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Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS)
Journal Pre-proof Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS) Kaiyuan Zheng, Chuantao Zheng, Junhao Li, Ningning Ma, Zidi Liu, Yaoyu Li, Yu Zhang, Yiding Wang, Frank K. Tittel PII:
S0039-9140(20)30132-6
DOI:
https://doi.org/10.1016/j.talanta.2020.120841
Reference:
TAL 120841
To appear in:
Talanta
Received Date: 15 January 2020 Revised Date:
11 February 2020
Accepted Date: 13 February 2020
Please cite this article as: K. Zheng, C. Zheng, J. Li, N. Ma, Z. Liu, Y. Li, Y. Zhang, Y. Wang, F.K. Tittel, Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS), Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2020.120841. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Kaiyuan Zheng: Conceptualization, Methodology, Software, Investigation, Writing Original Draft Chuantao Zheng: Formal analysis, Writing - Review & Editing, Project administration, Funding acquisition, Resources Junhao Li: Visualization, Investigation Ningning Ma: Data Curation, Formal analysis Zidi Liu: Software, Validation Yaoyu Li: Data Curation, Validation Yu Zhang: Writing- Reviewing and Editing, Resources Yiding Wang: Supervision, Writing- Reviewing and Editing, Funding acquisition, Resources Frank K Tittel: Funding acquisition, Writing- Reviewing and Editing
Graphical Abstracts
Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS)
Kaiyuan Zheng a, Chuantao Zheng a*, Junhao Li a, Ningning Ma a, Zidi Liu a, Yaoyu Li a, Yu Zhang a, Yiding Wang a, Frank K. Tittel b
a
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P.R. China
b
Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
*
Corresponding author. E-mail: [email protected] (Chuantao Zheng); Tel.: +86-13756090979
1
Abstract By feeding back the reflected light from the first cavity mirror to a single-/multi-pass gas cell via a multi-mode fiber, we demonstrated a novel gas-phase analytical scheme for methane (CH4) detection by combing fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS). This scheme has an electrical module and two optical sensing modules which are connected through both single- and multi-mode optical fibers. Long-distance gas sensing application was conducted for verifying the analytical ability of the demonstrated technique exploiting the two fiber-coupled optical modules. A detection limit of 3 parts-per-million in volume (ppmv) for an 84 s averaging time and a precision of 56 ppmv for a 150 s averaging time were achieved using FC-OA-ICOS and CR-WMS, respectively. Two different CH4 measurement ranges were achieved in the sensor system with a wide dynamic range from ~ 15 ppmv to ~ 12% for CH4 detection. Field monitoring of CH4 leakage was performed for environmental analysis under a static and mobile mode using the wireless-controlled vehicle-mounted gas sensor. The proposed gas sensing scheme with fiber-coupled dual optical modules demonstrates a good potential for long-distance field CH4 measurements, especially for those in hazardous environment where in-situ human observation is impossible. Keywords: Chemical gas sensor; Remote sensing analysis; Fiber-coupled; Off-axis integrated cavity output spectroscopy; Wavelength modulation spectroscopy
2
1. Introduction Chemical gas-phase analysis is significant in biological chemistry, geochemistry, as well as in environmental applications. Methane (CH4) is a colorless, flammable and explosive gas that poses significant safety hazards in natural gas transportation and coal mining [1]. Natural gas demand has been on the rise in the past few years and is expected to grow by 2.8% annually from 2001 to 2025 [2]. This growing demand is satisfied through a network of natural gas pipelines. However, natural gas pipelines leakage may cause long-term emissions of CH4 with a level of up to several percent, leading to healthy and environmental problems as well as a huge waste of energy [3]. In addition, electrical sparks may cause explosion when the CH4 concentration is above the low explosive limit (LEL) level of 4.9% [4]. Therefore there are additional requirements for the electrical components and devices in field measurements. In this manner, the development of a secure, sensitive and low-cost CH4 sensor system with a wide dynamic range from parts-per-million in volume (ppmv) levels to several percent (~ %) is of vital importance to atmospheric and industrial monitoring. Various techniques for CH4 detection have been reported in recent years, such as catalyst combustion [5,6], electrochemical [7], semiconductor [8,9], and infrared (IR) laser absorption spectroscopy [10–12]. Among these techniques, IR laser absorption spectroscopy is widely used for gas sensing due to its in-situ and highly-selective quantification without any sample preparation and accumulation [13,14]. Also, in the laser absorption spectroscopy, off-axis integrated cavity output spectroscopy (OA-ICOS) [15–17] and wavelength modulation spectroscopy (WMS) [18–20] are two effective methods for chemical analysis owing to advantages including fast response, high robustness and sensitive and selective detection capability [21,22]. Additionally, compared with a multi-pass gas cell (MPGC) commonly used in tunable diode laser absorption spectroscopy (TDLAS), a cavity used in OA-ICOS can achieve the same or even much longer effective path length with a more compact configuration and a relatively smaller cell volume. In our previous studies of the development and measurement of OA-ICOS sensor systems [17,23], the light beams were directly coupled into the cavity in an off-axis pattern without processing the reflected light from the first cavity mirror. Also, the cavity outputs were focused into a photodetector equipped with 3
electrical components, which was at the risk of electrical sparks and causing explosion in field application. Additionally, the sensor showed a small dynamic range for CH4 measurement, and further field applications for environmental analysis are not performed. On the basis of our previous work on OA-ICOS [17, 23] and in order to further enable sensitive and remote CH4 detection in hazard area, we propose a novel gas-phase analytical scheme by simultaneously using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS). There are two main approaches adopted: 1) A novel gas cell with fiber-in fiber-out coupling scheme was developed for the OA-ICOS, resulting in the following benefits: a) due to low optical loss, the laser beam for gas detection can transmit through an optical fiber with a length of a few hundred meters and even several kilometers; and b) the optical gas-cell can be placed in the field for gas detection and the electrical part in a secure area for data acquisition and processing. 2) The reflected light from the first cavity mirror is re-collected and injected into a fiber-coupled single-pass gas cell (SPGC). In this case, two analytical schemes (i.e. using a cavity and a SPGC) with different measurement ranges share the same laser source, current driver, temperature controller, data acquisition (DAQ) card and computer, leading to a compact and low-cost configuration. As a comparison to our previous work [17,23], the novel aspects of the hybrid detection scheme are: (1) An all-fiber-coupled optical cavity is developed with an input and two output using both single-mode fiber (SMF) and multi-mode fiber (MMF) for highly-efficient mode coupling. (2) The useless light of the OA-ICOS reflected by the first cavity mirror is re-collected and injected into a fiber-coupled SPGC (can also be a MPGC) for WMS detection, thereby avoiding the use of a fiber beam splitter and ensuring the maximum light intensity utilization for OA-ICOS, which is helpful for improving the sensitivity of the FC-OA-ICOS. (3) Field monitoring of CH4 leakage is performed in a static and mobile manner for environmental analysis using the two developed vehicle-mounted fiber-coupled optical modules based on the two techniques. 2. Experimental section As shown in Fig. 1(a), a novel fiber-coupled optical module is proposed based on FC-OA-ICOS and 4
CR-WMS. An all-fiber-coupled optical cavity was developed for CH4 detection, which is different from our previous scheme [23]. A single-mode fiber collimator (FC) is used in front of the cavity to align the round-shape laser beam with a diameter of 0.5 mm. The cavity length of the FC-OA-ICOS is 18 cm with a radius of curvature of 20 cm and a reflectivity of 99.35% for the mirror, leading to an effective path length of 27 m. The cavity output laser beam with a wavelength of 1653.7 nm is focused by an off-axis parabolic mirror (OAP) onto a FC-connected MMF. The light reflected by the first cavity mirror (HR1) is coupled into another long MMF via two plane mirrors (M2 and M3) and an achromatic lens (f = 6 cm). Light output from the MMF is coupled into a round-shape multi-mode FC with a diameter of 0.9 mm, and then transmitted into a SPGC (8 cm in length) with an effective path length of 8 cm. The output of the SPGC is focused on another FC-connected, long MMF using an achromatic lens (f = 6 cm), which is used for CH4 detection based on CR-WMS. Photograph of the fabricated fiber-coupled optical module is demonstrated in Fig. 1(b). *Proposed position for insertion of Fig. 1 For the FC-OA-ICOS, the efficiency of coupling the cavity output into the MMF (core diameter, 62.5 µm) is about 20%. This is because the angles of the off-axis laser beams are irregular, making it difficult for the achromatic lens to converge the output beams effectively. For the CR-WMS, the efficiency of coupling the reflected light from HR1 into the MMF is approximately 30%, while the coupling efficiency from the SPGC into the MMF is about 28%. Furthermore, in order to eliminate the influence of the leaked beam from the cavity to the SPGC measurement, an iris is added between M3 and the lens in front of the SPGC, which allows only the reflected light by the plane surface of HR1 to pass through and blocks the light leaked from the cavity. Fig. S1 shows the architecture of the all-fiber-coupled CH4 sensor, including an electrical module for signal generation, acquisition and processing, and two optical modules for CH4 sensing application in field #1 and field #2. The two optical modules are based on FC-OA-ICOS and CR-WMS, respectively. The laser beam firstly enters field #1 for CH4 sensing using FC-OA-ICOS technique. Light reflected by HR1 is then supplied to field #2 for CH4 detection exploiting the CR-WMS technique. Light exiting from 5
the two optical modules reached detector #1 (PDA10CS, Thorlabs) and #2 (PDA20H, Thorlabs) separately via two long-distance fibers. In order to maintain the cavity mirror reflectivity for field application, Teflon polytetrafluoroethylene (PTFE) filter can be used in the gas inlet to remove ambient impurities and aerosols. Also, a small purge flow of zero air or nitrogen (N2) can be used on both sides of the cavity to keep the HR mirrors clean. Other optical/electrical parameters and the processing method in the electrical domain can be found in Ref. [23]. 3. Results and discussion 3.1. Analytical performances of FC-OA-ICOS In order to improve the detection sensitivity, WMS and second harmonic (2f) detection were utilized in the FC-OA-ICOS. CH4 absorption line selection and modulation depth optimization was discussed in Ref. [23]. As shown in Fig. S2, the maximum 2f signal amplitude is observed at a modulation amplitude of 0.25 V, corresponding to a modulation coefficient of 3.6. Calibration of the CH4 sensor system was carried out by using diluted CH4 gas samples with six different concentration levels (0, 100, 200, 300, 400 and 500 ppmv), as shown in Fig. S3(a). Fig. 2(a) depicts the linear relation (correlation coefficient R2 = 0.99956) between the 2f signal amplitude (max(2f), in V) and the CH4 concentration (C, in ppmv), expressed as C = 36872.90 × max(2 f ) − 60.05
(1)
For further determining the detection limit of the FC-OA-ICOS, noise levels were recorded by passing pure N2 into the gas cell (Fig. S3(b)). An Allen-Werle analysis was employed to assess the detection limit, as shown in Fig. 2(b), which yields a precision of 15.51 ppmv with a 2 s averaging time and shows an optimum averaging time of 84 s corresponding to a detection limit of 3.05 ppmv. 3.2. Analytical performances of CR-WMS Similar to the analytical method used in FC-OA-ICOS, a 10000 ppmv CH4 sample was used in CR-WMS to optimize the modulation depth (Fig. S4). An optimum modulation amplitude of 0.25 V was obtained and selected for the CR-WMS, leading to a modulation coefficient of 3.6. In order to calibrate the CR-WMS sensor system, the 2f amplitudes over a period of ∼2.3 min were measured for different 6
CH4 concentration levels (0, 10000, 20000…50000 ppmv), as demonstrated in Fig. S5(a). Fig. 2(c) shows the relationship between the averaged 2f absorption signal (max(2f), in V) and the CH4 concentration (C, in ppmv), which is fitted by a linear curve (R2 = 0.9993) and written as C = 2519830 × max(2 f ) − 18179.31
(2)
To investigate the detection limit and long-term stability of the CR-WMS, a standard pure N2 sample was measured continuously for ∼50 min at a sampling period of ∼2 s, and the measured time-domain concentration variation is shown in Fig. S5(b). Fig. 2(d) depicts the Allan-Werle deviation analysis and a detection limit of 237 ppmv is obtained at a 2 s averaging time. The optimum averaging time for obtaining the minimum Allan variance is ∼150 s with a precision value of 56 ppmv. Consequently, in this hybrid sensing scheme, the dynamic detection range of FC-OA-ICOS is approximately from 15 to 7500 ppmv, while that of CR-WMS is ~ 237 ppmv – 12 %. In this case, two different CH4 measurement ranges are achieved in a single sensor system with a wide dynamic range from ppmv levels (~ 15 ppmv) to several percent (~ 12%) for CH4 detection, which expands the application of the sensor system. Furthermore, the oscillatory behavior in Fig. 2(b) after 84 s and that in Fig. 2(d) after 150 s are mainly dominated by the system drift, including the power fluctuation of the DFB laser, the temperature drift of the electronic components, as well as the instability of the detector and the gas mixing system due to long-term operation. *Proposed position for insertion of Fig. 2 3.3. Sensing applications of monitoring CH4 gas leakage 3.3.1. Static measurements For verifying the analytical applicability of the demonstrated technique, field monitoring of CH4 leakage was performed for environmental analysis. For static fix-point field measurements of CH4 leakage, the FC-OA-ICOS and CR-WMS sensing modules were mounted on two battery-powered vehicles that were remotely controlled. The photograph shown in Fig. 3(a) was taken when it was placed outside the State Key Laboratory of Integrated Optoelectronics on the Jilin University campus to monitor the CH4 gas leakage in field #1 and #2. For field #1, the laser beam input into the FC-OA-ICOS module 7
was from the DFB laser placed in the laboratory through a 300m-long optical fiber. The output of the FC-OA-ICOS module was divided into two parts, one part was sent back to detector #1 via an optical fiber for subsequent data processing. The other part was connected to the entrance of the CR-WMS module in field #2 using a 300m-long fiber. Also the output laser beam of the CR-WMS module was sent back to detector #2 placed in the laboratory through an optical fiber for data processing. In these two modules, all the components used for field detection are optical and mechanical components without electrical ones. Hence the two modules are intrinsically safe for CH4 detection at high concentration levels, which is of considerable interest for safety monitoring. The CH4 concentration levels of the cylinder used for field #1 (FC-OA-ICOS) and field #2 (CR-WMS) were 0.5% and 1%, respectively. The distances between the leakage point and this two sensors were ~ 1 m. Gas leakage monitoring was performed at a temperature of 28 °C and a wind speed of 1 m/s. As shown in Fig. 3(b), the FC-OA-ICOS module in field #1 was turned on in advance and one concentration value was obtained per 2 s. To simulate the CH4 leakage, the cylinder valve was opened at 45 s. After the gas diffusion for 5 s, the monitored CH4 concentration level began to rise. The gas concentration approached the maximum value after 55 s and still fluctuated due to the effect of wind and the uncertainty of diffusion. When the cylinder valve was closed at 90 s, the concentration began to drop accordingly. Additionally, the cylinder valve was reopened at 145 s and the sensor repeated the leakage monitoring process from 150 s to 210 s. Fig. 3(c) depicts the gas leakage process based on the CR-WMS module in field #2. The cylinder valve was opened at 45 s and 140 s, and two gas leakages were simulated during the period of 50 −110 s and 145−200 s. This field deployment proves that the sensor has the ability for unmanned, in-situ and intrinsically-safe detection of field CH4 leakage. The sensor can be used for coal mining alarms, natural gas pipeline leakage monitoring as well as for the application in the occasions where in-situ human observation is challenging. *Proposed position for insertion of Fig. 3 3.3.2. Mobile measurements For the developed CH4 sensor with optical modules mounted on two remotely-controlled vehicles, 8
unmanned mobile deployment measurement can be realized at different places simultaneously without the movement of the electrical module. We exploited this potential to carry out gas leakage experiments in field #1 and #2 separately with the two modules moving horizontally and longitudinally. Mobile CH4 leakage measurement was conducted in field #1 and #2 using the FC-OA-ICOS and CR-WMS sensor modules simultaneously. Photograph and results of mobile gas measurement in field #1 using FC-OA-ICOS are shown in Figs. 4(a) and (c), respectively. The vehicle moved from point A to B and then to C. The duration for this movement was 500 s. The peak concentration level of the leaked CH4 appeared at 110 s during the movement from point A to B. Then we detected the CH4 leakage from point B to C. The maximum concentration level was observed at 360 s, indicating that a CH4 leakage source was located nearby. Figs. 4(b) and (d) depict the mobile CH4 measurement in field #2 using CR-WMS. The vehicle moved from point D to E and then to F with a duration of 730 s. The maximum CH4 concentration value was observed at 200 s from point D to E and at 550 s from point E to F. In this manner, accurate gas leakage source can be located with the two sensing modules moving horizontally and longitudinally. This application is of vital significance for remediation of the filed natural gas pipeline leakages. *Proposed position for insertion of Fig. 4 4. Conclusions A novel all-fiber-coupled CH4 gas sensor combining dual sensing module of FC-OA-ICOS and CR-WMS was demonstrated for the first time to our knowledge. Detection limits of 3 and 56 ppmv were achieved using the proposed analytical scheme of FC-OA-ICOS and CR-WMS, respectively. CH4 leakage monitoring was performed statically and mobilely using the unmanned vehicle-mounted, fiber-coupled gas sensing modules with wireless remote control. The demonstrated dual-sensing method shows the advantages of different measurement ranges and long-distance mobile field measurement with a single sensor system of reduced size and cost without the degradation of sensitivity, selectivity and reliability. The reported method will work as a necessary supplement to the family of laser absorption spectroscopy for analytical chemistry. 9
Acknowledgements This work was supported by the National Key R&D Program of China (No. 2016YFD0700101), National Natural Science Foundation of China (Nos. 61775079, 61627823, 61960206004), Science and Technology Development Program of Jilin Province, China (Nos. 20180201046GX, 20190101016JH) and the National Science Foundation (NSF) ERC MIRTHE award and Robert Welch Foundation (No.C-0586). References [1] J. Hansen, M. Sato, R. Ruedy, A. Lacis, V. Oinas, Global warming in the twenty-first century: an alternative scenario, Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 9875–9880. [2] A. Demirbas, The importance of natural gas as a world fuel, Energy Sources Part B. 1 (2006) 413– 420. [3] N. Prakash, A. Ramachandran, R. Varma, J. Chen, C. Mazzolenid, K. Du, Near-infrared incoherent broad-band cavity enhanced absorption spectroscopy (NIR-IBBCEAS) for detection and quantification of natural gas components, Analyst 143 (2018) 3284–3291. [4] K.Y. Zheng, C.T. Zheng, N.N. Ma, Z.D. Liu, Y. Yang, Y. Zhang, Y.D. Wang, F.K. Tittel, Near-infrared broadband cavity-enhanced spectroscopic multigas sensor using a 1650 nm light emitting diode, ACS Sens. 4 (2019) 1899–1908. [5] A. Somov, A.M. Baranov, D. Spirjakin, A wireless sensor-actuator system for hazardous gases detection and control, Sens. Actuators A: Phys. 210 (2014) 157–164. [6] D. Nagai, M. Nishibori, T. Itoh, T. Kawabe, K. Sato, W. Shin, Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films, Sens. Actuators B: Chem. 206 (2015) 488–494. [7] X. Yu, R.H. Lv, F. Song, C.T. Zheng, Y.D. Wang, Pocket-sized non-dispersive infrared methane detection device using two-parameter temperature compensation, Spectrosc. Lett. 47 (2014) 30–37.
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Figure Captions Fig. 1. (a) Fiber-coupled optical sensing modules based on FC-OA-ICOS and CR-WMS. SMF: single-mode fiber. MMF: multi-mode fiber. FC: fiber collimator. HR: highly-reflective mirror. OAP: off-axis parabolic mirror. SPGC: single-pass gas cell. PD: photodetector. (b) Photograph of the fabricated fiber-coupled optical module. (Color online only) Fig. 2. (a) For FC-OA-ICOS, experimental data and fitting curve of the CH4 concentration versus max (2f). (b) Allan deviation plot as a function of averaging time. (c) For CR-WMS, measured max (2f) at different CH4 concentration levels. (d) Allan deviation plot versus the averaging time. (Color online only) Fig. 3. (a) All-fiber-coupled CH4 gas sensor in field #1 and #2 using the two techniques of FC-OA-ICOS and CR-WMS for static gas leakage monitoring on the campus of Jilin University. Monitoring results of CH4 leakage process based on the (b) FC-OA-ICOS and (c) CR-WMS. (Color online only) Fig. 4. Mobile measurements of monitoring CH4 gas leakage in (a) field #1 and (b) field #2. Mobile monitoring results in (c) field #1 based on FC-OA-ICOS and (d) field #2 based on CR-WMS, respectively. (Color online only)
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Figures
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Highlights
FC-OA-ICOS and CR-WMS were realized on a single sensor system.
All-fiber-coupled CH4 sensor was realized exploiting two optical modules.
The reflected light from OA-ICOS was re-used for WMS detection.
The sensor has long-distance gas detection ability with two measurement ranges.
In-situ field measurement of CH4 was conducted.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0039-9140(20)30132-6
DOI:
https://doi.org/10.1016/j.talanta.2020.120841
Reference:
TAL 120841
To appear in:
Talanta
Received Date: 15 January 2020 Revised Date:
11 February 2020
Accepted Date: 13 February 2020
Please cite this article as: K. Zheng, C. Zheng, J. Li, N. Ma, Z. Liu, Y. Li, Y. Zhang, Y. Wang, F.K. Tittel, Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS), Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2020.120841. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Kaiyuan Zheng: Conceptualization, Methodology, Software, Investigation, Writing Original Draft Chuantao Zheng: Formal analysis, Writing - Review & Editing, Project administration, Funding acquisition, Resources Junhao Li: Visualization, Investigation Ningning Ma: Data Curation, Formal analysis Zidi Liu: Software, Validation Yaoyu Li: Data Curation, Validation Yu Zhang: Writing- Reviewing and Editing, Resources Yiding Wang: Supervision, Writing- Reviewing and Editing, Funding acquisition, Resources Frank K Tittel: Funding acquisition, Writing- Reviewing and Editing
Graphical Abstracts
Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS)
Kaiyuan Zheng a, Chuantao Zheng a*, Junhao Li a, Ningning Ma a, Zidi Liu a, Yaoyu Li a, Yu Zhang a, Yiding Wang a, Frank K. Tittel b
a
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P.R. China
b
Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
*
Corresponding author. E-mail: [email protected] (Chuantao Zheng); Tel.: +86-13756090979
1
Abstract By feeding back the reflected light from the first cavity mirror to a single-/multi-pass gas cell via a multi-mode fiber, we demonstrated a novel gas-phase analytical scheme for methane (CH4) detection by combing fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS). This scheme has an electrical module and two optical sensing modules which are connected through both single- and multi-mode optical fibers. Long-distance gas sensing application was conducted for verifying the analytical ability of the demonstrated technique exploiting the two fiber-coupled optical modules. A detection limit of 3 parts-per-million in volume (ppmv) for an 84 s averaging time and a precision of 56 ppmv for a 150 s averaging time were achieved using FC-OA-ICOS and CR-WMS, respectively. Two different CH4 measurement ranges were achieved in the sensor system with a wide dynamic range from ~ 15 ppmv to ~ 12% for CH4 detection. Field monitoring of CH4 leakage was performed for environmental analysis under a static and mobile mode using the wireless-controlled vehicle-mounted gas sensor. The proposed gas sensing scheme with fiber-coupled dual optical modules demonstrates a good potential for long-distance field CH4 measurements, especially for those in hazardous environment where in-situ human observation is impossible. Keywords: Chemical gas sensor; Remote sensing analysis; Fiber-coupled; Off-axis integrated cavity output spectroscopy; Wavelength modulation spectroscopy
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1. Introduction Chemical gas-phase analysis is significant in biological chemistry, geochemistry, as well as in environmental applications. Methane (CH4) is a colorless, flammable and explosive gas that poses significant safety hazards in natural gas transportation and coal mining [1]. Natural gas demand has been on the rise in the past few years and is expected to grow by 2.8% annually from 2001 to 2025 [2]. This growing demand is satisfied through a network of natural gas pipelines. However, natural gas pipelines leakage may cause long-term emissions of CH4 with a level of up to several percent, leading to healthy and environmental problems as well as a huge waste of energy [3]. In addition, electrical sparks may cause explosion when the CH4 concentration is above the low explosive limit (LEL) level of 4.9% [4]. Therefore there are additional requirements for the electrical components and devices in field measurements. In this manner, the development of a secure, sensitive and low-cost CH4 sensor system with a wide dynamic range from parts-per-million in volume (ppmv) levels to several percent (~ %) is of vital importance to atmospheric and industrial monitoring. Various techniques for CH4 detection have been reported in recent years, such as catalyst combustion [5,6], electrochemical [7], semiconductor [8,9], and infrared (IR) laser absorption spectroscopy [10–12]. Among these techniques, IR laser absorption spectroscopy is widely used for gas sensing due to its in-situ and highly-selective quantification without any sample preparation and accumulation [13,14]. Also, in the laser absorption spectroscopy, off-axis integrated cavity output spectroscopy (OA-ICOS) [15–17] and wavelength modulation spectroscopy (WMS) [18–20] are two effective methods for chemical analysis owing to advantages including fast response, high robustness and sensitive and selective detection capability [21,22]. Additionally, compared with a multi-pass gas cell (MPGC) commonly used in tunable diode laser absorption spectroscopy (TDLAS), a cavity used in OA-ICOS can achieve the same or even much longer effective path length with a more compact configuration and a relatively smaller cell volume. In our previous studies of the development and measurement of OA-ICOS sensor systems [17,23], the light beams were directly coupled into the cavity in an off-axis pattern without processing the reflected light from the first cavity mirror. Also, the cavity outputs were focused into a photodetector equipped with 3
electrical components, which was at the risk of electrical sparks and causing explosion in field application. Additionally, the sensor showed a small dynamic range for CH4 measurement, and further field applications for environmental analysis are not performed. On the basis of our previous work on OA-ICOS [17, 23] and in order to further enable sensitive and remote CH4 detection in hazard area, we propose a novel gas-phase analytical scheme by simultaneously using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS). There are two main approaches adopted: 1) A novel gas cell with fiber-in fiber-out coupling scheme was developed for the OA-ICOS, resulting in the following benefits: a) due to low optical loss, the laser beam for gas detection can transmit through an optical fiber with a length of a few hundred meters and even several kilometers; and b) the optical gas-cell can be placed in the field for gas detection and the electrical part in a secure area for data acquisition and processing. 2) The reflected light from the first cavity mirror is re-collected and injected into a fiber-coupled single-pass gas cell (SPGC). In this case, two analytical schemes (i.e. using a cavity and a SPGC) with different measurement ranges share the same laser source, current driver, temperature controller, data acquisition (DAQ) card and computer, leading to a compact and low-cost configuration. As a comparison to our previous work [17,23], the novel aspects of the hybrid detection scheme are: (1) An all-fiber-coupled optical cavity is developed with an input and two output using both single-mode fiber (SMF) and multi-mode fiber (MMF) for highly-efficient mode coupling. (2) The useless light of the OA-ICOS reflected by the first cavity mirror is re-collected and injected into a fiber-coupled SPGC (can also be a MPGC) for WMS detection, thereby avoiding the use of a fiber beam splitter and ensuring the maximum light intensity utilization for OA-ICOS, which is helpful for improving the sensitivity of the FC-OA-ICOS. (3) Field monitoring of CH4 leakage is performed in a static and mobile manner for environmental analysis using the two developed vehicle-mounted fiber-coupled optical modules based on the two techniques. 2. Experimental section As shown in Fig. 1(a), a novel fiber-coupled optical module is proposed based on FC-OA-ICOS and 4
CR-WMS. An all-fiber-coupled optical cavity was developed for CH4 detection, which is different from our previous scheme [23]. A single-mode fiber collimator (FC) is used in front of the cavity to align the round-shape laser beam with a diameter of 0.5 mm. The cavity length of the FC-OA-ICOS is 18 cm with a radius of curvature of 20 cm and a reflectivity of 99.35% for the mirror, leading to an effective path length of 27 m. The cavity output laser beam with a wavelength of 1653.7 nm is focused by an off-axis parabolic mirror (OAP) onto a FC-connected MMF. The light reflected by the first cavity mirror (HR1) is coupled into another long MMF via two plane mirrors (M2 and M3) and an achromatic lens (f = 6 cm). Light output from the MMF is coupled into a round-shape multi-mode FC with a diameter of 0.9 mm, and then transmitted into a SPGC (8 cm in length) with an effective path length of 8 cm. The output of the SPGC is focused on another FC-connected, long MMF using an achromatic lens (f = 6 cm), which is used for CH4 detection based on CR-WMS. Photograph of the fabricated fiber-coupled optical module is demonstrated in Fig. 1(b). *Proposed position for insertion of Fig. 1 For the FC-OA-ICOS, the efficiency of coupling the cavity output into the MMF (core diameter, 62.5 µm) is about 20%. This is because the angles of the off-axis laser beams are irregular, making it difficult for the achromatic lens to converge the output beams effectively. For the CR-WMS, the efficiency of coupling the reflected light from HR1 into the MMF is approximately 30%, while the coupling efficiency from the SPGC into the MMF is about 28%. Furthermore, in order to eliminate the influence of the leaked beam from the cavity to the SPGC measurement, an iris is added between M3 and the lens in front of the SPGC, which allows only the reflected light by the plane surface of HR1 to pass through and blocks the light leaked from the cavity. Fig. S1 shows the architecture of the all-fiber-coupled CH4 sensor, including an electrical module for signal generation, acquisition and processing, and two optical modules for CH4 sensing application in field #1 and field #2. The two optical modules are based on FC-OA-ICOS and CR-WMS, respectively. The laser beam firstly enters field #1 for CH4 sensing using FC-OA-ICOS technique. Light reflected by HR1 is then supplied to field #2 for CH4 detection exploiting the CR-WMS technique. Light exiting from 5
the two optical modules reached detector #1 (PDA10CS, Thorlabs) and #2 (PDA20H, Thorlabs) separately via two long-distance fibers. In order to maintain the cavity mirror reflectivity for field application, Teflon polytetrafluoroethylene (PTFE) filter can be used in the gas inlet to remove ambient impurities and aerosols. Also, a small purge flow of zero air or nitrogen (N2) can be used on both sides of the cavity to keep the HR mirrors clean. Other optical/electrical parameters and the processing method in the electrical domain can be found in Ref. [23]. 3. Results and discussion 3.1. Analytical performances of FC-OA-ICOS In order to improve the detection sensitivity, WMS and second harmonic (2f) detection were utilized in the FC-OA-ICOS. CH4 absorption line selection and modulation depth optimization was discussed in Ref. [23]. As shown in Fig. S2, the maximum 2f signal amplitude is observed at a modulation amplitude of 0.25 V, corresponding to a modulation coefficient of 3.6. Calibration of the CH4 sensor system was carried out by using diluted CH4 gas samples with six different concentration levels (0, 100, 200, 300, 400 and 500 ppmv), as shown in Fig. S3(a). Fig. 2(a) depicts the linear relation (correlation coefficient R2 = 0.99956) between the 2f signal amplitude (max(2f), in V) and the CH4 concentration (C, in ppmv), expressed as C = 36872.90 × max(2 f ) − 60.05
(1)
For further determining the detection limit of the FC-OA-ICOS, noise levels were recorded by passing pure N2 into the gas cell (Fig. S3(b)). An Allen-Werle analysis was employed to assess the detection limit, as shown in Fig. 2(b), which yields a precision of 15.51 ppmv with a 2 s averaging time and shows an optimum averaging time of 84 s corresponding to a detection limit of 3.05 ppmv. 3.2. Analytical performances of CR-WMS Similar to the analytical method used in FC-OA-ICOS, a 10000 ppmv CH4 sample was used in CR-WMS to optimize the modulation depth (Fig. S4). An optimum modulation amplitude of 0.25 V was obtained and selected for the CR-WMS, leading to a modulation coefficient of 3.6. In order to calibrate the CR-WMS sensor system, the 2f amplitudes over a period of ∼2.3 min were measured for different 6
CH4 concentration levels (0, 10000, 20000…50000 ppmv), as demonstrated in Fig. S5(a). Fig. 2(c) shows the relationship between the averaged 2f absorption signal (max(2f), in V) and the CH4 concentration (C, in ppmv), which is fitted by a linear curve (R2 = 0.9993) and written as C = 2519830 × max(2 f ) − 18179.31
(2)
To investigate the detection limit and long-term stability of the CR-WMS, a standard pure N2 sample was measured continuously for ∼50 min at a sampling period of ∼2 s, and the measured time-domain concentration variation is shown in Fig. S5(b). Fig. 2(d) depicts the Allan-Werle deviation analysis and a detection limit of 237 ppmv is obtained at a 2 s averaging time. The optimum averaging time for obtaining the minimum Allan variance is ∼150 s with a precision value of 56 ppmv. Consequently, in this hybrid sensing scheme, the dynamic detection range of FC-OA-ICOS is approximately from 15 to 7500 ppmv, while that of CR-WMS is ~ 237 ppmv – 12 %. In this case, two different CH4 measurement ranges are achieved in a single sensor system with a wide dynamic range from ppmv levels (~ 15 ppmv) to several percent (~ 12%) for CH4 detection, which expands the application of the sensor system. Furthermore, the oscillatory behavior in Fig. 2(b) after 84 s and that in Fig. 2(d) after 150 s are mainly dominated by the system drift, including the power fluctuation of the DFB laser, the temperature drift of the electronic components, as well as the instability of the detector and the gas mixing system due to long-term operation. *Proposed position for insertion of Fig. 2 3.3. Sensing applications of monitoring CH4 gas leakage 3.3.1. Static measurements For verifying the analytical applicability of the demonstrated technique, field monitoring of CH4 leakage was performed for environmental analysis. For static fix-point field measurements of CH4 leakage, the FC-OA-ICOS and CR-WMS sensing modules were mounted on two battery-powered vehicles that were remotely controlled. The photograph shown in Fig. 3(a) was taken when it was placed outside the State Key Laboratory of Integrated Optoelectronics on the Jilin University campus to monitor the CH4 gas leakage in field #1 and #2. For field #1, the laser beam input into the FC-OA-ICOS module 7
was from the DFB laser placed in the laboratory through a 300m-long optical fiber. The output of the FC-OA-ICOS module was divided into two parts, one part was sent back to detector #1 via an optical fiber for subsequent data processing. The other part was connected to the entrance of the CR-WMS module in field #2 using a 300m-long fiber. Also the output laser beam of the CR-WMS module was sent back to detector #2 placed in the laboratory through an optical fiber for data processing. In these two modules, all the components used for field detection are optical and mechanical components without electrical ones. Hence the two modules are intrinsically safe for CH4 detection at high concentration levels, which is of considerable interest for safety monitoring. The CH4 concentration levels of the cylinder used for field #1 (FC-OA-ICOS) and field #2 (CR-WMS) were 0.5% and 1%, respectively. The distances between the leakage point and this two sensors were ~ 1 m. Gas leakage monitoring was performed at a temperature of 28 °C and a wind speed of 1 m/s. As shown in Fig. 3(b), the FC-OA-ICOS module in field #1 was turned on in advance and one concentration value was obtained per 2 s. To simulate the CH4 leakage, the cylinder valve was opened at 45 s. After the gas diffusion for 5 s, the monitored CH4 concentration level began to rise. The gas concentration approached the maximum value after 55 s and still fluctuated due to the effect of wind and the uncertainty of diffusion. When the cylinder valve was closed at 90 s, the concentration began to drop accordingly. Additionally, the cylinder valve was reopened at 145 s and the sensor repeated the leakage monitoring process from 150 s to 210 s. Fig. 3(c) depicts the gas leakage process based on the CR-WMS module in field #2. The cylinder valve was opened at 45 s and 140 s, and two gas leakages were simulated during the period of 50 −110 s and 145−200 s. This field deployment proves that the sensor has the ability for unmanned, in-situ and intrinsically-safe detection of field CH4 leakage. The sensor can be used for coal mining alarms, natural gas pipeline leakage monitoring as well as for the application in the occasions where in-situ human observation is challenging. *Proposed position for insertion of Fig. 3 3.3.2. Mobile measurements For the developed CH4 sensor with optical modules mounted on two remotely-controlled vehicles, 8
unmanned mobile deployment measurement can be realized at different places simultaneously without the movement of the electrical module. We exploited this potential to carry out gas leakage experiments in field #1 and #2 separately with the two modules moving horizontally and longitudinally. Mobile CH4 leakage measurement was conducted in field #1 and #2 using the FC-OA-ICOS and CR-WMS sensor modules simultaneously. Photograph and results of mobile gas measurement in field #1 using FC-OA-ICOS are shown in Figs. 4(a) and (c), respectively. The vehicle moved from point A to B and then to C. The duration for this movement was 500 s. The peak concentration level of the leaked CH4 appeared at 110 s during the movement from point A to B. Then we detected the CH4 leakage from point B to C. The maximum concentration level was observed at 360 s, indicating that a CH4 leakage source was located nearby. Figs. 4(b) and (d) depict the mobile CH4 measurement in field #2 using CR-WMS. The vehicle moved from point D to E and then to F with a duration of 730 s. The maximum CH4 concentration value was observed at 200 s from point D to E and at 550 s from point E to F. In this manner, accurate gas leakage source can be located with the two sensing modules moving horizontally and longitudinally. This application is of vital significance for remediation of the filed natural gas pipeline leakages. *Proposed position for insertion of Fig. 4 4. Conclusions A novel all-fiber-coupled CH4 gas sensor combining dual sensing module of FC-OA-ICOS and CR-WMS was demonstrated for the first time to our knowledge. Detection limits of 3 and 56 ppmv were achieved using the proposed analytical scheme of FC-OA-ICOS and CR-WMS, respectively. CH4 leakage monitoring was performed statically and mobilely using the unmanned vehicle-mounted, fiber-coupled gas sensing modules with wireless remote control. The demonstrated dual-sensing method shows the advantages of different measurement ranges and long-distance mobile field measurement with a single sensor system of reduced size and cost without the degradation of sensitivity, selectivity and reliability. The reported method will work as a necessary supplement to the family of laser absorption spectroscopy for analytical chemistry. 9
Acknowledgements This work was supported by the National Key R&D Program of China (No. 2016YFD0700101), National Natural Science Foundation of China (Nos. 61775079, 61627823, 61960206004), Science and Technology Development Program of Jilin Province, China (Nos. 20180201046GX, 20190101016JH) and the National Science Foundation (NSF) ERC MIRTHE award and Robert Welch Foundation (No.C-0586). References [1] J. Hansen, M. Sato, R. Ruedy, A. Lacis, V. Oinas, Global warming in the twenty-first century: an alternative scenario, Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 9875–9880. [2] A. Demirbas, The importance of natural gas as a world fuel, Energy Sources Part B. 1 (2006) 413– 420. [3] N. Prakash, A. Ramachandran, R. Varma, J. Chen, C. Mazzolenid, K. Du, Near-infrared incoherent broad-band cavity enhanced absorption spectroscopy (NIR-IBBCEAS) for detection and quantification of natural gas components, Analyst 143 (2018) 3284–3291. [4] K.Y. Zheng, C.T. Zheng, N.N. Ma, Z.D. Liu, Y. Yang, Y. Zhang, Y.D. Wang, F.K. Tittel, Near-infrared broadband cavity-enhanced spectroscopic multigas sensor using a 1650 nm light emitting diode, ACS Sens. 4 (2019) 1899–1908. [5] A. Somov, A.M. Baranov, D. Spirjakin, A wireless sensor-actuator system for hazardous gases detection and control, Sens. Actuators A: Phys. 210 (2014) 157–164. [6] D. Nagai, M. Nishibori, T. Itoh, T. Kawabe, K. Sato, W. Shin, Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films, Sens. Actuators B: Chem. 206 (2015) 488–494. [7] X. Yu, R.H. Lv, F. Song, C.T. Zheng, Y.D. Wang, Pocket-sized non-dispersive infrared methane detection device using two-parameter temperature compensation, Spectrosc. Lett. 47 (2014) 30–37.
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Figure Captions Fig. 1. (a) Fiber-coupled optical sensing modules based on FC-OA-ICOS and CR-WMS. SMF: single-mode fiber. MMF: multi-mode fiber. FC: fiber collimator. HR: highly-reflective mirror. OAP: off-axis parabolic mirror. SPGC: single-pass gas cell. PD: photodetector. (b) Photograph of the fabricated fiber-coupled optical module. (Color online only) Fig. 2. (a) For FC-OA-ICOS, experimental data and fitting curve of the CH4 concentration versus max (2f). (b) Allan deviation plot as a function of averaging time. (c) For CR-WMS, measured max (2f) at different CH4 concentration levels. (d) Allan deviation plot versus the averaging time. (Color online only) Fig. 3. (a) All-fiber-coupled CH4 gas sensor in field #1 and #2 using the two techniques of FC-OA-ICOS and CR-WMS for static gas leakage monitoring on the campus of Jilin University. Monitoring results of CH4 leakage process based on the (b) FC-OA-ICOS and (c) CR-WMS. (Color online only) Fig. 4. Mobile measurements of monitoring CH4 gas leakage in (a) field #1 and (b) field #2. Mobile monitoring results in (c) field #1 based on FC-OA-ICOS and (d) field #2 based on CR-WMS, respectively. (Color online only)
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Figures
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Highlights
FC-OA-ICOS and CR-WMS were realized on a single sensor system.
All-fiber-coupled CH4 sensor was realized exploiting two optical modules.
The reflected light from OA-ICOS was re-used for WMS detection.
The sensor has long-distance gas detection ability with two measurement ranges.
In-situ field measurement of CH4 was conducted.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: