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Abstract
For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm-1 located in ν3 band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor’s usability in environmental monitoring.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Trace detection of methane is a vital issue in fossil fuel mining and transportation, industrial process control, environmental research or exhaled breath analysis [1]. In many of these applications sub-ppm (parts per million) precision and compact sensor design are required. Sub ppm detection limits are usually achieved by using the strongest fundamental rotational-vibrational transitions of methane located near 3.3 µm which can be conveniently targeted with distributed feed-back (DFB) interband cascade lasers (ICLs). Some examples of methane detection in this spectral region have been reported, e.g., using GaSb laser at 3.27 µm [2], using GaSb ICL at 3.29 µm [3] or using ICL operating near 3.34 µm to measure not only methane, but also ethane [4]. In all three cases dense multi-pass cells (MPCs) were employed to increase the optical path length (thus sensitivity) while keeping footprint of the sensors small.
Although sensitivity enhancement using multi-pass cells is relatively simple and convenient, it comes with some limitations, e.g., related to weight, bulkiness and cost. Additionally, dense MPCs may be prone to opto-mechanical drifts which often significantly impede long term performance of the sensors [5]. These issues can be addressed by using hollow-core fibers/waveguides as gas cells. Different types of hollow-core fibers (HCFs) have been demonstrated in gas sensing setups, including waveguides with inner metallic layer, as well as microstructured hollow-core optical fibers. Mid-infrared guiding was reported in all-silica photonic bandgap hollow-core fibers (PBG-HCFs) back in 2005 [6]. More recently, significant progress has been made in fabrication of anti-resonant hollow-core fibers (AR-HCFs) which not only can have low bending losses but also very low attenuation in the mid-infrared, even when made from fused silica [7,8]. Another advantage of AR-HCFs is their relatively large core (typically from ∼40 to ∼100 micrometers), which makes it easier to fill the fiber core with gas mixture. Some authors also demonstrated that AR-HCFs can be spliced to standard polarization maintaining solid-core fibers [9] and that holes can be drilled on their side-walls in order to enable or improve gas flow through the fiber [10,11].
AR-HCFs have been demonstrated in gas sensing systems targeting numerous molecules, using various spectral regions and spectroscopic techniques. In [12] Kapit and Michel presented sensor that detects methane in the near-infrared spectral region (∼1.65 µm) using 5.1-m-long HCF. More recently, Bao et al. demonstrated gas sensing in the near-infrared and visible spectral regions (1.65 µm, 1.55 µm and 761 nm) using photothermal interferometry inside resonant cavity based on HCF [13]. In [14] Yao et al. presented HCF-based gas sensing at 2.3 µm, with carbon monoxide as target molecule. The same molecule was also detected in [15] using photothermal spectroscopy. Several examples of HCF-based gas sensing in the mid-infrared spectral region have also been demonstrated so far. Examples include nitric oxide detection at 5.26 µm presented in [16], nitrous oxide sensing near 3.6 µm demonstrated in [17] or ethylene spectroscopy at 10.5 µm inside chalcogenide HCF recently published by Hu et al. in [18].
Methane sensing using mid-infrared guiding HCFs have been reported in several papers. Direct absorption spectroscopy was applied to detect methane using low-cost broadband sources such as mid-IR light emitting diodes (LEDs) [19,20]. In those examples, methane was pumped into hollow-core capillary waveguides with a reflective metallic layer. Although the cost-efficiency and sensitivity of those setups is impressive, their performance is prone to impact of measurement conditions and other chemical compounds like water vapor. Successful CH4 detection near 3.3 µm has also been reported inside microstructured hollow-core optical fibers. Broadband methane detection was performed inside photonic bandgap hollow-core fibers using either a supercontinuum source [21] or an optical parametric oscillator [22]. Differential frequency generation (DFG) source at 3.33 µm and Kagome-lattice HCF have been applied to detect methane at sub-ppm levels using wavelength modulation spectroscopy (WMS) [23] and chirped laser dispersion spectroscopy (CLaDS) [24]. In both these works samples with relatively high methane concentration of 200 ppm were examined. In the first work, the estimated sensitivity was approximately 4 ppm and it was limited by interference fringes. In the CLaDS-based sensor the impact of fringes was reduced and obtained detection limit was improved to hundreds of ppb. The same DFG source was also used recently for methane detection inside anti-resonant HCF [25]. In this case the detection limit of 24 ppb was reported with averaging time of 40 seconds and using a sample with methane concentration of 100 ppm.
In this work we demonstrate high-sensitivity methane sensor comprising the interband cascade laser operating near 3.27 µm and an all-silica anti-resonant hollow-core fiber. The laser targets one of the strongest methane absorption lines near 3057.71 cm-1, where no significant overlap with water vapor transitions is observed. The performance of the sensor is presented for two different fiber lengths of 0.6 m and 7.5 m. The system’s sensitivity and response time is discussed. In terms of the detection limit, the sensor demonstrated here outperforms all systems mentioned earlier. It is also far more compact than previously reported systems relying on DFG sources [23–25]. We also show results of continuous monitoring of ambient methane and water vapor that demonstrates usefulness of the HCF-based configuration in practical applications.
2. Experimental setup
The schematic diagram of the experimental setup used in this work is depicted in Fig. 1(a), while the photograph of the setup is shown in Fig. 1(b). DFB ICL (from Nanoplus) operating near 3.27 µm was used to target the R(3) line (near 3057.71 cm-1) in ν3 band of methane which is one of the strongest absorption lines of this molecule, that is not overlapped by water vapor transitions. The collimated mid-IR beam from the laser was guided thorough the 100-mm-long gas cell with CaF2 windows (which was used for calibration) and subsequently coupled into the air core of the anti-resonant revolver-type HCF using gold-coated off-axis parabolic mirror with focal length of 2 inches. Additional gold-coated plane mirror was used to help with the alignment. The fiber was mounted on the fiber holder allowing for the adjustment in 5 axes. This specifically included XYZ translation and pitch and yaw control, because the precise adjustment of light coupling into the HCF was critical in suppressing optical fringes that may be present in the recorded spectra due to the interference of higher order spatial modes co-propagating in the HCF. The setup was designed with only reflective elements (i.e., no lenses) and aligned so that optical fringes were minimized. As a result, the typical efficiency of the light coupling into the AR-HCF in our setup was ∼10% (which was still more than sufficient to perform all measurements). The input of the HCF was kept at ambient pressure while the other end of the HCF was terminated with the tightened temporary fiber connector (B30126C3 from Thorlabs) and placed inside the tight housing which allowed forcing the gas flow through the HCF by applying the overpressure to this fiber end. Light transmitted through the fiber was detected with an MCT photodetector (PDAVJ5 from Thorlabs) that was integrated with the tight housing.
Fig. 1. Schematic of the hollow-core fiber based methane detector (a) and a photograph of the key part of the actual setup (b) (Note: pressure, temperature and humidity sensors have been used only in environmental CH4 sensing demonstration).
The fiber used for methane detection was an all-silica AR-HCF with six internal capillaries. The fiber was manufactured in the Institute of Microelectronics and Photonics, Poland (part of the Łukasiewicz Research Network) using the stack-and-draw method. The diameter of the inner hollow-core was ∼41 µm and the total area of air gap in the fiber cross-section (relevant when filling the fiber with gas) was ∼0.0048 mm2. Figure 2(a) shows the SEM picture of the fiber. The HCF used in this work has multiple transmission bands in the near- and mid-infrared, as demonstrated in Fig. 2(b). Transmission through two AR-HCF pieces (∼7.5 m and ∼0.6 m) was measured in order to estimate the attenuation of the fiber at 3.27 µm. The value of 1.13 dB/m was obtained, which is in good agreement with the data provided earlier for a HCF with very similar geometry [26]. During all measurements the HCF was coiled (as shown in Fig. 1(b)) with a diameter of approximately 20 cm which caused only very small (∼0.4 dB) power drop due to bending losses. We also did not observe any other negative effects of fiber bending (including bending-induced multimode propagation or mechanical damage of the fiber).
Fig. 2. SEM image of the anti-resonant hollow-core fiber structure (a); optical power transmitted through the fiber (supercontinuum was used as a source) (b).
3. Results and discussion
3.1. Direct absorption spectroscopy
The quasi linear wavelength tuning range of the ICL (obtained by applying saw-tooth modulation of the injection current) was from ∼3055 to ∼3061 cm-1. Within this spectral range we found not only the absorption line of methane (near 3057.71 cm-1) but also four water vapor peaks. This, however, has no significant impact on methane sensing. In typical atmospheric conditions absorption from the closest H2O line (near 3057.15 cm-1) is significantly weaker than that related to the methane transition. Stronger absorption lines of water vapor are located near 3056.35 cm-1 and 3059.92 cm-1, which is spectrally distant enough to have no impact on methane detection. The transmittance of the HCF filled with ambient air is demonstrated in the Fig. 3, together with the corresponding absorption spectrum simulated with HITRAN database (the model assumed methane and water vapor concentrations of 1.8 ppm and 0.51%, respectively). Good agreement between experiment and simulation was obtained.
Fig. 3. Transmission through 7.5-m-long HCF filled with ambient air (black) and simulation based on HITRAN database (red).
3.2. Wavelength modulation spectroscopy
In order to perform high-sensitivity methane detection, wavelength modulation spectroscopy (WMS) was employed. The ICL was modulated with signal comprising a 20 kHz sinewave superimposed on a low-frequency ramp (5 Hz) for periodic wavelength tuning. A lock-in amplifier (from Zurich Instruments) was used to demodulate signal from the MCT detector and retrieve its second harmonic component (2 × 20 kHz; so called 2f WMS signal). Modulation parameters, such as depth of the sinewave and modulation frequencies of ramp and sine components, were experimentally optimized.
The 2f WMS signals obtained for the three CH4 mixtures measured inside 0.6-m-long HCF mixtures are depicted in the Fig. 4(a). We used ambient air and calibrated mixtures of CH4 (5.6, 23.9, and 100 ppm) balanced with 21% oxygen/79% nitrogen. The measurements were performed with the gas flowing through the fiber (overpressure of ∼0.2 bar was used). As shown in Fig. 4(b), very good linear response of the sensor was obtained. The horizontal error bars in Fig. 4(b) represent the concentration uncertainty of the gas mixtures (±10%), while the vertical error bars height represents the magnitude of the optical fringe in the corresponding scans.
Fig. 4. Sample 2f WMS spectra obtained from 0.6-m-long HCF for three different concentrations of CH4 (a); linear dependency of 2f WMS amplitude from CH4 concentration (b).
The biggest drawback of the shorter, 0.6-m-long HCF, was the presence of strong optical fringes, shown in Fig. 4(a). This issue has been mitigated by using the longer, 7.5-m-long AR-HCF. Sample 2f WMS spectra are shown in Fig. 5. They were recorded for four gas mixtures: two calibrated CH4 mixtures of 5.6 ppm and 23.9 ppm, pure nitrogen and ambient air. With a broad tuning range, several water vapor lines could also be measured. One can notice that for the measurements with dry samples (nitrogen and CH4 mixtures) the strongest H2O line is still visible. Two factors contribute to the presence of this spectral feature. The first is the water vapor present in the short free-space section of the setup. The second is residual water vapor that was still present inside the HCF (H2O is very ‘sticky’ molecule). Based on the amplitude of the WMS signal we estimate that these two factor contributed approximately equally to the spectral feature visible near 3056.35 cm-1 and the amount of water vapor inside HCF was only ∼300 ppm (thus far less than 10% of ambient concentration). Nevertheless, these spectra suggest that hollow-core fiber may not be suitable for sensing of some ‘sticky’ molecules (e.g., ammonia).
It is also important to mention that with longer HCF and proper adjustment of the light coupling into the fiber optical fringes could be significantly reduced.
3.3. Response time
The time needed to fill the anti-resonant HCFs with a gas sample directly affects the response time of the HCF-based sensor. For the configuration presented in this work, filling time depended on the value of the applied overpressure and the length of the fiber. Figure 6 shows the results obtained using a 0.6-m-long piece of AR-HCF, with the overpressure of ∼0.2 bar. Three cycles of filling this AR-HCF with CH4 mixtures are shown and the gas exchange time of ∼13 s was achieved.
Fig. 6. 2f WMS amplitude at the line center versus time for three cycles of filling of the 0.6-m-long AR-HCF with different gas samples.
For the 7.5-m-long AR-HCF, the same small overpressure (∼0.2 bar) resulted in much longer filling time of ∼24 minutes. The most straightforward way to address this issue is by applying larger overpressure. Results obtained with 7.5-m-long HCF are shown in Fig. 7. In this experiment, the HCF was initially filled with nitrogen, and the 2f WMS amplitude was recorded as the gas sample (5.6 ppm of CH4) was inserted into the HCF at the selected input pressure (from 0.5 to 4 bar above ambient). The response time of the sensor (calculated as a time needed for the 2f WMS amplitude to saturate at its maximum value) drops from ∼24 minutes for 0.2 bar to less than 2 minutes for pressures higher than 2 bar (Fig. 7(b)).
Fig. 7. 2f WMS amplitude versus time when 7.5-m-long fiber was filled with 5.6 ppmv methane mixture at different pressures (a) and response time of the sensor based on 7.5-m-long AR-HCF versus pressure (b).
One can also notice that the saturated 2f WMS amplitude varies with applied pressure. This can be explained by the changes of the shape of the absorption line. Figure 8(a) shows methane absorption line measured for different values of overpressure. These absorption profiles agree well with absorption spectra generated using HITRAN database (shown in Fig. 8(b)). For simulation, we have assumed that the pressure inside the HCF is uniform and equals the average value between the pressure at the inlet and the outlet of the HCF (so, for example, overpressure of 0.5 bar, i.e., 1 bar at the outlet and 1.5 bar at the inlet, means that uniform pressure of 1.25 bar is used for simulation). This approach was suggested recently in [27] and it results in good agreement with measured signals. The simulated absorption profiles were used to calculate 2f WMS spectra can be calculated for the given wavelength modulation depth (Fig. 8(c). One can notice that the 2f WMS amplitude drops with the pressure increasing, which is primarily due to pressure-broadening of the absorption line. The 2f WMS amplitudes obtained numerically agree reasonably well with the measured ones, as shown in Fig. 8(d).
Fig. 8. Simulated absorption profiles of methane near 3057.71 cm-1 for used overpressure values (a), simulated 2f WMS spectra for used overpressure values (b) and the 2f WMS amplitude normalized to the amplitude at no overpressure (c).
3.4. Detection limit
Sensitivity of the gas sensing system was determined using the Allan-Werle deviation analysis [28] which is shown in Fig. 9. The setup comprised the 7.5-m-long AR-HCF filled with ambient air from the laboratory (concentration of methane was assumed to be 2 ppm, based on subsequent measurement with in-line calibration). In order to lock the wavelength of the ICL at the center of the methane transition at 3057.71 cm-1 part of the output beam was outcoupled (using CaF2 wedged beamsplitter) and sent through an additional reference gas cell containing 1% of CH4 (balanced with nitrogen). The signal after the reference cell was measured and demodulated using second input of the lock-in amplifier. The third harmonic component was retrieved and used as a feedback for wavelength stabilization (this type of wavelength locking is often implemented in spectroscopic systems that use WMS [29], photoacoustic spectroscopy [30] or Faraday rotation spectroscopy [31].
When the laser wavelength was locked at the transition center, the second harmonic signal from the HCF was registered for over 15 minutes with the acquisition rate of 100 samples/s. The lock-in amplifier’s demodulator bandwidth was set to 17.4 Hz. From the Allan-Werle plot, the detection limit can be estimated to be ∼3.96 ppb for averaging time of 1 s and ∼1.54 ppb for averaging time of 20 s. As shown in Table 1, this performance is comparable to many MPC-based CH4 detection systems with ICLs or laser diodes targeting methane lines in the 3.3 µm region [2–4,32,33] and outperforms the HCF-based sensors reported previously [19,20,23–25].
Table 1. Performance comparison of methane sensors operating between 3.15 and 3.45 µm that use either multi-pass cell (MPC) or hollow-core fiber (HCF)
3.5. Demonstration of atmospheric methane detection
The system was used in a scanned mode, i.e., full 2f WMS spectra were recorded (similar to those shown in Fig. 5). This allowed for retrieving information about concentration of not only methane but also water vapor. A 1f WMS signal was also recorded for power normalization of the 2f WMS signals (it will be referred as 2f/1f WMS). The DFB-ICL was modulated at 20 kHz (for WMS) with additional ramp at 0.5 Hz (for wavelength tuning). The subsequent scans were averaged and analyzed. The amplitudes of the 2f WMS signals from methane and water vapor transitions were retrieved not directly, as values of peaks near 3057.71 cm-1 (for methane) and 3056.35 cm-1 (for water vapor), but as differences between these peaks and the baseline value near 3059.5 cm-1 (where there was no absorption features). With this baseline subtraction we were able to remove drifts from the final data that were caused by small optical fringes still present in the recorded spectra. This was particularly critical for methane sensing because, as shown in Fig. 5, methane signal was much weaker than the signal from water vapor line (thus the impact of optical fringes was relatively larger for methane detection). A single data point was obtained every 5 minutes. More frequent measurements were possible but not used due to the long filling time of the HCF which, for the 0.2 bar overpressure that has been used, was ∼24 minutes. Prior to entering the housing the gas parameters (pressure, temperature and relative humidity) were measured, as shown in Fig. 1 (using AA01A Baratron manometer from MKS and TSP01 data logger from Thorlabs). With these parameters the concentration of water vapor could be calculated (using absolute humidity table and ideal gas model) which was subsequently used to validate the data obtained from WMS-based measurement.
For methane detection, the sensor was calibrated using a reference cell placed in the free-space section of the setup (as shown in Fig. 1). Calibration was performed before the beginning of the measurement and also 3 times during the sensor operation. The procedure included recording two signals: the one with cell being empty, and the second with the cell filled with the calibrated mixture of 100 ppm CH4 (±10%). Because the sensor was running with gas samples for which peak absorbance was below 0.03, the recorded 2f/1f WMS signals can be considered to depend linearly on concentration [35]. As a result, simple proportion between amplitudes of the two signals (with and without calibration gas) could be used to calculate the concentration of methane inside HCF. Additionally, methane concentration was corrected for temperature changes. The correction factor was determined based on numerical simulations using HITRAN database.
The 6-days-long measurement of H2O and CH4 concentrations in ambient air from outside of the laboratory was performed in Wrocław (Poland), in the early November of 2021. Figure 10 shows the amplitude of 2f/1f WMS amplitude from water vapor line. Very good agreement between this data and H2O concentration calculated from pressure/temperature/humidity measurement was obtained. We have also observed a delay of ∼30 minutes between the two measurements which is consistent with the gas filling time of the HCF.
Fig. 10. The measurement of ambient H2O concentration: ‘WMS’ plot represents amplitude of the 2f WMS signal from water vapor transition; ‘T,P,RH’ plot is H2O concentration calculated from temperature, pressure and relative humidity measurement.
Figure 11(a) shows retrieved concentration of methane. Values between ∼1.75 and ∼2.4 ppm were recorded. The diurnal cycle is clearly visible, as shown in Fig. 11(b). The mean values of methane concentration between 11pm and 5am and between 11am and 5pm were 2.2125 ppm (standard deviation 0.1193 ppm) and 2.0058 ppm (standard deviation 0.1707ppm), respectively. These results are consistent with other reports, e.g., [33,34]. It is also worth mentioning that during this almost one week-long continuous measurement there was no need for any re-alignment of the setup.
Fig. 11. (a) Methane concentration retrieved from 2f/1f WMS amplitude; (b) the same methane signal superimposed with the sinewave with a 1 day period.
4. Conclusions
In this paper we proposed highly sensitive and selective HCF-based optical sensor for methane detection. The presented setup is simple and compact. With the wide-band transmission of the HCF, the sensor is easy to adapt to target also other gaseous compounds in both near- and mid-infrared, provided that appropriate sources and detectors are available. In this work it was used to target methane at one of its strongest transitions at 3057.71 cm-1 using the tunable DFB ICL. Both direct absorption spectroscopy and wavelength modulation spectroscopy were demonstrated. Performance of the system using WMS has been characterized, ppb-level detection limit has been obtained and dynamic range of 3 orders of magnitude has been estimated. A scanned WMS system has been also implemented and used for almost week-long continuous monitoring of ambient methane and water vapor.
The longer, 7.5-m-long makes sub-ppm CH4 concentrations possible due to significant decrease in fringes magnitude and optical path increase. The disadvantageous impact of the optical fringes was reduced only by proper adjustment of the light coupling, so the fundamental mode of the HCF was excited at most. Further improvement could be achieved with custom mode-matching optical setup coupling light to the air core of the HCF, but it comes at the price of increased system complexity, size and cost. Although, the response time of the 7.5-m fiber was longer, it could be reduced when larger pressure difference between the two HCF ends is used. It was also shown, that the pressure applied to the one end of the fiber influences the 2f WMS amplitude, so this effect has to be taken into account during the measurements.
It was demonstrated, that with fiber as long as 7.5 m, the atmospheric concentration of CH4 can be detected using direct absorption spectroscopy and the result can be further greatly improved with wavelength modulation spectroscopy. Based on Allan deviation analysis sensor detection limit of 3.96 ppb for 1 s of averaging was obtained. This could be further reduced to 1.54 ppb for 20 s of averaging. The noise equivalent absorption coefficient (NEAC) was calculated using data from HITRAN database and the value of 7.63 × 10−8 was obtained, which corresponds to the fractional absorption of 5.995 × 10−5. This performance is comparable to MPC-based CH4 detection systems using ICLs targeting the strongest methane absorption lines [2–4] and is better than previously reported HCF-based methane sensors [19,20,23–25].
We have also presented the 6-days-long continuous monitoring of ambient air. Fluctuations of the concentrations of water vapor and methane were recorded. Diurnal changes in methane concentration were observed. Separate measurement of water vapor was also used to validate the performance of the sensor and confirm its suitability for applications that require continuous concentration monitoring.
Funding
Ministerstwo Edukacji i Nauki ('Diamentowy Grant' DI 2019 0026 49).
Acknowledgments
G.G. acknowledges Narodowe Centrum Nauki for financing his Ph.D. program with the Preludium Bis project (2019/35/O/ST7/04176). The Department of Optics and Photonics, Wroclaw University of Science and Technology, acknowledges the financial support within the National Laboratory for Photonics and Quantum Technologies (NLPQT) infrastructural project (POIR.04.02.00-00-B003/18), co-financed by the European Regional Development Fund (ERDF).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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