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Abstract
In this paper, we experimentally asses the performance of wavelength modulation spectroscopy-based spectrometers incorporating 1.3 m-long gas absorption cells formed by an antiresonant hollow core fiber (ARHCF) and a Kagome hollow core fiber. To evaluate the discrepancies with minimum methodology error, the sensor setup was designed to test both fibers simultaneously, providing comparable measurement conditions. Ethane (C2H6) with a transition located at 2996.88 cm−1 was chosen as the target gas. The experiments showed, that due to better light guidance properties, the ARHCF-based sensor reached a minimum detection limit of 4 ppbv for 85 s integration time, which is more than two times improvement in comparison to the result obtained with the Kagome fiber.
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1. Introduction
The rapid and consistent development of microstructured fiber technology, especially focused on the design and fabrication of hollow-core fibers (HCFs), has significantly broadened the application area of optical fibers. The unique guidance properties of HCFs, enabling, for example, robust delivery of high-energy pulsed laser light [1,2] and low-loss transmission over a broad spectral range up to mid-infrared (mid-IR) wavelengths [3–6], allowed their implementation in precise micromachining of various materials [7–9], novel configurations of high-power mid-IR gas laser sources [10–12], and efficient sensing of gaseous substances via laser-based spectroscopic techniques [13–16]. The HCFs can be relatively easily filled with a gas sample, serving as a low volume, robust, compact and versatile gas absorption cells (GACs). Experiments verifying detection of, e.g. greenhouse gases at trace concentration level using HCFs have been documented [17–19]. Since the detection capability of a vast number of laser-based detection techniques is correlated with the gas-beam interaction path length, access to optical path-length enhancing solutions is of high interest [20,21]. HCFs with lengths of up to several tens of meters have already been shown to fulfill this need and can be used as an alternative to the commonly implemented bulky and complex conventional optics-based system, such as multi-pass cells, yielding comparable, or superior detection limits [13,22–24].
To date, several different types of HCFs have been utilized in laser-based gas sensor setups. Amongst them the antiresonant hollow-core fibers (ARHCFs) and Kagome HCFs provide the guidance performance required to target the strongest transitions of, e.g., hydrocarbons in mid-IR wavelength region [15,25–27]. The light guidance mechanism of such fibers can be explained by means of the so-called antiresonant reflecting optical waveguide (ARROW) principle [28] and the inhibited coupling mechanism [29,30]. With proper design, these fibers can be fabricated with core diameters even exceeding 100 µm, while maintaining very low light-glass overlap, which is not achievable with e.g. conventional hollow-core photonics bandgap fibers (HC-PBGFs). These unique features render the fibers suitable for applications in mid-IR laser-based gas spectroscopy, as low attenuation and broad transmission windows can be achieved. The high interest in the development of gas sensors benefiting from the use of HCFs is confirmed by the continuously increasing number of published work within both the laser spectroscopy and fiber development communities [31,32].
Nikodem et al. reported in [33] a WMS-based methane (CH4) sensor utilizing a 1.3 m long Kagome HCF. The fiber used as a GAC allowed targeting a CH4 transition in the 3.3 µm wavelength band, resulting in the sensor minimum detection limit (MDL) of 505 parts per billion by volume (ppbv) at 10 ms integration time. This result was further improved by Krzempek et al. who modified the sensor by combining the Kagome HCF with chirped laser dispersion spectroscopy (CLaDS) technique [26]. In this configuration, the sensor reached an MDL of 65 parts per billion by volume (ppbv) for a 100 s integration time. It can be assumed that the improved detection capability resulted from the immunity of CLaDS to changes in the amplitude of the measured signal [34], which is observable in Kagome HCF-aided sensors due to the multimode characteristic of this type of fiber [12]. On the other hand, ARHCF-assisted gas sensors utilizing the WMS technique have been shown to deliver high sensitivity over a significantly broader spectral range. Jaworski et al. presented in [27] detection of CH4 at 3.334 µm inside a 1 m ARHCF with an MDL of 24 ppbv for a 40 s integration time. The operational wavelength range of ARHCF-aided gas sensors was increased by Nikodem et al. who reported nitrous oxide (N2O) detection at ∼ 4.54 µm combining a 3.2 m long revolver type ARHCF with a quantum cascade laser (QCL) and WMS technique [13]. The sensor was characterized by an MDL of 5.4 ppbv for a 1 s integration time. Jaworski et al. [17] and later Yao et al. [18] have demonstrated an application of ARHCFs made of borosilicate and tellurite glass, respectively, in sensors that targeted a strong transition of nitric oxide (NO) at 5.26 µm, delivering a sensitivity of up to 6 ppbv for a 30 s integration time [18]. Summary of the performance of the aforementioned HCF-aided gas sensors is presented in Table 1. The reported up to date results have confirmed suitability of both types of HCFs for gas sensing in the mid-IR. However, a direct comparison of the performance of the sensors utilizing Kagome HCF and ARHCF has not been investigated yet, making it impossible to distinguish which fiber is better suited for such applications. Both fibers require precise manufacturing infrastructure, thus experimental verification of their usability and parameters would benefit the researchers working on the development of such HCFs, justifying the efforts in perfecting their designs.
Table 1. Summary of the performance of the selected ARHCF- and Kagome HCF-based gas sensors.
In this work, we present the first direct experimental performance comparison of gas sensors utilizing an ARHCF- and a Kagome-HCF-based GACs. The developed sensor configuration allowed simultaneous detection of ethane (C2H6) inside two different types of HCFs placed in the separate measurement sections of the sensor. WMS signal extraction technique was used in the experiments. The HCFs used in the experiment enabled low-loss transmission of the laser beam within the 3.33 µm spectral band, however, representing different structures, complexity, bending sensitivities and light guidance characteristic. The performance of each sensor section was carefully, simultaneously characterized in order to determine its detection capability, stability, response linearity, and gas filling time. The performed research allowed us to investigate the influence of light guidance properties and the physical structure of two different types of HCF on the efficiency of WMS-based gas sensors.
2. Experimental setup
The schematic of the experimental setup of the sensor is depicted in Fig. 1. The C2H6 gas molecules were targeted at a wavelength of 2996.88 cm−1 (3336.9 nm) with the aid of a self-made difference frequency generation source (DFG) [33]. The DFG source consisted of two narrow-linewidth distributed feedback diode lasers (DFB) lasing at 1.064 µm (Eagleyard, model DFB-1064-0040-BFY02-0002) and 1.562 µm (Eblana, EP1562-5-DM-B01-FM). Their emissions were additionally optically amplified using custom-built polarization maintaining fiber amplifiers and subsequently combined in a wavelength division multiplexer (WDM), which was spliced to a 50 mm long fiber-coupled periodically polled lithium niobate waveguide chip (PPLN WG, NTT Electronics, model WD-3333-000-A-B-C). The PPLN WG was kept at a constant temperature of 65.5°C, which ensured phase matching conditions of the pump and signal beams. The DFG process delivered a single-mode output beam. To perform WMS-based spectroscopic signal retrieval, the 1.562 µm DFB laser wavelength was additionally modulated and swept with sinewave (fmod) and sawtooth ramp signals, respectively. This was achieved by providing proper voltage signals from an arbitrary function generator (Tektronix, model AFG3102C) to the modulation input of the laser controller (CLD1015, Thorlabs). The DFG-generated beam was collimated using a calcium fluoride (CaF2) plano-convex lens with a focal length of 20 mm and a germanium wedge was used to separate the unconverted near-infrared beams from the mid-IR measurement beam. The DFG emission was split into two beams using an germanium (Ge) wedge forming separate measurement paths within the sensor setup with nearly identical beam propagation conditions (with ∼ 50/50 optical power split ratio). The beams were coupled into the 1.3 m long GACs consisting of an ARHCF from [27] and a Kagome HCF from [33] using single CaF2 plano-convex lenses with 75 mm focal lengths in each sensor section, which allowed for matching the numerical aperture (NA) of the fibers (∼ 0.03). Filling of both fibers was achieved by gluing the near ends in specially developed gas filling chambers [23], allowing fast and efficient gas exchange in their cores. The output of each HCF was collimated and focused on mid-IR photodetectors (VIGO 1 – Vigo System S.A., model PVI-3TE-4, VIGO 2 – Vigo System S.A., model LabM-I-10.6) using a pair of CaF2 and Ge plano-convex lenses with focal lengths of 40 mm and 25.4 mm, respectively. Both photodetectors were characterized by a similar noise level at 3.4 µm wavelength (measurements are presented in following sections). The DC signal amplitude observed for each photodetector was ∼1 V (related to the mid-IR optical power focused onto the detectors), ensuring comparable initial measurement conditions for both fibers (measured with both fibers entirely filled with pure N2 at an averaged pressure at the level of 800 Torr). The electrical signals from the photodetectors were fed to two inputs of an UHFLI lock-in amplifier (Zurich Instruments, model UHFLI 600 MHz), which enabled their simultaneous demodulation at the 2nd harmonic of the sinewave frequency (2×fmod) modulating the wavelength of the DFG emission.
Fig. 1. Experimental setup of the sensor. 1.064 µm/1.562 µm - lasers with fiber amplifiers; FG – arbitrary function generator; WDM - wavelength division multiplexer; PPLN WG - periodically polled lithium niobate waveguide; CaF2 - calcium fluoride plano-convex lens, GW- germanium window, BS - beam splitter; M - mirror; IGC - input gas chamber; Ge – germanium plano-convex lens; VIGO1/VIGO2 – mid-IR photodetectors; PC – computer; BV – ball valve; PG – pressure gauge; VP – vacuum pump.
3. Mid-infrared gas detection
3.1 WMS measurement parameter optimization
The most crucial parameters in the WMS technique are frequency (fmod) and depth (amplitude) of the sinewave wavelength modulation applied to the gas molecules excitation source. In the DFG source used in the developed sensor, the modulation signal can be applied to each of the DFB lasers that take part in the nonlinear frequency conversion process. In our case, the 1.562 µm laser wavelength was modulated by the external sinewave signal applied to its driving current. As a result, the a modulation of the mid-IR radiation wavelength generated via the DFG process was observed.
To properly determine the optimal fmod, the DFG wavelength was tuned to the center of the selected C2H6 transition and both fiber GACs were filled with a certified mixture (Air Liquide) of 50 ppmv C2H6 in N2, maintaining a constant flow of the gas through both, with an averaged pressure in their cores at the level of 800 Torr. Using the UHFLI lock-in amplifier, the 2f WMS signal amplitudes were recorded for different values of fmod, within the range of 0.5–18 kHz. The results of the measurements are shown in Fig. 2(a). The optimal fmod was defined as the frequency at which the 2f WMS signal reached the highest amplitude for each fiber. In the case of the developed sensor, the optimal fmod was reached at 4.2 kHz for both GAC’s.
Fig. 2. Optimization of the WMS measurement parameters: (a) sinewave modulation frequency fmod; (b) sinewave modulation depth. Measurement and data acquisition details are provided in both graphs. The optimal values of both WMS parameters for each fiber are marked with blue dashed circles.
The optimal modulation depth (amplitude) was determined using a similar approach as discussed above. The sinewave modulation frequency was set to 4.2 kHz during the measurements. The 2f WMS signal amplitudes were recorded for different modulation depths within the 4–20.2 GHz range. The modulation depth was expressed in GHz according to the procedure described in [23]. The measurement results depicted in Fig. 2(b) indicate that the highest amplitudes of the 2f WMS signals were registered for the modulation depth of 11.78 GHz for both fibers, which makes this value optimal for this particular sensor configuration and measurement conditions.
The noticeable difference in the amplitude of the registered 2f WMS signals between the fibers is probably related to the change in light guidance conditions in the Kagome HCF due to the presence of C2H6 molecules at an averaged pressure of 800 Torr within its core. The responsivity of the VIGO 2 detector used with the Kagome HCF is less than 5% lower in comparison to VIGO 1 performance for the considered 2f WMS signal measurement conditions. We assume, that the presence of a noninert gas at a pressure different from the ambient value in the hollow part of Kagome HCF or ARHCF leads to a local change in the refractive index and can induce a distortion of a thin core walls (due to its e.g. pressure-induced bending), which directly impacts the position of the low-loss region (it shifts in wavelength) and transmission loss of the fiber [35]. The observed difference in the amplitudes of the registered 2f WMS signals between both fibers indicates that the loss in the Kagome HCF due to the presence of C2H6 molecules at the increased pressure inside its hollow region is approximately 2 dB greater in comparison to the ARHCF. In the WMS technique, the amplitude of the 2f signal is strongly dependent on the optical intensity of the measured beam, hence, higher fiber loss in turn leads to a lower spectroscopic signal level. In the case of C2H6 detection within ∼ 3.33 µm spectral range, the ARHCF shows greater immunity to gas and pressure changes within its structure than the Kagome HCF. We believe, that the vulnerability of the Kagome fiber to pressure and gas refractive index-related light guidance performance results in its increased attenuation and consequently lower amplitude of the signal registered by the photodetector – thus lower overall 2f signal is observed.
3.2 Sensor performance evaluation
The first part of the sensor performance evaluation focused on determining the gas filling time of the GAC formed by both HCFs. This parameter is independent of the detected gas sample, thus the experimental results can be considered as a guideline. The GAC were filled with the target gas using the overpressure-aided approach described in [23]. To measure the gas filling time for both fibers, the DFG source wavelength was tuned to the center of the targeted C2H6 transition and the sinewave modulation parameters were set at their optimal values determined in the previous experiments. The UHFLI lock-in amplifier allowed continuous registering of the 2f WMS signal amplitude levels for both the ARHCF and Kagome HCF GAC sensor sections simultaneously, minimizing the influence of the apparatus error on the performance assessment. At the beginning of the measurement, both fibers were first flushed with pure N2 and subsequently filled with a mixture of 50 ppmv C2H6 in N2. The input gas pressure was set to 840 Torr for both HCFs, while both fiber ends were kept opened to the laboratory air with the pressure of 760 Torr. According to the simulations of the gas filling process for both fibers, which was performed using the model and the procedure described in detail in [36], the averaged C2H6 pressure inside both HCFs was at the level of 800 Torr, which ensured the same measurement conditions for both sensor arms. Gas filling time was defined as the time within which the amplitude of the 2f WMS signal reaches 90% of the maximum value [23,37]. According to the measurement results shown in Fig. 3, the gas filling times were approximately 2.4 s and 7.8 s for Kagome HCF and ARHCF GAC’s, respectively. The gas filling time obtained with Kagome HCF with a core diameter of 116 µm was more than three times shorter compared to the ARHCF having a hollow core with a diameter of 84 µm. This is the direct result of a lower gas flow rate presented in the ARHCF core, which was at a level of 0.06 sccm compared to 0.34 sccm in the Kagome HCF core. Nevertheless, both fibers allow for relatively fast gas exchange within the sensor using only a small overpressure, hence without introducing any significant pressure-induced broadening of the gas transition, that could negatively impact the detection capability of the sensor.
Fig. 3. Gas filling profiles of the GAC utilizing the: ARHCF (red line) and Kagome HCF (black trace). Each fiber was first flushed with pure N2 and subsequently filled with 50 ppmv C2H6. The averaged gas pressure inside both fibers reached 800 Torr. The gas filling time was marked with red and grey areas for ARHCF and Kagome HCF, respectively. The DFG source wavelength was tuned to the center of the selected C2H6 transition during the measurement time.
To directly compare the gas detection performance of both sensor configurations, each fiber was filled with C2H6 samples with different concentrations at 800 Torr pressure (averaged value) ensuring constant flow of gas through their cores. Experimental gas samples were prepared by diluting a certified mixture of 50 ppmv C2H6 in pure N2 down to 200 ppbv, using a commercial gas mixer (Environics, model 4000). The GAC’s were flushed with N2 between each measurement, evacuating the previously measured gas from the hollow part of each fiber. Sinewave modulation parameters were set to their optimal values. The DFG source wavelength was additionally scanned through the entire C2H6 absorption line profile with the aid of a 50 mHz sawtooth ramp signal applied to the 1.562 µm DFB laser injection current. Full 2f WMS absorption spectra for different C2H6 concentrations were recorded and analyzed with the aid of the UHFLI lock-in amplifier, with data acquisition parameters set to the following values: sampling rate of 1.717 kSa/s, time constant (TC) of 9.321 ms and 24 dB/oct filter setting. The measured signals are depicted in Fig. 4. Both fibers enabled the registration of 2f WMS signal spectra for all prepared C2H6 samples. However, a higher amplitude of the measured signals was observed for the ARHCF-based gas absorption cell. We assume, that this is a result of pressure and gas-induced changes of the light guidance parameters (increased attenuation at the considered wavelength) of the Kagome HCF, as discussed above. Furthermore, higher 2f WMS signal distortions, can be observed in the Kagome HCF-based sensor part, especially in the signal trace for 200 ppbv C2H6. We believe that this is connected with the multimode guidance nature of this particular fiber, leading to a parasitic intermodal interference-induced modulation of the registered signal.
Fig. 4. 2f WMS signal spectra recorded for different concentrations of C2H6 inside: (a) ARHCF; (b) Kagome HCF. The insets on both graphs represent 2f WMS signal spectra for 200 ppbv C2H6 for better visibility (black plots show measured data, and red traces illustrate signal averaged every 50 samples for clarity).
The linearity of the response of each sensor configuration was determined by recording the 2f WMS signal amplitudes for 30 s for different concentrations of C2H6 flowing through both GAC’s. During this measurement, the DFG wavelength was tuned to the center of the target transition (2996.88 cm−1) and the sinewave modulation parameters were kept at their optimal values. Both fibers were flushed with N2 before switching to a different C2H6 sample. The UHFLI data acquisition parameters were maintained at the same values as mentioned in the earlier paragraph of this manuscript. The experimental results are presented in Fig. 5. To calculate the R2 coefficient, which describes the linearity of the sensor response, the averaged 2f WMS signal amplitudes for each measurement were plotted as a function of gas concentration inside each fiber. The insets included in Figs. 5(a) and 5(b) show that the R2 coefficient reached 0.9989 and 0.9987 for the ARHCF and Kagome HCF, respectively. Both fibers allow for high linearity of the sensor’s response with a slightly better performance observable for the ARHCF GAC, which in our opinion is associated with better modal stability of this fiber compared to the Kagome HCF. This is especially evident when the pressure and the gas type inside the Kagome fiber is varied.
Fig. 5. 2f WMS signal amplitudes plotted as a function of gas concentration: (a) ARHCF; (b) Kagome HCF. The values were recorder for 30 s for each concentration. The insets show averaged amplitudes of the 2f WMS signals along with a linear fit.
The minimum detection limit (MDL) was established by calculating the Allan deviation from the recorded 2f WMS signal noise for each sensor configuration independently. To carry out the experiment, both fibers were filled with pure N2 at 800 Torr pressure (averaged pressure inside both HCFs), causing its constant flow through both absorption cells during the measurement. The DFG source wavelength was tuned to the center of the selected C2H6 transition. Modulation parameters were set to the optimal values. The time constant (TC) on the UHFLI was set to 1.005 ms. The noise amplitudes of the 2f WMS signal for ARHCF and Kagome HCF were recorded for a 60 min period, as shown in Fig. 6(a). The measured dataset was used to calculate Allan deviation plots for both sensor configurations, which are shown in Fig. 6(b). The MDL for the ARHCF-based absorption cell reached 4 ppbv for an integration time of 85 s (noise level ∼ 3.2 µV), which corresponds to a noise equivalent absorption coefficient (NEA) of ∼ 9.4 × 10−8 cm−1. The application of the Kagome-HCF-based absorption cell allows obtaining an MDL of 9 ppbv for a 37 s integration time (noise level ∼ 4.7 µV) yielding a NEA of ∼ 2.1 × 10−7 cm−1. More than two times lower MDL and worse long-time stability of the Kagome-HCF-based sensor resulted in our opinion from the combination of multimode guidance characteristic of the fiber and, as we assume, its higher attenuation due to the presence of the measured gas at an increased pressure inside the hollow region of the fiber. As a consequence, the 2f WMS signal amplitude (measured for 50 ppmv C2H6 inside the ARHCF, shown in Fig. 4(b)) is lower in comparison to the ARHCF, which in addition to approximately 1.5 times higher noise level leads to the worse sensitivity of the Kagome HCF-based sensor. To assure, that the measurements are not affected by the methods and apparatus used in the experiments, we have tested VIGO detectors in each section of the sensor, by measuring their long-term noise. The results are plotted in Figs. 6(c) and 6(d). This concludes, that the discrepancy in the MDLs is resulting purely from the performance of each of the fibers. The deteriorated stability of the Kagome fiber is clearly observable in Fig. 6 (a). Detection limit estimation was performed using N2, thus errors resulting from e.g. dissimilar gas flow through the fibers could be neglected.
Fig. 6. Long-term sensor stability evaluation. (a) 2f WMS sensor noise amplitude registered for 60 min with the fibers filled with pure N2 and the DFG source wavelength tuned to the center of the C2H6 transition. (b) Allan deviation plots calculated from the registered background noise. (c) 2f WMS noise amplitude recorded over 60 min for both detectors with the DFG source turned OFF. (d) Allan deviation plots calculated for both detectors based on long-term noise assessment. Data acquisition, laser modulation, and experiment conditions are provided on the left-hand side graphs.
4. Conclusions
In this work, we report the first direct experimental comparison of the detection capability of a WMS-based sensor utilizing two dissimilar HCFs, i.e. ARHCF and Kagome HCFs forming low-volume and robust gas absorption cells. The developed sensor allowed simultaneous detection of C2H6 at a wavelength of 3336.9 nm within two separate sensing paths formed by the aforementioned fibers, thus allowing direct comparison of the sensing performance delivered by both fibers. Proper design of the experiment ensured minimized influence of the apparatus and methods used for this evaluation. The performed experiments showed that the GAC formed using an ARHCF delivers better overall long-term stability of the measurements, with significantly lower drift of the signal amplitude (Fig. 6(a)). By calculating the Allan deviation plots we can conclude, that the Kagome fiber-based GAC achieved an inferior MDL. We believe, that this is connected with the combination of two phenomena, i.e. multimode characteristic of the Kagome HCF and its reduced immunity to the presence of a noninert gas under increased pressure in its hollow region. The multimode guidance of this particular fiber leads to an intermodal interference within the measurement path, manifesting itself as a parasitic modulation of the retrieved spectroscopic signal, which directly impacts the stability of the sensor and results in the increased noise. We assume, that the presence of the C2H6 molecules under increased pressure inside the Kagome HCF causes a modification of the local refractive index and a deformation of its structure e.g. by bending of the thin core walls, which could lead to the change of the guidance performance of the fiber, i.e. to the increased loss within the considered wavelength band [35]. Therefore, the amplitude of the registered spectroscopic signals in the case of the Kagome HCF is reduced in contrast to that of the ARHCF. This, in addition to the greater noise level results in lower MDL obtainable with the Kagome HCF-based sensor. However, this hypothesis requires further investigation in order to entirely analyze the light guidance performance of the HCFs while both are being filled with a gas mixture at the increased pressure, which we aim to execute in the near future. The increased loss in the case of the Kagome HCF can be in principle partially resolved by increasing the optical power of the excitation beam coupled to the fiber while constructing a sensor based solely on this fiber type. However, the available optical power of the CW laser sources (mainly DFG and Interband Cascade Lasers) at ∼3.33 µm is limited to several milliwatts, therefore this method of addressing the issue is not a remedy. This will be especially evident in configurations where tens-of-meters-long GAC have to be implemented in the sensor, in order to reach the desired sensitivities. The abovementioned drawbacks of Kagome fibers will manifest strongly, with increased optical path-lengths and higher pressure differences. On the other hand, because of its larger core size, the Kagome HCF allows for more than three times shorter gas exchange time in comparison to the ARHCF, which improves the overall responsivity of this part of the developed sensor. The larger core size is beneficial in a sensor configuration where the gas filling process is realized by the overpressure-aided approach. However, in the ultimate out-of-lab implementation, the HCF-based gas sensor should allow diffusion-based gas exchange in the GAC. This is achieved by a modification of its structure in order to enable efficient gas exchange inside its core [38]. In comparison to Kagome HCFs, the ARHCFs, have a less complex cross-section structure (gaps between cladding capillaries), and thus can be relatively easily modified using a laser micromachining process. This allows fabricating gas diffusion channels along the fiber length without introducing additional transmission loss and modification of the light guidance properties [38]. Modification of the complex, few layer-based cladding of the Kagome HCF using ultrashort laser pulses is a very complex process which results in contaminating its hollow region with the post-process debris (silica glass), which significantly impacts the guidance performance of the fiber. Thus fabricating numerous gas diffusion channels in fibers with tens-of-meters-lengths is not considered for Kagome-based GAC’s. As for the achieved detection limits, the NEA coefficients of ∼ 9.4 × 10−8 cm−1 and ∼ 2.1 × 10−7 cm−1 obtained for ARHCF and Kagome HCF, respectively, are at the level presented in other types of WMS-based gas sensors that utilize HCFs in the mid-IR spectral band [13,16,39]. The experimental results and noted limitations presented in this paper conclude, that the ARHCFs are more overall better suited for the application to laser-based gas sensing. We believe, that further optimization of the ARHCF structure and its mid-IR light transmission properties will allow achieving detection limits currently reserved for complex gas detection systems.
Funding
Narodowe Centrum Nauki (M-ERA.NET 2 Call 2019, 2019/01/Y/ST7/00088); National Key Research and Development Program of China (2020YFB1312802); Chinese Academy of Sciences (Pioneer Hundred Talents Program, ZDBS-LY JSC020); National Natural Science Foundation of China (61935002); International Science and Technology Cooperation Programme (2018YFE0115600); South China University of Technology, Key Laboratory of Fiber Laser Materials and Applied Techniques (The Open Fund of the Guangdong Provincial); STI2030 – Major Projects (2022ZD0212100).
Acknowledgments
The authors would like to thank Piotr Bojęś for preparing ethane samples. Author contributions – Piotr Jaworski: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Visualization, Data curation, Writing – original draft, Writing – review & editing, Resources, Project administration, Funding acquisition. Dakun Wu: Resources, Writing – review & editing. Fei Yu: Resources, Writing – review & editing. Karol Krzempek: Writing – review & editing, Resources, Project administration, Funding acquisition.
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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