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Abstract
In this paper we report a mid-infrared sensor based on an anti-resonant hollow core fiber. A quantum cascade laser operating around 4.53 µm is used to target one of the strongest transition of nitrous oxide near 2203.7 cm−1. The system provides 1-second minimum detection limit at single parts-per-billion level using 3.2-m-long fiber with the response time of less than 30 seconds. Presented sensing approach shows a good perspective for compact and sensitive mid-infrared fiber-based spectrometers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-based gas sensing plays important role in various applications including environmental monitoring [1], medical diagnosis [2] or industrial process control [3]. In majority of gas sensing techniques sensitivity may be improved through increasing the effective sensing path length. This is typically accomplished with multi-pass cells of different types, e.g. Herriott [4] or cylindrical [5]. Unfortunately, long-path multi-pass cells are usually bulky. Some designs may be compact but they require large number of reflections which results in additional losses. Furthermore, multi-pass cells may be prone to significant thermal drifts [6]. Another way of increasing the sensing path length is to use hollow core fibers (HCFs) which can be filled with the gas of interest [7]. Different types of HCFs can be used for this purpose. Laser spectroscopy inside capillary HCFs with inner reflective coating has been demonstrated in [8–10]. HCF of this type has several advantages such as broad transmission and large inner diameter. However, multimode propagation or scattering on inner surface may lead to the presence of optical fringes. As a result, to obtain reasonable performance, the impact of optical fringes was suppressed by introducing mechanical vibrations in the setup [8] or through application of more sophisticated sensing techniques [9,10]. Photonic band-gap (PBG) HCF is another type of fiber which can be used for in-fiber gas sensing. Several examples can be found in literature, in various spectral regions, including ∼1.55 µm [11], ∼1.65 µm [12,13], ∼2 µm [14] and even ∼3.3 µm [15]. Unfortunately, PBG HCFs have two major drawbacks. Firstly, all reported examples which relied on simple absorption sensing, were affected by strong fringe patterns, which most likely were due to propagation of higher order spatial modes inside PBG fibers. The second drawback is related to relatively long gas diffusion times inside PBG HCFs. Diameter of their core is typically below 20-30 µm which results in diffusion times usually in the order of minutes, even for relatively short pieces of fiber. This may be acceptable, when HCF is used as reference gas cell which is filled only once and then sealed [16,17], but will be a limiting factor when gas of interest has to be continuously flown through the fiber. As we have shown recently [18,19] much better performance in this application can be expected from anti-resonant (AR) HCFs [20–24]. Light propagation in AR-HCFs relies on inhibited coupling between the core and cladding modes, not on the photonic band gap mechanism [25,26]. As a result, single mode operation can be maintained for relatively large core diameters (50 µm and more) and good light transmission can be obtained even in the spectral regions where material itself has high losses [27,28]. So far, AR-HCFs have been demonstrated to be suitable e.g. for high energy pulse delivery [29], light generation in the mid-infrared hollow fiber gas lasers [30,31] or as fiber-based reference gas cells for frequency stabilization [32–34]. In our previous work we have shown laser spectroscopy of methane at 3.4 µm inside a Kagome-type AR HCF [18,35]. More recently we have presented carbon dioxide detection near 2 µm inside a Revolver-type AR HCF [19]. In both cases ∼1.3-m-long fibers and standard absorption-based methods were used. We have shown that gas diffusion time can be short (below 10 seconds) which proved that AR HCFs may be used effectively for continuous gas sensing at trace levels.
All previous examples of gas sensing inside microstructure fibers were performed in the near-infrared spectral region or near 3.4 µm. Because mid-infrared gives access to strong, fundamental ro-vibrational molecular transitions, it is highly desired to be able to use also longer wavelengths, especially beyond 4 µm. In this paper, we address this issue by demonstrating mid-infrared absorption spectroscopy inside silica-based revolver-type AR HCF. The new fiber design enabled light transmission near 4.538 µm where detection of nitrous oxide (N2O) was performed.
2. Hollow core fiber
The AR HCF used in this work has been fabricated at the Institute of Electronic Materials Technology, using pure silica glass tubes and the stack-and-draw technique (detailed information about this fiber can be found in Ref. [36]). Microstructure of the fiber consists of seven nested capillaries. The structure was numerically optimized for the mid-infrared transmission near 4 µm. In an optimized design the air core diameter was 65 µm, the nested capillaries had diameters of 30 µm and 15 µm and their wall thickness was 0.9 µm. As shown in Fig. 1, numerical simulation predicted that the transmission window of optimized AR HCF will cover a broad spectral range from 3500 nm to 5000 nm.
Fig. 1. Calculated loss spectrum of the nested capillary anti-resonant hollow core fiber used in this work.
The cross-section of the fabricated HCF is shown in Fig. 2 and its dimensions are very close to the numerically optimized design. The fiber has an outer diameter of 162 µm and the air core diameter of 62 µm. The outer and inner capillary diameters are 29 µm and 16 µm and their wall thickness is 1.6 µm and 0.9 µm, respectively. The fiber was protected with an acrylic coating to allow mechanical withstanding of a bending radius even as low as 5 mm.
Fig. 2. A cross-section of the anti-resonant hollow core fiber with nested capillaries obtained with scanning electron microscopy.
The fabricated AR HCF can guide two modes in the considered range above 4 µm and the fundamental mode can be selectively excited. Because the expected transmission window was very broad we have measured the attenuation at two wavelengths. Near 4.0 µm the lowest confinement losses down to 0.5 dB/m have been measured using an optical parametric oscillator source and a Fourier transform infrared spectrum analyzer. Approximately 6 dB/m was measured near 4.54 µm using a quantum cascade laser (QCL) and a mercury-cadmium-tellurite (MCT) detector. In both cases a cut back technique was implemented.
Bending losses were also measured and an attenuation of 0.5 dB/m was obtained for over 3 full turns with a 40 mm radius (more details can be found in [36]). A nested capillary structure was responsible for such a dramatic reduction of the bending losses, which so far were considered as one of the main challenge in mid-infrared silica-based AR HCFs [37,38].
3. Experimental setup
Figure 3(a) shows the schematic of the experimental setup which was used to demonstrate mid-infrared spectroscopy inside a HCF.
Fig. 3. (a) Experimental setup: QCL – quantum cascade laser, PD – photodetector, HCF – hollow core fiber; (b) Output power vs. laser current characteristic of QCL used in this study.
A distributed feed-back (DFB) QCL operating near 4.538 µm (∼2203.7 cm−1) was used as the source (Thorlabs, model QD4500CM1). It was driven with a low-noise current driver (Wavelength Electronics, model QCL500). A Virtual Bench device (from National Instruments) was used to modulate the laser current and acquire the signal from the detector. The QCL characteristic (output power vs. current) is shown in Fig. 3(b). Light from the laser was collimated using an aspheric lens with anti-reflective coating (Thorlabs, model C037TME-E) and an off-axis parabolic mirror (2-inch focal length, gold coated) was used to couple the mid-infrared radiation into 3.2-m-long HCF. The outlet of the fiber was placed inside a custom-made gas-tight housing with a gas tube connector and a translation stage that enabled precise alignment in the X and Y directions. A MCT detector (Thorlabs, model PDAVJ5) was integrated with the housing and the HCF end was placed approximately 1 mm from the detector window (without any additional lenses). A small air-pumped was used to reduce the pressure inside the tight-housing (approximately 50 Torr below ambient) and force gas to flow through the HCF. When the air-pump was switched on the amount of N2O inside the fiber could be increased above an ambient level by releasing N2O close to its inlet.
4. Gas sensing demonstration
Two measurement approaches were implemented in the setup: direct laser absorption spectroscopy (DLAS) and wavelength modulation spectroscopy (WMS). In DLAS the emission wavelength of the laser is swept across molecular transition. The concentration can be subsequently retrieved e.g. by comparing the measured absorption line profile with the spectrum simulated using spectroscopic database such as HITRAN [39]. In WMS [40,41] the emission wavelength is modulated using a sinusoidal signal at fm and the signal from the detector is analyzed in order to retrieve its component at 2×fm (additional ramp may be used to retrieve full 2f WMS spectrum). When trace concentrations are detected the amplitude of the 2f WMS signal at the absorption line center is proportional to the molecular concentration.
The main advantage of DLAS is its simplicity and calibration-free nature. In contrast, retrieving the molecular concentration in WMS is not straightforward and typically requires additional calibration [41]. However, because the 2×fm signal at the transition center is proportional to the number density, WMS is a convenient approach for continuous monitoring of gas, e.g. in gas leak detection.
4.1 Direct laser absorption spectroscopy
Figure 4 shows spectra recorded using DLAS. A saw-tooth 1 kHz signal was used to modulate the laser injection current from 310 mA down to 230 mA. With this current modulation the emission wavelength was changed by approximately 1 cm−1 (it was measured with a germanium etalon with a free spectral range of ∼2.94 GHz placed between the QCL and the detector. The transmission of the etalon is shown in Fig. 3(a)). Sample DLAS spectra (averaged for 100 scans) recorded when HCF was filled with different gas samples are presented in Figs. 3(b) and 3(c) (baseline was removed during fitting process). For both concentrations (∼470 ppbv and 2.0 ppmv were retrieved based on spectral fitting using data from the HITRAN database; fitted spectra are also shown) similar fit residuals are obtained, with standard deviation near the peak of the absorption line below 2×10−3 and with no clearly visible fringe pattern.
Fig. 4. (a) Optical fringes recorded using a Ge etalon (FSR – free spectral range) show ∼1 cm−1 tuning range of the QCL; (b) and (c) direct laser absorption spectra recorded inside the AR HCF (solid line) for two different N2O concentrations and spectra fitted using the HITRAN database (dashed line). Fit residuals are shown below.
4.2 Wavelength modulation spectroscopy
For WMS measurements the laser injection current was modulated at fm = 2.5 kHz and the signal at 2×fm = 5 kHz was retrieved using custom LabVIEW program. A single data point could be acquired within 10 ms, which includes 4 ms of actual signal acquisition and 6 ms of signal processing time. WMS signals were recorded for different wavelength modulation amplitudes in order to find the optimum experimental parameters. Sample spectra are shown in Fig. 5(a) (corresponding DLAS spectrum was shown in Fig. 4(c)) and they are in good agreement with simulation based on data from HITRAN (Fig. 5(b)).
Fig. 5. (a) 2f WMS signals recorded for different wavelength modulation amplitudes and (b) corresponding simulation based on HITRAN database.
As mentioned earlier, the main advantage of WMS is that it allows for continuous concentration monitoring through recording of only the amplitude of 2f WMS signal after the laser wavelength is adjusted to the center of the target transition. This approach (comparing to full spectral scanning) is more prone to drifts but it simplifies data processing and enables more effective signal averaging. Moreover, it can be used to study the performance of the systems, including estimation of the minimum detection limit (MDL) and measurement of the response time of the system.
4.3 Detection limit analysis
In order to characterize the detection limit of the system, the HCF was filled with a gas mixture of N2O and laboratory air. The N2O concentration was estimated to be ∼3 ppmv (based on spectral fitting using the HITRAN database). Subsequently, the laser wavelength was adjusted to the peak of the transition. 2f WMS amplitude time series was acquired at 100 Hz and was used to calculate the detection limit as a function of the averaging time [42]. The Allan-Werle plot is shown in Fig. 6. For an averaging time of 1-second a minimum detection limit (MDL) of ∼5.4 ppbv is obtained (1.2×10−4).
4.4 Response time
The response time in HCF-based sensing systems is typically measured as the time needed to fully fill the fiber with gas. In this work another, equivalent method was used. When the ambient air was flowing through the fiber N2O was released close to the fiber inlet. The response time was measured as the time that N2O which entered the HCF needed to pass through the entire fiber and leave it. Figure 7 shows the recorded signal. Every release time was below 1 second. An additional fan was used to make sure that the nitrous oxide does not accumulate in the free-space section of the setup, i.e. between QCL and HCF. As shown in Fig. 7, anytime N2O was released an immediate increase of 2f WMS amplitude was observed and it typically took ∼23 seconds for the gas sample to reach its other end.
Fig. 7. Left: 2f WMS signal amplitude recorded over ∼5 minutes during which N2O was released three times close to the HCF inlet. Right: data recorded during first release shown in detail. For a 3.2-m-long fiber a response time of ∼23 seconds was observed.
5. Discussion
In this paper we have demonstrated a spectroscopic system capable of detecting nitrous oxide inside an anti-resonant hollow core fiber with a 1-second minimum detection limit of ∼5.4 ppbv (where the typical ambient concentration of N2O is between 300 and 400 ppbv). This detection limit corresponds to a fractional absorption of ∼1.2×10−4 which is comparable to our recent result obtained using a different anti-resonant fiber and a laser diode at 2 µm for carbon dioxide sensing. We believe that further improvements in the short-term sensitivity are still possible. For example, in the WMS mode only 40% of time was used for actual signal acquisition and 60% of time was used for signal processing. This can be improved by using a standard lock-in amplifier for WMS signal retrieval (the current setup uses a Virtual Bench device for signal acquisition and a custom LabVIEW program for signal processing). Switching to a thermoelectrically cooled detector may also reduce the noise and improve the detection sensitivity.
A response time of 23 seconds is far less than typically demonstrated for PBG-type fibers [13–14], but more than we have obtained previously for AR HCFs [18,19,35]. However, in both previous publications much shorter pieces of fiber were used (1.30 m in [18] and 1.35 m in [19] vs. 3.20 m in the results presented in this manuscript). A shorter response time with the presented fiber may be obtained when a higher pressure difference between HCF outlet and inlet is used. Nevertheless, the presented experiment suggests that with longer pieces of fiber (e.g. 10 meters and more) it may be challenging to keep the sensor response time below 30-60 seconds.
6. Conclusions
In conclusion, we have demonstrated mid-infrared laser spectroscopy inside an anti-resonant hollow core fiber. To our best knowledge, this is the first ever experimental demonstration of QCL-based gas sensing inside a microstructure fiber. We have used a 4.53 µm laser to target one of the strongest transition of nitrous oxide near 2203.7 cm−1. The system demonstrated a 1-second minimum detection limit at a single parts-per-billion level using 3.2-m-long fiber with a response time of less than 30 seconds. Most importantly, the hollow core fiber used in this setup has small bending losses which makes the reported sensing approach a promising solution for compact mid-infrared laser spectrometers.
Funding
Narodowe Centrum Nauki (UMO-2016/21/B/ST7/02249); Fundacja na rzecz Nauki Polskiej (First TEAM/2016-1/1).
Acknowledgments
MN and GG acknowledge the Ministry of Science and Higher Education for supporting the Faculty of Fundamental Problems of Technology (funding for restructurization, application number 386399).
Disclosures
The authors declare no conflicts of interest.
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