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Piezoelectric effect-based detector for spectroscopic application
Optics and Lasers in Engineering 115 (2019) 141–148
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Piezoelectric effect-based detector for spectroscopic application Jingsong Li∗, Ningwu Liu, Junya Ding, Sheng Zhou, Tianbo He, Lei Zhang Laser Spectroscopy and Sensing Laboratory, Anhui University, 230601 Hefei, China
a r t i c l e
i n f o
Keywords: Remote sensing and sensors Quantum cascade laser Piezoelectric detector Spectroscopic applications
a b s t r a c t A standoff laser spectroscopy sensor based on a broadband tunable external cavity quantum cascade laser (ECQCL) and a piezoelectric effect-based detector was developed for simultaneous detection of multiple chemicals. Instead of using a standard infrared detector, a custom quartz crystal tuning fork (QCTF) with a high resonant frequency (∼75 kHz) was adopted as a light detector for laser signal collection. To explore the capabilities of this technique, the impact of the position of incident light beam excitation with respect to the QCTF on the signal amplitude, resonant frequency and Q-factor was observed in detail. In addition, the influence of incident light intensity and pressure on the intrinsic property of the QCTF was also systematically investigated. Finally, the ECQCL sensor was successfully demonstrated for the standoff detection of plumes for three volatile organic compounds (VOCs) (i.e., alcohol, acetone and ether) at a distance of 40 m, which proves the applicability of the technique for the detection of leak plumes in security fields.
1. Introduction Among the various robust and sensitive gas detection techniques, laser absorption spectroscopy (LAS) techniques have been proved to be an extremely effective analysis tool for quantitative analysis and component identification of trace gaseous species, especially for detection in the mid-infrared (MIR) spectral region, where many molecules of atmospheric interest, including their isotopes, have their fundamental strong absorption fingerprint region, which is typically several orders of magnitude higher than that of the near-IR overtone or combination bands. Recently, a significant improvement was achieved with the advent of high-performance quantum cascade lasers (QCLs) [1], which extend LAS as an emerging analytical technique in many interdisciplines such as atmospheric chemistry, clinical medicine, and combustion diagnosis [2–6]. Most recently, broadly tunable external cavity quantum cascade lasers (ECQCLs) have been used to provide alternative laser sources for MIR sensing of larger molecules with broad and unresolved spectral absorption features. There have been many publications demonstrating ECQCLs with a maximum tunable range of over 760 cm−1 [7–9]. Although commercially available ECQCLs usually cover a spectral range of ∼ 300 cm−1 , most operate under a pulsed model, typically Alpes Lasers (SA, Switzerland), Block Engineering (Marlborough, MA, USA) and Daylight Solutions (Poway, CA, USA), with the wide wavelength tunability of ECQCLs enabling one to access the fundamental vibrational bands of many chemical agents, which are well-suited for trace explosive, chemical warfare agent, and toxic industrial chemical detection and even spectroscopic study of heavy molecules [10–15].
∗
Corresponding author. E-mail address: [email protected] (J. Li).
In MIR laser spectroscopy, an infrared mercury cadmium telluride (MCT) detector (typical VIGO Systems) is commonly used for optical signal detection [16]. However, the detectivity of this kind of semiconductor photodetector shows a significant inverse proportion to the wavelength response bandwidth. Recently, sensitive microcantilevers and microphones have been employed for standoff chemical detection [17,18]. For this issue, a new type of photoelectric detection technology based on a standard quartz crystal tuning fork (QCTF) with a resonance frequency f0 of ∼ 32.8 kHz and characterized by a typical geometry of 3700 𝜇m × 600 𝜇m × 300 𝜇m (length × width × thickness) was recently proposed in our laboratory [19], which shows the significant features of a no cutoff wavelength limit, exceptional high quality factor, small size, and immunity to various low-frequency noise as well as cost-effectiveness. In our technique, similar to the photoelectric effect in a semiconductor detector, we utilize the piezoelectric effect and resonance effect of the QCTF to gauge the light intensity. For the standoff detection technique, there are many challenges, such as spectral interference and broadening effect at atmospheric pressure, the influence of air turbulence and the effective absorption path length of the vapor plume. In this paper, an ECQCL gas sensor system based on a custom QCTF with a resonant frequency over twofold higher than that obtained for the standard QCTF was demonstrated for standoff detection of hazardous volatile organic compounds (VOCs) vapors. To explore and demonstrate the capabilities of the technique, the impact of the position of the incident light beam excitation with respect to QCTF on the signal amplitude and Q-factor, as well as the influence of the incident light intensity and pressure on the intrinsic property of the QCTF, were also systematically investigated. Primary laboratory assessment for open-path detection of three typical VOCs plume (i.e., alcohol, acetone and ether) at a standoff
https://doi.org/10.1016/j.optlaseng.2018.11.020 Received 28 May 2018; Received in revised form 22 November 2018; Accepted 23 November 2018 0143-8166/© 2018 Elsevier Ltd. All rights reserved.
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Optics and Lasers in Engineering 115 (2019) 141–148
Fig. 1. 3D drawing of the QCTF and the laser beam excitation positions.
distance of 40 m was also presented by combining a fixed-wavelength detection method.
“middle gap” excitation mode. On the vertical coordinate y-axis, an optimal height of approximately 3 mm was finally determined after numerous experiments. Based on this finding, the four different excitation modes mentioned above were investigated in detail at the same horizontal level. As an example, Fig. 3 presents the observed and simulated resonant profiles for the QCTF at different excitation modes. As expected, the optimal resonance frequency is always constant, while the Q factor and signal amplitude of the QCTF shows a significant dependence on the excitation modes. As an example, the statistical results for the four different QCTFs are summarized in Table 1. Overall, the middle excitation mode provided the maximal signal amplitude. For clarity, Fig. 4 illustrates the variation in the Q factor as a function of signal amplitude at the center frequency f0 . The data show an obvious inverse relationship between the maximal signal amplitude at f0 and the Q factor for the QCTF. Although the active area in the side excitation mode is minimum (0.3 × 0.5 mm2 ), the maximal signal amplitude is not the weakest for a certain QCTF. Therefore, the optimal excitation mode should be carefully determined for the selected QCTF in practical application. In addition, the influence of the incident light intensity and environmental pressure on the intrinsic property of the QCTF was also investigated. For this experiment, a custom QCTF with better performance under the middle excitation mode was selected. Fig. 5 shows the dependence of the QCTF detector response on incident laser power. A good linear dependence of the QCTF detector signal on the laser power was found in the range from 0 to 8 mW. From this result, we can conclude that a minimal light power change of several μW levels can be detected by the QCTF detector; currently, the detectivity was mainly limited by the noise of the preamplifier circuit. For observing a pressure effect, the QCTF was placed in a homemade stainless steel cell with two CaF2 windows in which the pressure can be regulated. Fig. 6 illustrates the dependence of the intrinsic property of the QCTF on environmental pressure. With decreasing pressure, the Q value and signal amplitude varies inversely with the operating pressure, while the optimal resonant frequency f0 shows a good linear response to pressure. Linear regression leads to the equation of f0 (Hz) = 75,019.59– 0.0225 × P (mbar) with a regression coefficient of R2 = 0.998 for pressure between 0 and 1 atm. A maximum quality factor (Q ∼ 16,800) and signal amplitude at a resonance frequency of f0 = 75,020 Hz were obtained in a vacuum environment. Therefore, the fabrication of the QCTF detector in a vacuum cavity will provide better performance.
2. QCTF photodetector QCTF is commonly employed as an acoustic wave transducer instead of the microphone used in conventional PAS, namely, quartz-enhanced photoacoustic spectroscopy (QEPAS) or its variation [20,21]. In the case of the QEPAS technique, the light beam is directly transmitted through the QCTF prongs gap, with the optimal incident point commonly located at a distance of approximately 1 mm between the laser beam and the top of the QCTF prongs [22,23], as shown in Fig. 1(left). Here, the transmitted laser light was vertically focused onto the surface of the QCTF prongs. Therefore, the optimal incentive location also needs to be carefully determined. A series of custom tuning forks were employed to investigate their intrinsic characteristics. The dimensions of the QCTF were as follows: length, width, and thickness equal to 2200 𝜇m, 400 𝜇m, and 300 𝜇m, respectively, as depicted in Fig. 1(right). To test the QCTFs, a near-infrared diode laser model was used as the excitation light source. The diode laser used was fiber-coupled and operated at a fixed emission wavelength, with the optical fiber end coupled to a beam collimator, which was mounted on a 3-axis mount. A fibercoupled visible red indicator light was used to facilitate beam alignment. We performed the QCTF experiments by applying a sinusoidal modulation to the diode laser driving current and then scanning the modulation frequency around the QCTF resonance frequency. A homemade low-noise transimpedance preamplifier circuit with a feedback resistor of 10 MΩ was used to magnify the piezoelectric current generated by the QCTF and transform it into a voltage signal. Unlike a custom method based on the lock-in amplifier technique, here, the vibration intensity signal of the QCTF was directly extracted by fast Fourier transform (FFT) analysis, as shown in Fig. 2. From this result, we can obtain a long-term measurement precision of 1.3‰. In view of the geometrical symmetry of the QCTF, only front and left-side excitation modes were investigated, which are generally classified into four different excitation modes, namely, left side (black rectangle), left prong (green circle), middle gap (red circle) and right prong (pink circle); the coordinate system of the QCTF was defined as shown in Fig. 1. The output laser beam was directed into a 0.5 mm-diameter light spot onto the surface of the QCTF by a fiber-coupled focuser. Since the gap between the QCTF prongs was approximately 0.2 mm, the incident laser beam actually motivated both prongs in the case of the 142
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Optics and Lasers in Engineering 115 (2019) 141–148
Fig. 2. Signal processing procedure used for testing the QCTF photodetector.
Fig. 3. Experimentally observed and theoretically calculated resonant profile for the QCTF at different excitation modes.
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Table 1 Measured and calculated characteristic parameters for the QCTFs. QCTF 1 Position Middle Left Right Side Middle Left Right Side
Fitted f0 (Hz) 75,173.3 75,172.3 75,173.8 75,173.7 QCTF 3 75,559.6 75,560.2 75,559.7 75,560.2
QCTF 2 Fitted y (V) 1.7843 1.3854 1.5615 1.5426
Measured f0 (Hz) 75,172 75,172 75,172 75,172
Measured y (V) 1.8236 1.4704 1.5681 1.5454
Q 7345 7812 7245 7287
1.8469 0.9341 0.8601 1.3488
75,560 75,560 75,560 75,560
2.0214 1.0084 0.9196 1.4477
7641 10,254 9903 8616
Fitted f0 (Hz) 74,865.2 74,866.2 74,865.1 74,866.9 QCTF 4 74,994.2 74,994.2 74,994.3 74,994.3
Fitted y (V) 1.589 0.8453 1.0482 1.9385
Measured f0 (Hz) 74,864 74,865 74,864 74,865
Measured y (V) 1.6909 0.8663 1.1056 2.0103
Q 5382 6623 6426 4657
2.2108 1.4915 1.1731 1.3231
74,994 74,994 74,994 74,994
2.4178 1.6015 1.2441 1.4124
7235 7510 8736 8172
from 1130 cm−1 to 1437 cm−1 (i.e., between 6.96 and 8.85 𝜇m), pulse width can be set from 20 to 350 ns with a pulse repetition rate of up to 3 MHz while maintaining a duty cycle up to ∼15%. As shown in Fig. 1 of reference [19], the higher the pulse repetition rate, the higher the ECQCL output power. The ECQCL laser beam was directly collimated by a 6 × beam expander (BE06R, Thorlabs) to decrease the divergence after passing through the beam splitter, which was used to coalign a visible red light with the ECQCL beam to facilitate beam alignment. Then, the ECQCL beam was launched onto a distant object, with the collimated beam reflected back by a reflector unit on a tripod positioned at a known distance from the optical unit. The reflected and scattered light is finally collected and focused onto the surface of the QCTF detector via a ZnSe lens with a focal length of 300 mm and a diameter of 85 mm. Finally, the QCTF detector signal was transferred to a digital acquisition board (NI USB-6259, 1.25 MHz sampling rate) and synchronously analyzed using a notebook-based Labview program. One major challenge for standoff detection is the spectral interference due to absorption features other than the species of interest, especially atmospheric H2 O, which varies with high dynamic range (approximately 0.17 ppm-4.5%) and has rich absorption features in the whole infrared range. A spectral simulation for three typical VOCs based on the Pacific Northwest National Laboratory (PNNL) [24] and the HITRAN databases [25] within the ECQCL wavelength tuning range and potential absorption of atmospheric water vapor is shown in Fig. 8. The data show that the spectral window between 1150 cm−1 and 1300 cm−1 is well suited for simultaneous standoff detection of acetone, alcohol and ether plume.
Fig. 4. The experimentally measured Q factor vs. the QCFT signal amplitude at f0 .
3. QCTF- and ECQCL-based standoff VOC sensor A photograph of our QCTF- and ECQCL-based VOC sensor is shown in Fig. 7. A detailed description of the laser source has already been described elsewhere [19]. Here, we focus on recent advances and improvements that make the sensor suitable for standoff detection of VOCs with fast response. The pulsed ECQCL emission wavelength can be tuned
Fig. 5. Dependence of the QCTF detector signal amplitude on incident laser power.
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Fig. 6. The experimentally measured QCFT resonant profile, signal amplitude at f0 , f0 and Q factor as a function of operating pressure. Fig. 7. Photograph of the QCTF- and ECQCL-based VOC sensor (left) and a reflector unit (right).
resulting in a corresponding suppression of system detection noise [28]. In our previous work [29], it takes us approximately 20s to acquire a whole scanning between 1150 cm−1 and 1300 cm−1 for the wavelengthsweeping detection mode, which was not satisfied to eliminate the fluctuations in a transient chemical plume, as shown in Fig. 8 (bottom panel).
The second significant challenge for standoff detection is the influence of air turbulence caused by atmospheric winds and the evaporation process of VOCs, which requires that the monitoring system should have high spectral acquisition rates [26,27]. Generally, the rapid wavelengthsweeping method enables measurement of changing concentrations in a turbulent plume and reduces the influence of atmospheric turbulence,
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Fig. 8. Simulation of three typical VOCs (acetone, alcohol and ether) and H2 O absorption spectra within the ECQCL tuning range shown together with the experimentally measured spectrum as well as the fitting.
In view of both issues mentioned above, in this study, the ECQCL sensor was operated at fixed emitting wavelength modes. As demonstrated in Fig. 8, the acetone, alcohol and ether absorption spectra within the spectral range from 1150 to 1300 cm−1 show an obvious cross absorption effect; however, they still show unique absorption peak features within this spectral region. Therefore, three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 were selected for identifying ether, acetone and alcohol, respectively. Details of the experimental evaluation are presented in the next section.
provides a high speed response to reduce the influence of air turbulence. To generate a continuous plume, alcohol, acetone and ether solutions with a stated purity of 99.7%, 99.5% and 99.7%, respectively, were prepared in an open vessel (diameter 16 cm and height 2.5 cm), respectively, which were placed at a height of ∼1 m above ground level and a distance of 40 m from the sensor system. In view of their different absorption line intensities and volatilization rates, solution volumes of 20 ml, 40 ml and 100 ml were specially prepared for ether, acetone and alcohol, respectively, with a corresponding vapor pressure at room temperature of 25 °C is 535.6 Torr, 230 Torr and 59.3 Torr. Theoretically, the proposed standoff spectroscopy detection technique is based on the principle of direct absorption spectroscopy. Three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 were specially selected for identifying ether, acetone and alcohol, respectively, with a higher data sampling rate tested experimentally for standoff detection of conventional hazardous chemicals. First, the ECQCL emitting wavelength was fixed at 1150 cm−1 for identifying ether and the potential influence of other species, as shown in Fig. 10 (upper panel), the air background and different VOC plumes were measured in real-time with a 30-min time interval and 10 Hz sampling rate. This measurement procedure was repeated several times to evaluate system repeatability. As shown by the data, the QCTF detector signal shows a maximal response (in unit of voltage) under the condition of no VOC plume (i.e., air background). Once the VOC plume is inserted into the laser beam path, the reflected light will be attenuated depending on the target absorption level; thus, the QCTF detector signal will decrease. The experimental results show that ether absorption at a wavelength of 1150 cm−1 is very significant, with acetone and alcohol absorption at this wavelength completely negligible, which is coincident with the theoretical simulation demonstrated in Fig. 8. Note that one challenge for stand-off detection is that the leaking or volatile plume commonly
4. Results and discussion In the following experiments, a custom QCTF with a quality factor in ambient air of Q ∼ 9400 and a resonance frequency of f0 = 74.998 kHz was used, with the middle excitation mode employed to provide the maximum signal amplitude. A detailed comparison with a standard MCT detector (PVMI-4TE-10.6, Vigo Systems) was primarily investigated, with the laser output power at a pulse width of 300 ns and repetition rate of 74.998 kHz also recorded by using a power meter (Nova II, Ophir Photonics). For this experiment, the ECQCL laser source was placed at a distance of 20 cm outside the detectors. Fig. 9 presents the raw background signals measured within the ECQCL tuning range by using three detectors. Note that each signal was recorded with a single scan acquisition without any signal averaging and wavelength calibration. We can see that each set of signals show good reproducibility; however, the results for both the QCTF detector and power meter show a better signal-to-noise ratio (SNR) than that obtained for the MCT detector, especially for some absorption peaks due to atmospheric water vapor. Due to unknown factors, a strange oscillation effect for the power meter was found to always occur at ∼ 1215 cm−1 . To evaluate system performance, standoff detection of VOC plume based on a fixed wavelength method was proposed in this work, which 146
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Optics and Lasers in Engineering 115 (2019) 141–148
Fig. 9. ECQCL laser intensity measured by different detectors: Vigo detector (upper panel), QCTF (middle panel) and Ophir power meter (bottom panel).
occupies an unknown proportion of the path length [30]. Therefore, the QCTF detector signal (in units of voltage) is not transformed into an absorption coefficient (cm−1 ). Finally, the measurement procedure was applied to the other two wavelengths (1217 cm−1 and 1250 cm−1 ) for identifying acetone and alcohol, respectively, with the influence of a nontarget also evaluated. The results are demonstrated in Fig. 10 (bottom panel and middle panel), respectively. For clarity, the average results corresponding to each time interval are also shown in Fig. 10 (right panels). In the cases of 1217 cm−1 and 1250 cm−1 , the acetone and alcohol absorptions are obviously dominant, respectively. However, ether still shows certain absorption interference, mainly due to the influence of its strong absorption band, as seen in the simulated spectra shown in Fig. 8. On the whole, each VOC component can be effectively identified by using the proposed fixed-wavelength detection method, according to their different absorbance depth. In view of the effects of both atmospheric and plume turbulence, the sensor system still shows good stability and reproducibility during the continuous test carried out over one hour. On the other hand, this result proves that the proposed fixed-wavelength detection method can be effectively used for identifying ether, acetone and alcohol, respectively, for security applications, where the quantitative result is usually not of great concern, such as chemical VOCs leakage detection and warning.
vices. The results indicate that the QCTF detector shows slightly better performance compared to the commercial infrared MCT detector used in this work. To explore and demonstrate the capabilities of the technique, the impact of incident light beam excitation position with respect to QCTF on signal amplitude and Q-factor, as well as the influence of incident light intensity and pressure on the intrinsic property of QCTF, were also investigated in detail. Overall, the results show that the QCTF can be efficiently excited in four different modes as defined in this study, with the maximal signal amplitude at f0 inversely proportional to the Q factor for a certain QCTF; therefore, the optimal excitation mode should be carefully determined for the selected QCTF in practical applications. Moreover, with decreasing operating pressure, the Q value and signal amplitude shows an inverse dependence on pressure, while the optimal resonant frequency shows a good linear response to pressure and incident light intensity. In view of the effects of both atmospheric and plume turbulence, the ECQCL sensor was operated at a fixed emitting wavelength mode, with three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 successfully selected for identifying ether, acetone and alcohol, respectively. Primary laboratory assessment for standoff detection of the mixing plume for three VOCs at a distance of 40 m (limited by the laboratory environment) was successfully demonstrated. The experimental results show that standoff detection for VOC chemicals at a distance of over one hundreds of meters can be achieved. Future work will aim at extending measurements to longer ranges in outdoor environments and using diffuse reflection from topographic scattering targets, as well as optimizing QCTF selection (with higher resonant frequency or fabrication in a vacuum environment) and the signal processing method and detection scheme (such as using a high speed data acquisition board) to reduce the effects of both atmospheric and plume turbulence.
5. Conclusions In conclusion, an ECQCL gas sensor based on a new type of photodetector has been demonstrated as a versatile and powerful approach for standoff detection of vapor chemicals. Unlike the common QEPAS and other spectroscopic techniques, where standard QCTFs with a resonance frequency f0 ∼ 32.8 kHz (or lower for enhancing the PA signal) and MCT detectors are generally used, a custom QCTF with higher resonant frequency (∼75 kHz) was employed as a light detector for laser signal collection, and compared with the sophisticated photoelectric de147
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Optics and Lasers in Engineering 115 (2019) 141–148
Fig. 10. Real-time measurement of VOC (ether, alcohol and acetone) plumes with a 10 Hz sampling rate (left panel) and the corresponding average results (right panel).
Acknowledgments
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Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Piezoelectric effect-based detector for spectroscopic application Jingsong Li∗, Ningwu Liu, Junya Ding, Sheng Zhou, Tianbo He, Lei Zhang Laser Spectroscopy and Sensing Laboratory, Anhui University, 230601 Hefei, China
a r t i c l e
i n f o
Keywords: Remote sensing and sensors Quantum cascade laser Piezoelectric detector Spectroscopic applications
a b s t r a c t A standoff laser spectroscopy sensor based on a broadband tunable external cavity quantum cascade laser (ECQCL) and a piezoelectric effect-based detector was developed for simultaneous detection of multiple chemicals. Instead of using a standard infrared detector, a custom quartz crystal tuning fork (QCTF) with a high resonant frequency (∼75 kHz) was adopted as a light detector for laser signal collection. To explore the capabilities of this technique, the impact of the position of incident light beam excitation with respect to the QCTF on the signal amplitude, resonant frequency and Q-factor was observed in detail. In addition, the influence of incident light intensity and pressure on the intrinsic property of the QCTF was also systematically investigated. Finally, the ECQCL sensor was successfully demonstrated for the standoff detection of plumes for three volatile organic compounds (VOCs) (i.e., alcohol, acetone and ether) at a distance of 40 m, which proves the applicability of the technique for the detection of leak plumes in security fields.
1. Introduction Among the various robust and sensitive gas detection techniques, laser absorption spectroscopy (LAS) techniques have been proved to be an extremely effective analysis tool for quantitative analysis and component identification of trace gaseous species, especially for detection in the mid-infrared (MIR) spectral region, where many molecules of atmospheric interest, including their isotopes, have their fundamental strong absorption fingerprint region, which is typically several orders of magnitude higher than that of the near-IR overtone or combination bands. Recently, a significant improvement was achieved with the advent of high-performance quantum cascade lasers (QCLs) [1], which extend LAS as an emerging analytical technique in many interdisciplines such as atmospheric chemistry, clinical medicine, and combustion diagnosis [2–6]. Most recently, broadly tunable external cavity quantum cascade lasers (ECQCLs) have been used to provide alternative laser sources for MIR sensing of larger molecules with broad and unresolved spectral absorption features. There have been many publications demonstrating ECQCLs with a maximum tunable range of over 760 cm−1 [7–9]. Although commercially available ECQCLs usually cover a spectral range of ∼ 300 cm−1 , most operate under a pulsed model, typically Alpes Lasers (SA, Switzerland), Block Engineering (Marlborough, MA, USA) and Daylight Solutions (Poway, CA, USA), with the wide wavelength tunability of ECQCLs enabling one to access the fundamental vibrational bands of many chemical agents, which are well-suited for trace explosive, chemical warfare agent, and toxic industrial chemical detection and even spectroscopic study of heavy molecules [10–15].
∗
Corresponding author. E-mail address: [email protected] (J. Li).
In MIR laser spectroscopy, an infrared mercury cadmium telluride (MCT) detector (typical VIGO Systems) is commonly used for optical signal detection [16]. However, the detectivity of this kind of semiconductor photodetector shows a significant inverse proportion to the wavelength response bandwidth. Recently, sensitive microcantilevers and microphones have been employed for standoff chemical detection [17,18]. For this issue, a new type of photoelectric detection technology based on a standard quartz crystal tuning fork (QCTF) with a resonance frequency f0 of ∼ 32.8 kHz and characterized by a typical geometry of 3700 𝜇m × 600 𝜇m × 300 𝜇m (length × width × thickness) was recently proposed in our laboratory [19], which shows the significant features of a no cutoff wavelength limit, exceptional high quality factor, small size, and immunity to various low-frequency noise as well as cost-effectiveness. In our technique, similar to the photoelectric effect in a semiconductor detector, we utilize the piezoelectric effect and resonance effect of the QCTF to gauge the light intensity. For the standoff detection technique, there are many challenges, such as spectral interference and broadening effect at atmospheric pressure, the influence of air turbulence and the effective absorption path length of the vapor plume. In this paper, an ECQCL gas sensor system based on a custom QCTF with a resonant frequency over twofold higher than that obtained for the standard QCTF was demonstrated for standoff detection of hazardous volatile organic compounds (VOCs) vapors. To explore and demonstrate the capabilities of the technique, the impact of the position of the incident light beam excitation with respect to QCTF on the signal amplitude and Q-factor, as well as the influence of the incident light intensity and pressure on the intrinsic property of the QCTF, were also systematically investigated. Primary laboratory assessment for open-path detection of three typical VOCs plume (i.e., alcohol, acetone and ether) at a standoff
https://doi.org/10.1016/j.optlaseng.2018.11.020 Received 28 May 2018; Received in revised form 22 November 2018; Accepted 23 November 2018 0143-8166/© 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. 3D drawing of the QCTF and the laser beam excitation positions.
distance of 40 m was also presented by combining a fixed-wavelength detection method.
“middle gap” excitation mode. On the vertical coordinate y-axis, an optimal height of approximately 3 mm was finally determined after numerous experiments. Based on this finding, the four different excitation modes mentioned above were investigated in detail at the same horizontal level. As an example, Fig. 3 presents the observed and simulated resonant profiles for the QCTF at different excitation modes. As expected, the optimal resonance frequency is always constant, while the Q factor and signal amplitude of the QCTF shows a significant dependence on the excitation modes. As an example, the statistical results for the four different QCTFs are summarized in Table 1. Overall, the middle excitation mode provided the maximal signal amplitude. For clarity, Fig. 4 illustrates the variation in the Q factor as a function of signal amplitude at the center frequency f0 . The data show an obvious inverse relationship between the maximal signal amplitude at f0 and the Q factor for the QCTF. Although the active area in the side excitation mode is minimum (0.3 × 0.5 mm2 ), the maximal signal amplitude is not the weakest for a certain QCTF. Therefore, the optimal excitation mode should be carefully determined for the selected QCTF in practical application. In addition, the influence of the incident light intensity and environmental pressure on the intrinsic property of the QCTF was also investigated. For this experiment, a custom QCTF with better performance under the middle excitation mode was selected. Fig. 5 shows the dependence of the QCTF detector response on incident laser power. A good linear dependence of the QCTF detector signal on the laser power was found in the range from 0 to 8 mW. From this result, we can conclude that a minimal light power change of several μW levels can be detected by the QCTF detector; currently, the detectivity was mainly limited by the noise of the preamplifier circuit. For observing a pressure effect, the QCTF was placed in a homemade stainless steel cell with two CaF2 windows in which the pressure can be regulated. Fig. 6 illustrates the dependence of the intrinsic property of the QCTF on environmental pressure. With decreasing pressure, the Q value and signal amplitude varies inversely with the operating pressure, while the optimal resonant frequency f0 shows a good linear response to pressure. Linear regression leads to the equation of f0 (Hz) = 75,019.59– 0.0225 × P (mbar) with a regression coefficient of R2 = 0.998 for pressure between 0 and 1 atm. A maximum quality factor (Q ∼ 16,800) and signal amplitude at a resonance frequency of f0 = 75,020 Hz were obtained in a vacuum environment. Therefore, the fabrication of the QCTF detector in a vacuum cavity will provide better performance.
2. QCTF photodetector QCTF is commonly employed as an acoustic wave transducer instead of the microphone used in conventional PAS, namely, quartz-enhanced photoacoustic spectroscopy (QEPAS) or its variation [20,21]. In the case of the QEPAS technique, the light beam is directly transmitted through the QCTF prongs gap, with the optimal incident point commonly located at a distance of approximately 1 mm between the laser beam and the top of the QCTF prongs [22,23], as shown in Fig. 1(left). Here, the transmitted laser light was vertically focused onto the surface of the QCTF prongs. Therefore, the optimal incentive location also needs to be carefully determined. A series of custom tuning forks were employed to investigate their intrinsic characteristics. The dimensions of the QCTF were as follows: length, width, and thickness equal to 2200 𝜇m, 400 𝜇m, and 300 𝜇m, respectively, as depicted in Fig. 1(right). To test the QCTFs, a near-infrared diode laser model was used as the excitation light source. The diode laser used was fiber-coupled and operated at a fixed emission wavelength, with the optical fiber end coupled to a beam collimator, which was mounted on a 3-axis mount. A fibercoupled visible red indicator light was used to facilitate beam alignment. We performed the QCTF experiments by applying a sinusoidal modulation to the diode laser driving current and then scanning the modulation frequency around the QCTF resonance frequency. A homemade low-noise transimpedance preamplifier circuit with a feedback resistor of 10 MΩ was used to magnify the piezoelectric current generated by the QCTF and transform it into a voltage signal. Unlike a custom method based on the lock-in amplifier technique, here, the vibration intensity signal of the QCTF was directly extracted by fast Fourier transform (FFT) analysis, as shown in Fig. 2. From this result, we can obtain a long-term measurement precision of 1.3‰. In view of the geometrical symmetry of the QCTF, only front and left-side excitation modes were investigated, which are generally classified into four different excitation modes, namely, left side (black rectangle), left prong (green circle), middle gap (red circle) and right prong (pink circle); the coordinate system of the QCTF was defined as shown in Fig. 1. The output laser beam was directed into a 0.5 mm-diameter light spot onto the surface of the QCTF by a fiber-coupled focuser. Since the gap between the QCTF prongs was approximately 0.2 mm, the incident laser beam actually motivated both prongs in the case of the 142
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Fig. 2. Signal processing procedure used for testing the QCTF photodetector.
Fig. 3. Experimentally observed and theoretically calculated resonant profile for the QCTF at different excitation modes.
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Table 1 Measured and calculated characteristic parameters for the QCTFs. QCTF 1 Position Middle Left Right Side Middle Left Right Side
Fitted f0 (Hz) 75,173.3 75,172.3 75,173.8 75,173.7 QCTF 3 75,559.6 75,560.2 75,559.7 75,560.2
QCTF 2 Fitted y (V) 1.7843 1.3854 1.5615 1.5426
Measured f0 (Hz) 75,172 75,172 75,172 75,172
Measured y (V) 1.8236 1.4704 1.5681 1.5454
Q 7345 7812 7245 7287
1.8469 0.9341 0.8601 1.3488
75,560 75,560 75,560 75,560
2.0214 1.0084 0.9196 1.4477
7641 10,254 9903 8616
Fitted f0 (Hz) 74,865.2 74,866.2 74,865.1 74,866.9 QCTF 4 74,994.2 74,994.2 74,994.3 74,994.3
Fitted y (V) 1.589 0.8453 1.0482 1.9385
Measured f0 (Hz) 74,864 74,865 74,864 74,865
Measured y (V) 1.6909 0.8663 1.1056 2.0103
Q 5382 6623 6426 4657
2.2108 1.4915 1.1731 1.3231
74,994 74,994 74,994 74,994
2.4178 1.6015 1.2441 1.4124
7235 7510 8736 8172
from 1130 cm−1 to 1437 cm−1 (i.e., between 6.96 and 8.85 𝜇m), pulse width can be set from 20 to 350 ns with a pulse repetition rate of up to 3 MHz while maintaining a duty cycle up to ∼15%. As shown in Fig. 1 of reference [19], the higher the pulse repetition rate, the higher the ECQCL output power. The ECQCL laser beam was directly collimated by a 6 × beam expander (BE06R, Thorlabs) to decrease the divergence after passing through the beam splitter, which was used to coalign a visible red light with the ECQCL beam to facilitate beam alignment. Then, the ECQCL beam was launched onto a distant object, with the collimated beam reflected back by a reflector unit on a tripod positioned at a known distance from the optical unit. The reflected and scattered light is finally collected and focused onto the surface of the QCTF detector via a ZnSe lens with a focal length of 300 mm and a diameter of 85 mm. Finally, the QCTF detector signal was transferred to a digital acquisition board (NI USB-6259, 1.25 MHz sampling rate) and synchronously analyzed using a notebook-based Labview program. One major challenge for standoff detection is the spectral interference due to absorption features other than the species of interest, especially atmospheric H2 O, which varies with high dynamic range (approximately 0.17 ppm-4.5%) and has rich absorption features in the whole infrared range. A spectral simulation for three typical VOCs based on the Pacific Northwest National Laboratory (PNNL) [24] and the HITRAN databases [25] within the ECQCL wavelength tuning range and potential absorption of atmospheric water vapor is shown in Fig. 8. The data show that the spectral window between 1150 cm−1 and 1300 cm−1 is well suited for simultaneous standoff detection of acetone, alcohol and ether plume.
Fig. 4. The experimentally measured Q factor vs. the QCFT signal amplitude at f0 .
3. QCTF- and ECQCL-based standoff VOC sensor A photograph of our QCTF- and ECQCL-based VOC sensor is shown in Fig. 7. A detailed description of the laser source has already been described elsewhere [19]. Here, we focus on recent advances and improvements that make the sensor suitable for standoff detection of VOCs with fast response. The pulsed ECQCL emission wavelength can be tuned
Fig. 5. Dependence of the QCTF detector signal amplitude on incident laser power.
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Fig. 6. The experimentally measured QCFT resonant profile, signal amplitude at f0 , f0 and Q factor as a function of operating pressure. Fig. 7. Photograph of the QCTF- and ECQCL-based VOC sensor (left) and a reflector unit (right).
resulting in a corresponding suppression of system detection noise [28]. In our previous work [29], it takes us approximately 20s to acquire a whole scanning between 1150 cm−1 and 1300 cm−1 for the wavelengthsweeping detection mode, which was not satisfied to eliminate the fluctuations in a transient chemical plume, as shown in Fig. 8 (bottom panel).
The second significant challenge for standoff detection is the influence of air turbulence caused by atmospheric winds and the evaporation process of VOCs, which requires that the monitoring system should have high spectral acquisition rates [26,27]. Generally, the rapid wavelengthsweeping method enables measurement of changing concentrations in a turbulent plume and reduces the influence of atmospheric turbulence,
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Fig. 8. Simulation of three typical VOCs (acetone, alcohol and ether) and H2 O absorption spectra within the ECQCL tuning range shown together with the experimentally measured spectrum as well as the fitting.
In view of both issues mentioned above, in this study, the ECQCL sensor was operated at fixed emitting wavelength modes. As demonstrated in Fig. 8, the acetone, alcohol and ether absorption spectra within the spectral range from 1150 to 1300 cm−1 show an obvious cross absorption effect; however, they still show unique absorption peak features within this spectral region. Therefore, three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 were selected for identifying ether, acetone and alcohol, respectively. Details of the experimental evaluation are presented in the next section.
provides a high speed response to reduce the influence of air turbulence. To generate a continuous plume, alcohol, acetone and ether solutions with a stated purity of 99.7%, 99.5% and 99.7%, respectively, were prepared in an open vessel (diameter 16 cm and height 2.5 cm), respectively, which were placed at a height of ∼1 m above ground level and a distance of 40 m from the sensor system. In view of their different absorption line intensities and volatilization rates, solution volumes of 20 ml, 40 ml and 100 ml were specially prepared for ether, acetone and alcohol, respectively, with a corresponding vapor pressure at room temperature of 25 °C is 535.6 Torr, 230 Torr and 59.3 Torr. Theoretically, the proposed standoff spectroscopy detection technique is based on the principle of direct absorption spectroscopy. Three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 were specially selected for identifying ether, acetone and alcohol, respectively, with a higher data sampling rate tested experimentally for standoff detection of conventional hazardous chemicals. First, the ECQCL emitting wavelength was fixed at 1150 cm−1 for identifying ether and the potential influence of other species, as shown in Fig. 10 (upper panel), the air background and different VOC plumes were measured in real-time with a 30-min time interval and 10 Hz sampling rate. This measurement procedure was repeated several times to evaluate system repeatability. As shown by the data, the QCTF detector signal shows a maximal response (in unit of voltage) under the condition of no VOC plume (i.e., air background). Once the VOC plume is inserted into the laser beam path, the reflected light will be attenuated depending on the target absorption level; thus, the QCTF detector signal will decrease. The experimental results show that ether absorption at a wavelength of 1150 cm−1 is very significant, with acetone and alcohol absorption at this wavelength completely negligible, which is coincident with the theoretical simulation demonstrated in Fig. 8. Note that one challenge for stand-off detection is that the leaking or volatile plume commonly
4. Results and discussion In the following experiments, a custom QCTF with a quality factor in ambient air of Q ∼ 9400 and a resonance frequency of f0 = 74.998 kHz was used, with the middle excitation mode employed to provide the maximum signal amplitude. A detailed comparison with a standard MCT detector (PVMI-4TE-10.6, Vigo Systems) was primarily investigated, with the laser output power at a pulse width of 300 ns and repetition rate of 74.998 kHz also recorded by using a power meter (Nova II, Ophir Photonics). For this experiment, the ECQCL laser source was placed at a distance of 20 cm outside the detectors. Fig. 9 presents the raw background signals measured within the ECQCL tuning range by using three detectors. Note that each signal was recorded with a single scan acquisition without any signal averaging and wavelength calibration. We can see that each set of signals show good reproducibility; however, the results for both the QCTF detector and power meter show a better signal-to-noise ratio (SNR) than that obtained for the MCT detector, especially for some absorption peaks due to atmospheric water vapor. Due to unknown factors, a strange oscillation effect for the power meter was found to always occur at ∼ 1215 cm−1 . To evaluate system performance, standoff detection of VOC plume based on a fixed wavelength method was proposed in this work, which 146
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Fig. 9. ECQCL laser intensity measured by different detectors: Vigo detector (upper panel), QCTF (middle panel) and Ophir power meter (bottom panel).
occupies an unknown proportion of the path length [30]. Therefore, the QCTF detector signal (in units of voltage) is not transformed into an absorption coefficient (cm−1 ). Finally, the measurement procedure was applied to the other two wavelengths (1217 cm−1 and 1250 cm−1 ) for identifying acetone and alcohol, respectively, with the influence of a nontarget also evaluated. The results are demonstrated in Fig. 10 (bottom panel and middle panel), respectively. For clarity, the average results corresponding to each time interval are also shown in Fig. 10 (right panels). In the cases of 1217 cm−1 and 1250 cm−1 , the acetone and alcohol absorptions are obviously dominant, respectively. However, ether still shows certain absorption interference, mainly due to the influence of its strong absorption band, as seen in the simulated spectra shown in Fig. 8. On the whole, each VOC component can be effectively identified by using the proposed fixed-wavelength detection method, according to their different absorbance depth. In view of the effects of both atmospheric and plume turbulence, the sensor system still shows good stability and reproducibility during the continuous test carried out over one hour. On the other hand, this result proves that the proposed fixed-wavelength detection method can be effectively used for identifying ether, acetone and alcohol, respectively, for security applications, where the quantitative result is usually not of great concern, such as chemical VOCs leakage detection and warning.
vices. The results indicate that the QCTF detector shows slightly better performance compared to the commercial infrared MCT detector used in this work. To explore and demonstrate the capabilities of the technique, the impact of incident light beam excitation position with respect to QCTF on signal amplitude and Q-factor, as well as the influence of incident light intensity and pressure on the intrinsic property of QCTF, were also investigated in detail. Overall, the results show that the QCTF can be efficiently excited in four different modes as defined in this study, with the maximal signal amplitude at f0 inversely proportional to the Q factor for a certain QCTF; therefore, the optimal excitation mode should be carefully determined for the selected QCTF in practical applications. Moreover, with decreasing operating pressure, the Q value and signal amplitude shows an inverse dependence on pressure, while the optimal resonant frequency shows a good linear response to pressure and incident light intensity. In view of the effects of both atmospheric and plume turbulence, the ECQCL sensor was operated at a fixed emitting wavelength mode, with three different wavelengths at 1150 cm−1 , 1217 cm−1 and 1250 cm−1 successfully selected for identifying ether, acetone and alcohol, respectively. Primary laboratory assessment for standoff detection of the mixing plume for three VOCs at a distance of 40 m (limited by the laboratory environment) was successfully demonstrated. The experimental results show that standoff detection for VOC chemicals at a distance of over one hundreds of meters can be achieved. Future work will aim at extending measurements to longer ranges in outdoor environments and using diffuse reflection from topographic scattering targets, as well as optimizing QCTF selection (with higher resonant frequency or fabrication in a vacuum environment) and the signal processing method and detection scheme (such as using a high speed data acquisition board) to reduce the effects of both atmospheric and plume turbulence.
5. Conclusions In conclusion, an ECQCL gas sensor based on a new type of photodetector has been demonstrated as a versatile and powerful approach for standoff detection of vapor chemicals. Unlike the common QEPAS and other spectroscopic techniques, where standard QCTFs with a resonance frequency f0 ∼ 32.8 kHz (or lower for enhancing the PA signal) and MCT detectors are generally used, a custom QCTF with higher resonant frequency (∼75 kHz) was employed as a light detector for laser signal collection, and compared with the sophisticated photoelectric de147
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Fig. 10. Real-time measurement of VOC (ether, alcohol and acetone) plumes with a 10 Hz sampling rate (left panel) and the corresponding average results (right panel).
Acknowledgments
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