of such a sensor system. For an integration time of 800 s, a minimum detection limit (MDL) of 21 ppb was obtained. To demonstrate the practical application of the reported compact and portable QEPAS sensor, a continuous measurement of CO from the combustion of three kinds of burning materials was performed.
1. Introduction Carbon monoxide (CO) is a highly toxic gas, which is emitted from incomplete burning of natural gas, carbon containing fuels and organic matter. CO can cause hypoxia in humans as a result of excessive exposure to CO. A safety limitation of a 10 ppm CO concentration for an 8 h period was adopted by the National Air Quality Standard of the UK government [1]. In addition, a certain concentration results in climate change and global warming. From a medical perspective, CO is a physiological tracer for diverse diseases such as asthma, Alzheimer’s and inflammatory lung disease [2,3]. Therefore, there is a need to develop a sensitive, portable sensor system for continuous CO monitoring. Optical detection techniques used for CO sensing, include tunable diode laser absorption spectroscopy (TDLAS) [4–8] and quartz-enhanced photoacoustic spectroscopy (QEPAS) [9–12], have several merits, such as a fast response as well as a high selectivity. The TDLAS method requires a large size of the multi pass cell for improving the sensor sensitivity and the numerous optical components which are
E-mail address: [email protected] (Y. Ma). https://doi.org/10.1016/j.optlastec.2019.02.030 Received 5 December 2018; Accepted 4 February 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
needed for laser beam alignment. The QEPAS technique, a significant innovation of photoacoustic spectroscopy, utilizes a commercial quartz crystal oscillator as the acoustic detector, which has a small size and excellent immunity to ambient acoustic noise. Therefore, QEPAS is well suited for sensitive measurements and practical applications in trace gas sensing [13–19]. For CO detection based on QEPAS, different laser sources were adopted, such as a quantum cascade laser ([email protected] μm) [10,11] and a distributed feedback (DFB) diode laser (@2.33 μm and 1.57 μm) [9,12,15]. However, due to the fact that the optical structure used discrete components, the CO-QEPAS sensors were on experimental stages. The sensor size was large and not suitable for practical applications. The size of the optical unit can be significantly reduced by using a Grin lens, which can transmit the diode laser radiation in the communication band [20]. However, there does not exist a similar optical element with a small size to deliver diode laser radiation beyond 2 μm. Therefore, the optical elements are still using block optical lenses for a CO-QEPAS sensor. Hence, for practical applications, a compactness of the optical structure is critical and a miniaturized, integrated,
Optics and Laser Technology 115 (2019) 129–133
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stable and sensitive CO-QEPAS system must be developed. This manuscript reports a compact, portable, rechargeable and sensitive CO gas sensor based on a QEPAS technique. The acoustic detection module (ADM) was assembled by miniaturizing the optical structure. The optical unit includes an ADM and a reference cell as well as the electrical module, which forms a compact and portable COQEPAS sensor system. A quartz turning fork (QTF) with a resonant frequency of 30.72 kHz was utilized and two micro resonators (mRs) were applied to improve the acoustic wave amplitude. To further enhance the QEPAS signal, water vapor was mixed into the measured gas in order to accelerate the vibrational translation rate. Furthermore, an Allan deviation analysis was used to compare the integrated ADM and discrete optical structure, which showed that the integrated ADM has an improved stability. The CO concentration released from three kinds of combustion materials were measured continuously, which demonstrated that the reported portable and compact CO-QEPAS sensor system has an excellent performance capability in real world applications.
integrated ADM. A high optical power serves to improve the QEPAS sensor performance. Therefore, a 2.33 μm continuous, distributed feedback (DFB) diode laser with a maximum output power of ∼6 mW was used as the excitation source. A FBS with a splitting ratio of 1/9 was applied to divide the laser output. Ninety percent of laser power was injected into the integrated ADM and the rest was transmitted to the reference cell. The fiber-coupled output laser was collimated and focused by a fiber collimator (FC) with a focal length of 11 mm and a plano-convex lens with a focal length of 30 mm, respectively. The FC was fixed with the plano-convex lens by a metallic sleeve. A microscope was used to assemble the micro resonators (mR). The laser beam was focused to pass through the QTF prongs and the mRs without touching. The beam size at the focal point in the mRs was ∼40 μm. The ADM was manufactured from an aluminum alloy with a dimension of 50 × 30 × 21 mm3. The gas chamber inside the ADM was sealed by a quartz glass window with room temperature, vulcanized silicone rubber and the gas path was guided by gas pipelines. The total weight of this ADM including the additional elements was only 71 g.
2. The architecture of the integrated ADM and sensor system
3. Experimental optimization
2.1. The sensor configuration
In this experiment, wavelength modulation spectroscopy associated with a harmonic demodulation method was employed to analyze the piezoelectric signal generated by the QTF. The 2nd harmonic signal (2f) was demodulated as the QEPAS system signal. A 5% CO:N2 target gas was flushed into the ADM with a flow rate of 120 mL/min. Compared to the usually used 32.768 kHz QTF, a QTF with a lower resonance frequency f0 of 30.72 kHz was used. The experiments were conducted with identical conditions and are shown in Fig. 3. It can be seen that compared to a standard 32.768 kHz QTF, the 2f signal amplitude was improved by a factor of 1.2 times for that of the 30.72 kHz QTF. Therefore, in the subsequent investigations, the 30.72 kHz QTF was adopted. An effective improvement of the QEPAS sensor signal is obtained when two cylindrical tubes were employed as mRs, which have the ability of accumulating acoustic energy. The optimum length of mR should be within λs/4 ∼ λ s/2, where λs is the acoustic wavelength. Due to the acoustic speed of 340 m/s in the air and the modulated frequency of 30.72 kHz, the calculated optimal length range of the mR is 2.8 ∼ 5.5 mm. Therefore, the mRs made from stainless steel with lengths of 3 mm, 4 mm, 5 mm, 5.5 mm were investigated experimentally. The inner diameter of the mRs is 0.5 mm and the outer diameter is 1.27 mm. The gap between the QTF surface and the end of mRs was 25 μm, and the distance from QTF tips to the axis of mR was 0.7 mm.
The system configuration and internal structure of the reported sensor containing two floors is shown in Fig. 1. Floor I is the electrical circuit unit, which includes the diode laser driver, the digital-to-analog (DA) and analog-to-digital (AD) converters, the transimpedance amplifier (TA) and a custom lock-in amplifier. The power supply system consists of a rechargeable battery with the capability of operating the sensor for 12 h and which makes it suitable for outdoor applications. Floor II is the optical unit, which is composed of a fiber beam splitter (FBS) with a splitting ratio of 1/9, an integrated ADM, a CO reference cell with an optical path length of 30 mm (Wavelength Reference™, 30% CO:N2@200 Torr) and an InGaAs PIN photodiode (PD, G12183010 K, Hamamatsu, Japan). A front panel is used for parameter setting and display. The power source connector, gas pipelines and the data communication interface connecting with the PC were mounted behind the front panel. The sensor has a size of 250 × 250 × 50 mm3 and a weight of 3.2 kg. 2.2. The integrated acoustic detection module Fig. 2 depicts a 3D model and the assembled configuration of the
Fig. 1. Schematic of the CO sensor. (a) System configuration; (b) Sensor exterior with the internal structure. FBS: fiber beam splitter, ADM: acoustic detection module, TA: transimpedance amplifier, PD: photodiode, PC: personal computer. 130
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Fig. 2. The designed 3D model and assembled configuration of the integrated ADM. (a) The designed 3D model, 1—quartz glass window, 2—quartz tuning fork (QTF), 3—micro resonator (mR), 4—plano-convex lens, 5—fiber collimator (FC), 6—metallic sleeve, 7—gas pipelines; (b) Assembled configuration of the integrated ADM.
modulation depth of 0.36 cm−1 were employed in the subsequent experiments. An enhancement of the QEPAS signal can be achieved by improving the vibration-translation (V-T) relaxation rate of the CO molecule and water vapor (H2O) is an effective molecule catalyst. The CO sample was humidified by passing through a water path with a constant length. The concentration of the water vapor was determined by means of a direct absorption method. Experimentally, as shown in Fig. 5, an improvement of ∼6 fold was obtained when the 1.01% H2O vapor was added in the CO:N2 target gas. Furthermore, pure nitrogen (N2) was flushed into the ADM to evaluate the background noise in such a sensor system, which is depicted in Fig. 5. The 1σ value of background noise was 1.4 μV, which resulted in a minimum detection limit (MDL) of 9.1 ppm for a 1 s integration time. The corresponding normalized noise equivalent absorption (NNEA) coefficient evaluating the sensitivity of sensor was calculated to be 1.8 × 10−8 cm−1 W/Hz−1/2. Fig. 3. 2f signal for two QTFs with resonance frequency f0 of 30.72 kHz and 32.768 kHz.
4. Performance assessment of the CO-QEPAS sensor To evaluate the long-term stability of the reported CO-QEPAS sensor with a compact structure, an Allan deviation analysis was performed for the system with a discrete optical configuration and an integrated ADM, respectively. The gas chamber was flushed with pure N2 at a constant flow rate, and the measurements lasted for more than two hours which are shown in Fig. 6. It can be seen that a MDL of 930 ppb can be achieved with an integration time of 300 s in a discrete optical configuration, which indicated an improvement of the MDL by one order of magnitude, and it is almost equal to those reported in Refs. [21–23]. Furthermore, the MDL was improved to 21 ppb when the integration
Fig. 4. 2f signal amplitude with different mRs as a function of laser modulation depth.
Furthermore, the wavelength modulation depth of the diode laser should be optimized for wavelength modulation spectroscopy in order to achieve the highest QEPAS signal level. The 2f signal amplitude with different laser modulation depths corresponding to four kinds of mRs is depicted in Fig. 4. A maximum signal amplitude was achieved when mRs with a length of 5 mm were used. The signal amplitude improved with increasing modulation depth and this trend does not change until the modulation depth of the diode laser was higher than 0.36 cm−1. Therefore, mRs tubes with a length of 5 mm and a laser wavelength
Fig. 5. 2f signal with and without water vapor as well as the sensor system background noise. 131
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Fig. 8. Continuous monitoring of the CO concentration emitted by combustion of three materials.
Fig. 6. Allan deviation analysis for the sensor system with discrete optical structure and an integrated ADM.
5. Conclusions
time exceeds 800 s for the CO-QEPAS sensor system with an integrated ADM. In addition, the MDL was 130 ppb with a sensor stability of 100 s. Therefore, compared to a MDL of 9.1 ppm reported in Section 3, finally a ∼430 folds maximum improvement for the MDL illustrated that such a sensor system with a compact structure could suppress the system instability effectively and results in an excellent stability and a longterm measurement capability. In this work, the 3f signal, which is demodulated from the reference cell was used to lock the wavelength of laser output to the targeted CO absorption peak. The measured 2f and 3f signals are shown in Fig. 7. The null point between the maximum value and the minimum values of the 3f signals located at the peak of 2f signal at the wavenumber axis was used to lock the diode laser wavelength. Therefore, the peak value of 2f signal can be continuously obtained. To preliminarily assess the capability of the CO-QEPAS sensor in a practical application, a gas pump was utilized to draw the exhaust gas released from the combustion chamber into the sensor’s ADM with a gas flow rate of 120 mL/min. The CO concentration produced by three types of materials combustion was continuously measured. Fig. 8 shows the continuous measurement results for the combustion of a cigarette, timber and cable sheath sequentially. It can be seen that the carbonaceous organic material such as tobacco and timber are more likely to release CO (> 150 ppm) during combustion and the CO concentration for the combustion of industrial polymer cable sheath is low with a ∼50 ppm concentration. The power consumption of the reported CO-QEPAS sensor system during continuous measurements was also monitored, which showed a low power dissipation of ∼4.2 W.
In conclusion, a portable and sensitive CO gas sensor based on a QEPAS technique was demonstrated. The ADM has a compact and integrated configuration containing the optics and acoustic detection components with a dimension of 50 × 30 × 21 mm3 and a mass of 71 g. The QEPAS system was assembled and resulted in a 250 × 250 × 50 mm3, 3.2 kg portable sensor system. A QTF with a low f0 of 30.72 kHz was utilized as the photoacoustic detector. The length of mR tubes and the wavelength modulation depth of the diode laser were optimized experimentally. Water vapor was employed as the associated V-T relaxation rate improving catalyst, which resulted in a MDL of 9.1 ppm for a 1 s integration time and the calculated NNEA was 1.8 × 10−8 cm−1 W/Hz−1/2. An Allan deviation analysis was used to evaluate and compare the stability for the reported integrated sensor and a discrete structural sensor system. With an integration time of 800 s, the MDL was improved to be 21 ppb for the integrated CO-QEPAS sensor. Compared to a discrete sensor system, the significant improvement indicated that this compact and portable sensor system has an excellent stability and a long-term measurement capability. Finally, continuous monitoring was realized by a laser wavelength locking technique based on 3f signal demodulation. The CO concentration levels from three types of burning materials was measured, which demonstrated the practical application of the reported sensor. With a ppb level detection limitation, a low power consumption of ∼4.2 W, a light weight of 3.2 kg, a rechargeable battery capable of a 12 h running time, an excellent stability and a compact, portable configuration, the reported CO-QEPAS sensor has applications in fire detection, mining accidents and industrial process control based on balloon and unmanned aerial vehicle (UAV) platforms.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 61505041 and 61875047); the Natural Science Foundation of Heilongjiang Province of China (Grant No. JJ2019YX0173), the Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-Q18052); the Application Technology Research and Development Projects of Harbin (No. 2016RAQXJ140); and the Fundamental Research Funds for the Central Universities. Frank K. Tittel gratefully acknowledges the financial support from the US National Science Foundation (NSF) ERC MIRTHE award and a grant C-0586 from the Welch Foundation. Fig. 7. The measured 2f and 3f QEPAS signal. 132
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References
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133
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A portable gas sensor for sensitive CO detection based on quartz-enhanced photoacoustic spectroscopy
T
Ying Hea, Yufei Maa, Yao Tonga, Xin Yua, Frank K. Tittelb a b
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150001, China Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
H I GH L IG H T S
low power consumption, rechargeable CO-QEPAS sensor was demonstrated. • AA compact, minimum detection limit shows an excellent sensitivity. • TheppbCOlevel concentration from combustion was measured for a practical application. •
A R T I C LE I N FO
A B S T R A C T
Keywords: QEPAS ppb-level detection Compact CO sensor
A compact, portable and rechargeable sensor platform based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was demonstrated for the sensitive detection of carbon monoxide (CO). The acoustic detection module and the circuit components were both integrated which formed a sensor system with a weight of 3.2 kg and dimensions of 250 × 250 × 50 mm3. Water vapor was added to the gas mixture in order to accelerate the vibration translation rate of CO molecules and improve the CO-QEPAS signal performance. An Allan deviation analysis was performed to assess the long-term performance of such a sensor system. For an integration time of 800 s, a minimum detection limit (MDL) of 21 ppb was obtained. To demonstrate the practical application of the reported compact and portable QEPAS sensor, a continuous measurement of CO from the combustion of three kinds of burning materials was performed.
1. Introduction Carbon monoxide (CO) is a highly toxic gas, which is emitted from incomplete burning of natural gas, carbon containing fuels and organic matter. CO can cause hypoxia in humans as a result of excessive exposure to CO. A safety limitation of a 10 ppm CO concentration for an 8 h period was adopted by the National Air Quality Standard of the UK government [1]. In addition, a certain concentration results in climate change and global warming. From a medical perspective, CO is a physiological tracer for diverse diseases such as asthma, Alzheimer’s and inflammatory lung disease [2,3]. Therefore, there is a need to develop a sensitive, portable sensor system for continuous CO monitoring. Optical detection techniques used for CO sensing, include tunable diode laser absorption spectroscopy (TDLAS) [4–8] and quartz-enhanced photoacoustic spectroscopy (QEPAS) [9–12], have several merits, such as a fast response as well as a high selectivity. The TDLAS method requires a large size of the multi pass cell for improving the sensor sensitivity and the numerous optical components which are
E-mail address: [email protected] (Y. Ma). https://doi.org/10.1016/j.optlastec.2019.02.030 Received 5 December 2018; Accepted 4 February 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
needed for laser beam alignment. The QEPAS technique, a significant innovation of photoacoustic spectroscopy, utilizes a commercial quartz crystal oscillator as the acoustic detector, which has a small size and excellent immunity to ambient acoustic noise. Therefore, QEPAS is well suited for sensitive measurements and practical applications in trace gas sensing [13–19]. For CO detection based on QEPAS, different laser sources were adopted, such as a quantum cascade laser ([email protected] μm) [10,11] and a distributed feedback (DFB) diode laser (@2.33 μm and 1.57 μm) [9,12,15]. However, due to the fact that the optical structure used discrete components, the CO-QEPAS sensors were on experimental stages. The sensor size was large and not suitable for practical applications. The size of the optical unit can be significantly reduced by using a Grin lens, which can transmit the diode laser radiation in the communication band [20]. However, there does not exist a similar optical element with a small size to deliver diode laser radiation beyond 2 μm. Therefore, the optical elements are still using block optical lenses for a CO-QEPAS sensor. Hence, for practical applications, a compactness of the optical structure is critical and a miniaturized, integrated,
Optics and Laser Technology 115 (2019) 129–133
Y. He, et al.
stable and sensitive CO-QEPAS system must be developed. This manuscript reports a compact, portable, rechargeable and sensitive CO gas sensor based on a QEPAS technique. The acoustic detection module (ADM) was assembled by miniaturizing the optical structure. The optical unit includes an ADM and a reference cell as well as the electrical module, which forms a compact and portable COQEPAS sensor system. A quartz turning fork (QTF) with a resonant frequency of 30.72 kHz was utilized and two micro resonators (mRs) were applied to improve the acoustic wave amplitude. To further enhance the QEPAS signal, water vapor was mixed into the measured gas in order to accelerate the vibrational translation rate. Furthermore, an Allan deviation analysis was used to compare the integrated ADM and discrete optical structure, which showed that the integrated ADM has an improved stability. The CO concentration released from three kinds of combustion materials were measured continuously, which demonstrated that the reported portable and compact CO-QEPAS sensor system has an excellent performance capability in real world applications.
integrated ADM. A high optical power serves to improve the QEPAS sensor performance. Therefore, a 2.33 μm continuous, distributed feedback (DFB) diode laser with a maximum output power of ∼6 mW was used as the excitation source. A FBS with a splitting ratio of 1/9 was applied to divide the laser output. Ninety percent of laser power was injected into the integrated ADM and the rest was transmitted to the reference cell. The fiber-coupled output laser was collimated and focused by a fiber collimator (FC) with a focal length of 11 mm and a plano-convex lens with a focal length of 30 mm, respectively. The FC was fixed with the plano-convex lens by a metallic sleeve. A microscope was used to assemble the micro resonators (mR). The laser beam was focused to pass through the QTF prongs and the mRs without touching. The beam size at the focal point in the mRs was ∼40 μm. The ADM was manufactured from an aluminum alloy with a dimension of 50 × 30 × 21 mm3. The gas chamber inside the ADM was sealed by a quartz glass window with room temperature, vulcanized silicone rubber and the gas path was guided by gas pipelines. The total weight of this ADM including the additional elements was only 71 g.
2. The architecture of the integrated ADM and sensor system
3. Experimental optimization
2.1. The sensor configuration
In this experiment, wavelength modulation spectroscopy associated with a harmonic demodulation method was employed to analyze the piezoelectric signal generated by the QTF. The 2nd harmonic signal (2f) was demodulated as the QEPAS system signal. A 5% CO:N2 target gas was flushed into the ADM with a flow rate of 120 mL/min. Compared to the usually used 32.768 kHz QTF, a QTF with a lower resonance frequency f0 of 30.72 kHz was used. The experiments were conducted with identical conditions and are shown in Fig. 3. It can be seen that compared to a standard 32.768 kHz QTF, the 2f signal amplitude was improved by a factor of 1.2 times for that of the 30.72 kHz QTF. Therefore, in the subsequent investigations, the 30.72 kHz QTF was adopted. An effective improvement of the QEPAS sensor signal is obtained when two cylindrical tubes were employed as mRs, which have the ability of accumulating acoustic energy. The optimum length of mR should be within λs/4 ∼ λ s/2, where λs is the acoustic wavelength. Due to the acoustic speed of 340 m/s in the air and the modulated frequency of 30.72 kHz, the calculated optimal length range of the mR is 2.8 ∼ 5.5 mm. Therefore, the mRs made from stainless steel with lengths of 3 mm, 4 mm, 5 mm, 5.5 mm were investigated experimentally. The inner diameter of the mRs is 0.5 mm and the outer diameter is 1.27 mm. The gap between the QTF surface and the end of mRs was 25 μm, and the distance from QTF tips to the axis of mR was 0.7 mm.
The system configuration and internal structure of the reported sensor containing two floors is shown in Fig. 1. Floor I is the electrical circuit unit, which includes the diode laser driver, the digital-to-analog (DA) and analog-to-digital (AD) converters, the transimpedance amplifier (TA) and a custom lock-in amplifier. The power supply system consists of a rechargeable battery with the capability of operating the sensor for 12 h and which makes it suitable for outdoor applications. Floor II is the optical unit, which is composed of a fiber beam splitter (FBS) with a splitting ratio of 1/9, an integrated ADM, a CO reference cell with an optical path length of 30 mm (Wavelength Reference™, 30% CO:N2@200 Torr) and an InGaAs PIN photodiode (PD, G12183010 K, Hamamatsu, Japan). A front panel is used for parameter setting and display. The power source connector, gas pipelines and the data communication interface connecting with the PC were mounted behind the front panel. The sensor has a size of 250 × 250 × 50 mm3 and a weight of 3.2 kg. 2.2. The integrated acoustic detection module Fig. 2 depicts a 3D model and the assembled configuration of the
Fig. 1. Schematic of the CO sensor. (a) System configuration; (b) Sensor exterior with the internal structure. FBS: fiber beam splitter, ADM: acoustic detection module, TA: transimpedance amplifier, PD: photodiode, PC: personal computer. 130
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Fig. 2. The designed 3D model and assembled configuration of the integrated ADM. (a) The designed 3D model, 1—quartz glass window, 2—quartz tuning fork (QTF), 3—micro resonator (mR), 4—plano-convex lens, 5—fiber collimator (FC), 6—metallic sleeve, 7—gas pipelines; (b) Assembled configuration of the integrated ADM.
modulation depth of 0.36 cm−1 were employed in the subsequent experiments. An enhancement of the QEPAS signal can be achieved by improving the vibration-translation (V-T) relaxation rate of the CO molecule and water vapor (H2O) is an effective molecule catalyst. The CO sample was humidified by passing through a water path with a constant length. The concentration of the water vapor was determined by means of a direct absorption method. Experimentally, as shown in Fig. 5, an improvement of ∼6 fold was obtained when the 1.01% H2O vapor was added in the CO:N2 target gas. Furthermore, pure nitrogen (N2) was flushed into the ADM to evaluate the background noise in such a sensor system, which is depicted in Fig. 5. The 1σ value of background noise was 1.4 μV, which resulted in a minimum detection limit (MDL) of 9.1 ppm for a 1 s integration time. The corresponding normalized noise equivalent absorption (NNEA) coefficient evaluating the sensitivity of sensor was calculated to be 1.8 × 10−8 cm−1 W/Hz−1/2. Fig. 3. 2f signal for two QTFs with resonance frequency f0 of 30.72 kHz and 32.768 kHz.
4. Performance assessment of the CO-QEPAS sensor To evaluate the long-term stability of the reported CO-QEPAS sensor with a compact structure, an Allan deviation analysis was performed for the system with a discrete optical configuration and an integrated ADM, respectively. The gas chamber was flushed with pure N2 at a constant flow rate, and the measurements lasted for more than two hours which are shown in Fig. 6. It can be seen that a MDL of 930 ppb can be achieved with an integration time of 300 s in a discrete optical configuration, which indicated an improvement of the MDL by one order of magnitude, and it is almost equal to those reported in Refs. [21–23]. Furthermore, the MDL was improved to 21 ppb when the integration
Fig. 4. 2f signal amplitude with different mRs as a function of laser modulation depth.
Furthermore, the wavelength modulation depth of the diode laser should be optimized for wavelength modulation spectroscopy in order to achieve the highest QEPAS signal level. The 2f signal amplitude with different laser modulation depths corresponding to four kinds of mRs is depicted in Fig. 4. A maximum signal amplitude was achieved when mRs with a length of 5 mm were used. The signal amplitude improved with increasing modulation depth and this trend does not change until the modulation depth of the diode laser was higher than 0.36 cm−1. Therefore, mRs tubes with a length of 5 mm and a laser wavelength
Fig. 5. 2f signal with and without water vapor as well as the sensor system background noise. 131
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Fig. 8. Continuous monitoring of the CO concentration emitted by combustion of three materials.
Fig. 6. Allan deviation analysis for the sensor system with discrete optical structure and an integrated ADM.
5. Conclusions
time exceeds 800 s for the CO-QEPAS sensor system with an integrated ADM. In addition, the MDL was 130 ppb with a sensor stability of 100 s. Therefore, compared to a MDL of 9.1 ppm reported in Section 3, finally a ∼430 folds maximum improvement for the MDL illustrated that such a sensor system with a compact structure could suppress the system instability effectively and results in an excellent stability and a longterm measurement capability. In this work, the 3f signal, which is demodulated from the reference cell was used to lock the wavelength of laser output to the targeted CO absorption peak. The measured 2f and 3f signals are shown in Fig. 7. The null point between the maximum value and the minimum values of the 3f signals located at the peak of 2f signal at the wavenumber axis was used to lock the diode laser wavelength. Therefore, the peak value of 2f signal can be continuously obtained. To preliminarily assess the capability of the CO-QEPAS sensor in a practical application, a gas pump was utilized to draw the exhaust gas released from the combustion chamber into the sensor’s ADM with a gas flow rate of 120 mL/min. The CO concentration produced by three types of materials combustion was continuously measured. Fig. 8 shows the continuous measurement results for the combustion of a cigarette, timber and cable sheath sequentially. It can be seen that the carbonaceous organic material such as tobacco and timber are more likely to release CO (> 150 ppm) during combustion and the CO concentration for the combustion of industrial polymer cable sheath is low with a ∼50 ppm concentration. The power consumption of the reported CO-QEPAS sensor system during continuous measurements was also monitored, which showed a low power dissipation of ∼4.2 W.
In conclusion, a portable and sensitive CO gas sensor based on a QEPAS technique was demonstrated. The ADM has a compact and integrated configuration containing the optics and acoustic detection components with a dimension of 50 × 30 × 21 mm3 and a mass of 71 g. The QEPAS system was assembled and resulted in a 250 × 250 × 50 mm3, 3.2 kg portable sensor system. A QTF with a low f0 of 30.72 kHz was utilized as the photoacoustic detector. The length of mR tubes and the wavelength modulation depth of the diode laser were optimized experimentally. Water vapor was employed as the associated V-T relaxation rate improving catalyst, which resulted in a MDL of 9.1 ppm for a 1 s integration time and the calculated NNEA was 1.8 × 10−8 cm−1 W/Hz−1/2. An Allan deviation analysis was used to evaluate and compare the stability for the reported integrated sensor and a discrete structural sensor system. With an integration time of 800 s, the MDL was improved to be 21 ppb for the integrated CO-QEPAS sensor. Compared to a discrete sensor system, the significant improvement indicated that this compact and portable sensor system has an excellent stability and a long-term measurement capability. Finally, continuous monitoring was realized by a laser wavelength locking technique based on 3f signal demodulation. The CO concentration levels from three types of burning materials was measured, which demonstrated the practical application of the reported sensor. With a ppb level detection limitation, a low power consumption of ∼4.2 W, a light weight of 3.2 kg, a rechargeable battery capable of a 12 h running time, an excellent stability and a compact, portable configuration, the reported CO-QEPAS sensor has applications in fire detection, mining accidents and industrial process control based on balloon and unmanned aerial vehicle (UAV) platforms.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 61505041 and 61875047); the Natural Science Foundation of Heilongjiang Province of China (Grant No. JJ2019YX0173), the Financial Grant from the Heilongjiang Province Postdoctoral Foundation (Grant No. LBH-Q18052); the Application Technology Research and Development Projects of Harbin (No. 2016RAQXJ140); and the Fundamental Research Funds for the Central Universities. Frank K. Tittel gratefully acknowledges the financial support from the US National Science Foundation (NSF) ERC MIRTHE award and a grant C-0586 from the Welch Foundation. Fig. 7. The measured 2f and 3f QEPAS signal. 132
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