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Abstract
In this paper, we present a laser-based sensing inside anti-resonant hollow core fiber. A distributed feedback laser diode operating near 2004 nm and a 1.35-m-long silica-based fiber are used to demonstrate carbon dioxide detection with sensitivity down to ~5 ppmv. Gas exchange time as low as 5 seconds is obtained. This performance was achieved in a very simple optical configuration, without any mirrors or lenses in the setup.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-based trace gas detection typically relies on measuring absorption of light that interacts with matter in the vicinity of molecular resonance. The most common way of improving sensitivity of laser-based sensing instrumentation is to increase the length of an interaction path between light wave and gas sample. This is frequently realized using multi pass cells (MPCs) of different types [1–3]. Unfortunately, MPCs are often prone to opto-mechanical drifts which can significantly affect the stability of the instrument [4]. Moreover, in many cases MPC is the largest part of the setup which may prevent reduction of the size of the sensor.
Hollow core fiber (HCF) is an interesting alternative to MPCs. HCF may serve as a cell which can be filled with the gas of interest. Hangauer et al. demonstrated laser spectroscopy setup which was using a long capillary with reflective inner coating [5]. Similar type of fiber was used by Li et al. in [6] for nitrous oxide sensing. Unfortunately, with this type of HCF multimode propagation or scattering on inner surface may lead to the presence of optical fringes which are most likely caused by the interference of higher order spatial modes. This phenomena severely limits the performance of majority of spectroscopic systems. Photonic bandgap (PBG) photonic crystal fibers (PCFs) is another type of HCF which may be used for gas detection. Several examples of their applications can be found in the literature, including the gas detection in the near-infrared [7,8] and the mid-infrared [9] spectral regions. However, PBG fibers have two drawbacks. Strong fringes are usually present in the recorded spectra. As a result, a reasonable performance using PBG HCF as a gas cell were reported only when some sophisticated sensing techniques were used [10–12]. Moreover, the hollow core in PBG fiber is relatively small. This results in very long gas diffusion times, typically more than few minutes even for short pieces of fiber [7,13].
In the last few years some examples of laser-based spectroscopy inside anti-resonant (AR-)HCFs were presented. The advantages of AR-HCFs (comparing to PBG-HCFs) include larger diameter of the hollow core (which potentially results in shorter gas exchange time) and reduced coupling between the cladding modes and the core modes (which possibly could lead to smaller amplitude of the optical fringes) [14–17]. In [18] acetylene-filled Kagome-type HCF was used for frequency-locking of fiber laser operating near 1.53 µm. Similar C2H2-filed HCF was used in [19] for frequency stabilization of an optical frequency comb source. More recently, Curtis et al. demonstrated laser frequency stabilization at 2051 nm using AR-HCF filled with CO2 [20]. In these three papers, the HCF was filled with gas and used as a reference cell. In gas sensing application usually gas needs to be continuously pushed through the fiber. This was recently demonstrated in [21] where methane sensing at 3.33 µm inside the Kagome-type HCF and using absorption spectroscopy was described. A minimum detection limit of few parts-per-million (ppm) that corresponds to fractional absorption of ~3 × 10−3 was obtained. It was demonstrated that thanks to large core diameter AR-HCF can be filled with sample gas relatively fast (in less than 10 seconds). Moreover, some AR-HCFs designs enable light transmission in the mid-infrared (where strong molecular transitions are located) despite high material loss of silica glass [22,23]. State-of-the-art AR-HCF fabrication technology enables hyperspectral transmission windows with ultra-low attenuation down to 10-20 dB/km, which further strengthens their advantages in physical systems [24], and specifically make AR-HCFs well-suited for real-time chemical sensing.
In this paper we demonstrate laser absorption spectroscopy at 2 µm inside 1.35-m-long revolver AR HCF. This spectral region enables accessing relatively strong transitions of carbon dioxide (CO2) from the R branch of its 2ν1 + ν3 band [25,26]. Using extremely simple optical configuration (without any mirrors or lenses) and integration time of 3 seconds a minimum detection limit of single ppmv of CO2 was obtained. Moreover, the gas exchange time of only ~5 seconds was demonstrated. This performance is much better than presented very recently in [27] where using PBG HCF and similar wavelength region a detection limit of 2% of CO2 and a gas exchange time of 10 minutes were obtained. It also outperforms previously demonstrated system based on Kagome-type AR HCF operating near 3.3 µm [21].
2. Experimental setup
Figure 1 shows the layout of the experimental setup. Discrete Mode (DM) laser diode (from Eblana Photonics) operating near 2004 nm was used as a source. To couple light from a laser diode into HCF both fibers were cleaved using standard fiber cleaver (Fitel, model S326). Both fibers were aligned under the microscope using manual 3-axis translation stage. The gap between two fiber ends was set to approximately 0.25 mm. We typically obtained coupling efficiency (from standard single-mode fiber into HCF) between 50 and 55%. The second end of HCF was also cleaved and placed inside a tight housing, approximately 1 mm from the window of the thermoelectrically cooled InGaAs photodetector (Thorlabs, model PDA10DT-EC). With approximately 2.5 mW emitted from the laser diode and with 20 dB gain setting of the detector (responsivity of ~19.5 kV/W) the output signal up to few volts could be obtained, even without focusing the light on the photodiode. As a result, no additional optical components (mirrors or lenses) were used in the setup. Using output of a small pump we were able to fill HCF with a gas sample by producing small (~100 Torr) overpressure inside tight housing.
Fig. 1 The schematic diagram of the experimental setup. Output fiber from laser diode and HCF were aligned using 3-axis translation stage. The tight housing had volume of approximately 20 ml. HCF end was placed in holder that enabled translation in two axis (XY) in order to align the fiber end and a detector. A laser driver used in these experiments was LDTC0520 from Wavelength Electronics. A Virtual Bench device from National Instruments was used as a function generator and a scope, both controlled using PC with LabVIEW-based software..
Virtual Bench device (from National Instruments) was used to control laser diode (bias current + modulation) and acquire signal from photodetector. It enabled performing both tunable diode absorption spectroscopy (TDLAS) in which laser current is modulated with a ramp signal and direct absorption spectrum is recorded directly, and wavelength modulation spectroscopy (WMS) [28,29] when additional sinusoidal modulation at fm is used and molecular concentration is retrieved from the second harmonic component of the photodetector signal (at 2 × fm).
3. Anti-resonant hollow core fiber
The AR-HCF developed for this work has been drawn using pure silica glass tubes (Suprasil F300, Heraeus). The fiber preform was composed of 7 capillaries with outer diameter of 2.80 mm and thickness of 0.25 mm, distributed uniformly inside a large capillary with internal diameter of 14.7 mm and a wall thickness of 2.6 mm (Fig. 2). After a two-step fiber drawing process, we achieved a fiber structure with an air core diameter of 70 μm. Inner capillaries had an outer diameter of ~21 μm and their wall thickness was 2.4 μm. The total diameter of the fiber was 160 μm.
Fig. 2 Scanning electron microscopy images of the AR-HCF with 7 inner capillaries made of silica glass.
Transmission spectrum of the fabricated AR-HCF has been measured in the near-infrared range (1200 nm to 2400 nm) using a supercontinuum source and an optical spectrum analyzer. We have identified two transmission windows spanning from 1350 nm to 1600 nm and from 1790 nm to 2220 nm (shown in Fig. 3). The transmission losses (measured using the cut back technique) were 6 dB/m across the window near 1550 nm and 2 dB/m across the window around 2000 nm.
Fig. 3 Optical spectrum measured using supercontinuum source shows two transmission windows of the fiber.
4. Experimental results
4.1 Line selection
Figure 4(a) shows spectrum of carbon dioxide (concentration of 400 ppmv and path length of 135 cm was assumed) near 2004 nm simulated using HITRAN database [30]. By changing the laser temperature we could access several of these transitions, as shown in Fig. 4(b). The absorption line near 4991.2 cm−1 has the smallest interference with water vapor (simulated spectrum is also shown in Fig. 4(a)) thus it was used in all further experiments.
Fig. 4 (a) absorption lines of carbon dioxide and water vapor simulated using HITRAN database; (b) measured 2f WMS spectra for different laser temperatures. Dashed line – signal from 10-cm-long gas cell filled with 1% of CO2, solid line – signal from HCF filled with ambient air.
WMS spectra shown in Fig. 4(b) were recorded by applying modulation of the laser current at fm = 2.5 kHz and with subsequent signal demodulation at 2 × fm. Two spectra are presented for each laser temperature. One was recorded using 10-cm-long gas cell filled with ~1% of CO2 at atmospheric pressure Another was measured using HCF-based cell filled with ambient air.
4.2 Detection limit and gas exchange time
Two 2f WMS spectra are shown in Fig. 5(a): red one (magnified 20 times) was recorded when HCF was filed with ambient air, black trace was measured after gas containing high CO2 concentration was pushed into the fiber. Corresponding direct absorption spectra are shown in Fig. 5(b). For ambient air measurement absorption line is hardly noticeable, whereas for sample with high CO2 content it is clearly visible. This direct absorption scan was used to estimate the amount of CO2 inside the fiber to be approximately 1.5%.
Fig. 5 (a) 2f WMS signals recorded when HCF was filled with ambient air (solid line, magnified 20 times) and ~1.5% of CO2 (dashed line); (b) corresponding direct absorption spectra (for ambient air measurement absorption line is hardly visible).
In WMS, molecular concentration can be monitored by setting the laser wavelength to the center of the absorption line and recording the amplitude of the 2f WMS signal. Figure 6(a) shows the signal recorded when HCF was being flushed with ambient air or with gas mixture containing ~1.5% of CO2. Part of recorded data when HCF was filled with ambient air (from the beginning of the measurement up to ~800 seconds) was used to perform the Allan deviation analysis. An Allan-Werle plot is shown in Fig. 6(b) (ambient CO2 concentration of 400 ppmv was assumed). For the integration time of 3 seconds a minimum detection limit of 5 ppmv was obtained (it was calculated based on the variance of the mean value that was predicted from the Allan variance analysis, as described by Werle et al. in [31]). This corresponds to the minimum detectable fractional absorption (MDFA) of 1 × 10−4 (based on HITRAN database). With longer averaging times Allan-Werle plot stays flat. We believe that no further improvement is primarily due to presence of optical fringes and due to drift of laser wavelength (no active line-locking was used). Nevertheless, to the best of our knowledge, MDFA of 1 × 10−4 is better than demonstrated so far for any other sensing system that uses hollow core fiber combined with simple absorption spectroscopy technique (comparison is shown in Table 1). Moreover, AR-HCF used in this study enabled very fast gas exchange, as shown in Fig. 6(a) and in details in Fig. 6(c) and Fig. 6(d). The gas filling time was found to be approximately 5 seconds.
Fig. 6 (a) 2f WMS signal amplitude recorded over ~15.5 minutes. During first ~14 minutes ambient air was pushed through HCF. This time series was used to create Allan-Werle plot shown in (b) (we assumed ambient CO2 concentration of 400 ppmv); (c-d) when gas mixture was switched between ~1.5% of CO2 and ambient air gas exchange times of ~5 seconds were observed.
Table 1. Comparison between results obtained in this work and report in the literature for different wavelengths and HCF types (anti-resonant and photonic band-gap)
4.3 Optical fringes
Performance of most laser spectrometers is affected by optical fringes [31,32]. In standard instruments these unwanted effects can be limited using good anti-reflective coatings and by avoiding parallel surfaced within the setup. When HCF is used for spectroscopy the fringes are also generated due to presence of higher order modes which propagate inside the fiber. This effect is particularly significant in PBG-HCFs. Attenuation of higher order modes in AR-HCFs is stronger thus smaller amplitudes of optical fringes may be expected. However, as shown in Fig. 7, this significantly depends on coupling conditions into the HCF. Figure 7 shows two 2f WMS spectra (solid line and long-dashed line) recorded within one minute. Between the two measurements we have slightly realigned single-mode fiber from the laser diode. Also shown (with short-dashed line) is the 2f WMS signal obtained using conventional gas cell containing 1% of CO2. After normalizing its amplitude we subtracted this signal from two spectra measured using HCF. As a result, it becomes clear that one spectral scan (long-dashed line) contains fringes with much higher amplitude comparing to the other (solid line). Interestingly, in both cases the total power reaching the detector was very similar. Therefore, it must be concluded that aligning the system to maximize both the coupling efficiency and power recorded by the detector does not guarantee that reduction of optical fringes will be obtained. It is also worth noticing that typically, when the setup was left for one or two days, the optical fringes similar to shown in Fig. 7 with long-dashed line were present. After small realignment these unwanted features could be easily removed. Spectrum similar to one shown with solid line was obtained and the setup would have remained stable (i.e. with no visible changes in the fringe amplitude or position) typically for few hours.
Fig. 7 Top: two 2f WMS spectra (solid line and long-dashed line) recorded for different coupling conditions between single-mode fiber from laser diode and HCF. Also plotted (short-dashed line) is a ‘reference measurement’ which shows spectrum recorded using conventional gas cell. Bottom: optical fringes visible after subtracting reference measurement from two spectra acquired using HCF.
5. Conclusions
In this paper, a 2 µm laser-based spectrometer using anti-resonant hollow core fiber was presented. A minimum detection limit of ~5 ppmv of CO2 was estimated based on Allan-Werle analysis for the integration time of 3 seconds (this corresponds to the minimum detectable fractional absorption of 1 × 10−4). A gas exchange time of only 5 seconds was obtained. This was achieved using 1.35-m-long fiber and a very simple optical configuration (without any mirrors or lenses in the setup).
Table 1 summarizes the obtained performance and compares it with similar configurations reported in the literature. When MDFA × HCF length is considered as a figure of merit, presented setup outperforms previously demonstrated configuration by more than order of magnitude. This is primarily due to combination of the use of the single-mode revolver-type AR HCF with large core diameter and the simplicity of the setup. Comparable gas exchange time was obtained only in [21] where also Anti-Resonant HCF was used as well (Kagome-type).
Despite significant improvements optical fringes are still visible in the presented system and ultimately they will affect its long-term performance and accuracy. Nevertheless, amplitude of fringes is much lower that demonstrated previously in the setups based not only on PBG-type HCFs [7,9,13,27] but also on Kagome-type HCFs [21]. More importantly, thanks to relatively low losses of Revolver-type AR-HCFs, presented performance gives perspective for simple and highly compact fiber-based chemical sensors with long optical path lengths and sensitivity comparable to conventional laser-based spectrometers based on multi-pass cells [26].
Funding
Narodowe Centrum Nauki, NCN (OPUS grant, UMO-2016/21/B/ST7/02249); Foundation for Polish Science (First TEAM programme co-financed by the European Union under the European Regional Development Fund, First TEAM/2016-1/1). Foundation for Polish Science (FNP) under START program—scholarship of K. Markowski.
Acknowledgments
The authors are grateful to Jaroslaw Sotor from Wroclaw University of Science and Technology for lending the laser diode used in this work. 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).
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