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Abstract
Antiresonant hollow-core fiber (AR-HCF) exhibits unprecedented optical performance in low transmission attenuation, broad transmission bandwidth, and single spatial mode quality. However, due to its lower numerical aperture, when utilizing the Fiber-Enhanced Raman Spectroscopy (FERS) principle for gas detection, the efficiency of AR-HCF in collecting Raman signals per unit length is significantly lower than that of hollow-core photonic crystal fiber. Nonetheless, AR-HCF effectively suppresses higher-order modes and offers bandwidth in hundreds of nanometers. By increasing the length of AR-HCF, its advantages can be effectively harnessed, leading to a considerable enhancement in the system's ability for low-concentration gas detection. We combine the nodeless antiresonant hollow-core fiber and Raman spectroscopy for enhanced Raman gas sensing in a forward scattering measurement configuration to investigate the attenuation behavior of the silica background signals. The silica background attenuation behavior enables the low baseline of the gas Raman spectroscopy and extends the integration time of the system. In addition, a convenient spatial filtering method is investigated. A multimode fiber with a suitable core diameter was employed to transmit the signal so that the fiber end face plays the role of pinhole, thus filtering the silica signal and reducing the baseline. The natural isotopes 12C16O2, 13C16O2, and 12C18O16O in ambient air can be observed using a 5-meter-long AR-HCF at 1 bar with a laser output power of 1.8 W and an integration time of 300 seconds. Limits of detection have been determined to be 0.5 ppm for 13C16O2 and 1.2 ppm for 12C16O2, which shows that the FERS with AR-HCF has remarkable potential for isotopes and multigas sensing.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Raman spectroscopy as a “fingerprint” for specific molecules [1] can identify vibrational and rotational modes of each molecule, shows excellent capabilities as an analytical tool and bears great potential for multigas sensing in the fields of human breath analysis (H2, CH4, N2, O2, CO2) [2–4], environmental sensing of climate-relevant gases [5], industrial gas concentration detection (CH4, C2H6, C3H8, CO2, H2S) [6,7]. However, weak Raman scattering efficiency and low signal intensity in the traditional free-space experimental setup limit the detection capability. Hollow-core optical fiber [8] with a unique cross-sectional structure that has excellent optical properties, such as low dispersion, low nonlinearity [9], and low time delay, has been provided to be the ideal place for light-analyte interaction. Its prolonged path length has greatly increased the yield of gas Raman signals. Fiber-enhanced Raman spectroscopy (FERS) [10], which combines the analysis prowess of Raman spectroscopy with the excellent optical properties of hollow core optical fiber, is a powerful and promising analytical technique for many applications.
Metal-coated capillary (MCC) [11] and hollow-core fibers are the main research objects in fiber-enhanced Raman sensing. For MCC [12], the inner diameters usually range from 300 µm to 1 mm. Large inner diameters allow quick gas exchange and the propagation of multimodes. However, its substantial losses and worse optical properties due to the multiple reflections and the significant fluorescence background limit its detection capability [13,14]. A cap with a central aperture of 0.8 mm was used to cover the capillary wall face and block the strong fluorescence at the edges of the capillary. The limits of detection (LOD) for trace gases are less than 100 ppm level [15].
Two main types are commonly mentioned for hollow-core fiber: hollow-core photonic crystal fiber (HC-PCF) and antiresonant hollow-core fiber (AR-HCF). Although the small core diameter hampers the quick exchange of gases, the low transmission attenuation and large numerical aperture characteristics of HC-PCF make it perfectly suitable for Raman signal generation and collection [4, 16, 17]. However, the persistent silica signal remains challenging in utilizing HC-PCF for long-term, low-concentration, multi-component gas detection. Digital spatial filtering and pinhole spatial filtering [3,18,19] are investigated to remove the silica Raman signal and further reduce the LOD of gases. S. Hanf et al. achieved the methane detection limit as low as 0.2 ppm in 2014, with an additional pinhole for spatial filtering on HC-PCF [2].
In recent years, as the understanding of antiresonant reflection in optical conduction mechanism deepens [20,21], AR-HCFs with better optical performance, such as very low dispersion, high damage threshold, and ultra-low nonlinear response [22] have been continuously optimized and applied to more fields [5]. The thin glass walls surrounding the hollow core in an AR-HCF function as Fabry–Perot resonators, resonating with the unwanted higher-order modes and not interfering with the fundamental mode [23]. Only the fundamental mode is confined by the cladding. Compared with HC-PCF, AR-HCF has advantages in gas Raman sensing for its unprecedented low surface scattering loss [24], less silica Raman signal interference due to the less optical overlap with silica core surrounding, and broader light transmission range. Meanwhile, the large aperture of the AR-HCF makes the coupling efficiency of the laser higher and the gas diffusion faster. These advantages show that AR-HCF possesses excellent potential in terms of fiber-enhanced Raman spectroscopy. In 2019, Knebl A. et al. [25] used AR-HCF to measure Carbon dioxide and oxygen isotopes at preset concentrations and achieve the LOD for CO2 at 25 ppm at 30 s integration time and with the input power of 1.3 W. They point out that only a small fraction of optical power is guided in the glass structures of the fiber and produces negligible levels of silica Raman signals, thus no extra spatial filtering is necessary, and this portable FERS system can be used for monitoring of multi-component gas [26],which shows its potential for isotope gas detection and dynamic gas analysis. Fu Wan et al. conducted extensive research in the field of low-concentration gas detection using AR-HCF. By employing fibers of 0.27 m and 2 m in length, they achieved detection limits of 83.9 ppm and 9.4 ppm for CO2 gas, respectively in 2021 and 2023 [27,28]. However, AR-HCF still has its limitations. In comparison to HC-PCF, it possesses a smaller numerical aperture (NA), resulting in a lower Raman signal collection efficiency per unit length of AR-HCF than HC-PCF. Therefore, in the case of shorter fiber lengths, HC-PCF has an advantage over AR-HCF in terms of detection limits. The lower loss of the AR-HCF and the suppression of silica signals allow it to achieve lower detection limits consistently over longer lengths.
In this paper, We explored the advantages of AR-HCF in increasing length. A series of experiments were conducted using a nodeless AR-HCF [29] with a maximum length of 5 meters. A calculation for determining the maximum effective length of the AR-HCF for multi-component gas detection is proposed. The Raman signal intensity was quantitatively analyzed as a function of the AR-HCF length. Furthermore, the attenuation of silica Raman signals along the fiber was demonstrated. A decay curve was fitted to illustrate the attenuation of undesired intrinsic silica Raman signals, suggesting that longer fibers are required for silica signal suppression. In terms of the received signal, a convenient method was investigated to replace the traditional pinhole filtering. A multimode fiber with a 10 µm core diameter plays the role of a pinhole, effectively filtering out the silica signal and reducing the baseline. Finally, based on a 5-meter AR-HCF, FERS was demonstrated for CO2 detection with a concentration of approximately 400 ppm in air. The LODs for both isotopes were determined, which were 0.5 ppm and 1.2 ppm for 13C16O2 and 12C16O2, respectively.
2. Experimental setup
The optical setup of the FERS gas sensing system is shown in Fig. 1, which is in a forward Raman scattering measurement configuration since forward Raman scattering has higher signal yields than backward under the same conditions [4,30]. The laser is a 532 nm All-Solid-State laser (CNI: MLL-F-532, 0 to 1.8 W CW output). The bandpass filter F1 (Semrock: LL01-532-12.5) removes all stray light except the 532 nm laser. Then, the 532 nm laser is coupled into the AR-HCF (26 µm core diameter, attenuation: 80 dB/km@532 nm, band width:440 to 1200 nm, the cross-section schematic is shown in Fig. 1 [29] with the aspheric lenses L1 (Thorlabs: AL2550-A, f = 50 mm). The forward Raman signals are collimated by the achromatic lens L2 (Thorlabs:AC254-50-A-ML, f = 50 mm), and the mirrors (M1, M2) deflect and direct the light beam. The laser and Rayleigh scattering are suppressed by long-pass laser filter F2 (Semrock:LP03-532RE-25) before the achromatic lens L3 (Thorlabs:AC127-025-A-ML,f = 25 mm) focuses the Raman signal into the multimode fiber MF (105 µm core diameter, NA = 0.1). One end of the MF is fixed in a fiber connector on a spectrometer from Princeton Instrument (HRS-300) equipped with deep cooling CCD (PIXIS-100BX). At a slit size of 20 µm, the resolution of the grating with 600 lines/mm and 1800 lines/mm are 8.72 cm-1 and 1.93 cm-1 at 532 nm, respectively. In the experiment, the fibers are placed in a straight line as much as possible, ensuring that the radius of curvature is larger than 50 cm.
Fig. 1. Experimental setups for FERS. The filter (F1) filters out stray light other than 532 nm. The Raman signals were collected by the multimode fibers (105 µm core diameter), which connect to the slit plane of the spectrometer (Princeton Instrument HRS-300). A long-pass filter (F2) deflected the remaining 532 nm laser and Rayleigh-scattered light. Inset: The 3D model and the microscope image of the AR-HCF cross-section. The cladding with an outer diameter of 200 µm consists of six tubes with a thickness of 210 nm, a tube diameter of 12 µm, an inter-tube gap distance of 7 µm, and a center hollow core diameter of 26 µm. All tubes are attached to the inner wall of the cladding, and the entire core is filled with air including the space between and within the silica tubes.
3. Results and discussion
Figure 2(a) shows the Raman spectra of ambient nitrogen (2331 cm-1) and oxygen (1556 cm-1) with 1 m, 2.5 m, and 5 m fiber length (integration time: 1 s, laser power: 400 mW). The length dependence of N2 and O2 Raman signals is illustrated in Fig. 2(b). The intensities of the Raman signals increase linearly to the fiber length in the range of 1∼5 m.
Fig. 2. (a) Raman Spectrum of air with the fiber length of 1 m, 2.5 m, 5 m (integration time:1s, the input laser power: 400 mW). (b)The length-dependent N2 and O2 Raman signals. Inset: the calculated length-dependent gas signal intensity.
The intensity of the forward Raman scattering is decided by attenuation at the wavelengths of the laser and the Raman signals, as the Raman signals are simultaneously excited and attenuated along the hollow core fiber [31,32]. The effective length of the fiber is a metric that indicates the effect of gain over the actual length of the fiber, which can be derived from the fiber's physical length and the attenuation, based on eq.1, where ${{\boldsymbol \alpha }_{\boldsymbol l}}$ represents the fiber attenuation at the laser wavelength (532 nm), ${{\boldsymbol \alpha }_{\boldsymbol g}}$ represents attenuation at the nitrogen gas Raman peak (607 nm), Lf is the AR-HCF's physical length, and Leff is the effective length. [18]
The attenuation of the laser excitation (532 nm) is 0.08 dB/m, and the attenuations at the wavelength of the Raman peak of nitrogen (607 nm or 2331 cm-1 shift from 532 nm) are 0.1 db/m[29]. The inset in Fig. 2(b) shows the relationship between the physical length and effective length of the AR-HCF, which can be obtained by substituting the attenuation of the fiber into Eq. (1). As shown in the figure, the optimal length of the AR-HCF is 11 m, at which the effective length reaches the maximum of 4.1 m.
For FERS, the intensities of the Raman gas signals depend on gas concentration, integration time, and laser power. The evident linearities between Raman intensity of O2 and N2 signals and laser power and integration time are shown in Fig. 3.
Fig. 3. The intensities of the fiber-enhanced Raman signals of ambient nitrogen (2331 cm-1) and oxygen (1556 cm-1) with increasing laser power (a) and integration time (b).
Silica Raman signals in the range from 700 cm-1 to 1100cm-1 affect the detection of the target gas with characteristic peaks at the range such as the rotational mode S0(1), S0(2), S0(3) of H2 at the Raman shift of 587 cm-1, 814 cm-1, 1034 cm-1 [16]. When the laser propagates through the AR-HCF, the presence of higher-order modes causes the laser's modal field to overlap with the microstructure within the HCF. This results in a continuous generation of silica signals along the length of the fiber. The ability of AR-HCF to suppress higher-order modes ensures that the signal only comes from the fiber end face during laser-fiber coupling. Figure 4 (a) illustrates the Silica Raman signals at 1 m (integration time 20 s, the input laser power is 400 mW). As higher-order modes are suppressed along the fiber length when it increases from 1 meter to 5 meters, the silica Raman signals generated from the end-face will be attenuated rapidly along the AR-HCF cladding. According to Fig. 4(b), showing the relationship between the silica Raman signal and the length of the fiber, the silica Raman signals decay in an exponential trend of 0.5 db/m. As the fiber length increases, the silica Raman signals will completely attenuate in the silica cladding. Based on the results above, AR-HCF exhibits a self-filtering effect along its length for silica Raman signals. Figure 4(c) shows the Raman spectroscopy ranging from 700 cm-1 to 1200 cm-1 with integration time of 1s. Within this range, both silica Raman signals and baseline heights exhibit a noticeable decreasing trend with the increase in optical fiber length.
Fig. 4. (a) Silica Raman signals at 1 m without Spatial filtering (integration time: 20 s, input laser power: 400 mW) (b) the attenuation curve of the silica signals (c) Silica Raman signals at different lengths of 1 m, 2.5 m, 5 m (integration time:1 s, the input laser power: 400 mW)
In addition to the self-filtering effect of AR-HCF, pinhole filtering is further introduced into the system to reduce noise and silica Raman signal when the fiber length is not long enough to eliminate the effects of silica. Given that the silica Raman signal distributes in the fiber cladding, a routine strategy is to use pinholes to filter the silica signal in the fiber cladding while allowing the gas signal in the core to pass through. Multimode fibers with a core diameter of 10 µm were chosen for transmitting the Raman signal to the spectrometer. This approach allows for a simplified optical path and is more mechanically robust than using a pinhole. Based on the ratio of focal lengths of lenses L2 and L3 in Fig. 1, the Raman signals emitted from AR-HCF, after passing through lenses L2 and L3, generate an image on the end-face of multifiber with a diameter of approximately 13 µm. Unlike the regular multimode fiber that receives all Raman signals, the 10 µm core diameter fiber acts like a pinhole, effectively filtering out the silica Raman signals. The schematic is depicted in Fig. 5(a). Figures 5(b) and 5(c) represent the Raman spectra obtained using multimode fiber with 105 µm core and multimode fiber with 10µm core diameter, respectively, using 400 mW laser power and 5 seconds integration time. As shown in Fig. 5(b), the Raman spectrum obtained using fiber with 105 µm fiber core exhibits a prominent signal from Silica in the 600–1000 cm-1 range. However, the silica signal is effectively filtered out by using a fiber with a 10 µm fiber core. It can be concluded that receiving the Raman signals of gases using multimode fiber with a 10 µm fiber core will contribute to extracting the pure gas signal, excluding interference of the fiber microstructure on the gas signals.
Fig. 5. Spatial filtering in FERS: (a) Schematic diagram of spatial filtering. (b) Silica Raman signals without spatial filtering (integration time:5 s, the input laser power is 400 mW) (c) Silica Raman signals are nearly eliminated after spatial filtering (integration time: 5 s, input laser power: 400 mW)
To test the system's detection capability, we employ a 5-meter-long optical fiber, and the detection object is carbon dioxide in the air. With an integration time of 5 s, ambient CO2 (∼0.04%) Raman shifts of 1285 cm-1 and 1388 cm-1 that corresponds to the two Fermi dyad of the major natural isotope 12C16O2 can be easily detected, as shown in Fig. 6(a). When the integration time is up to 300 s, as shown in Fig. 6(b), the other much weaker Raman bands of CO2 could be detected, demonstrating the high sensitivity of our fiber-enhanced Raman spectroscopy system. The isotopes 12C16O2, 13C16O2, and 12C18O16O can be observed in the ambient CO2 Raman spectroscopy. The respective peaks for 13C16O2 at 1370 cm-1 and 1267 cm-1 (overlaid with a hot band of 12C16O2 at 1266cm-1), the hot bands of 13C16O2 show up at 1410 cm-1 and the hot bands of 13C16O2 at 1389cm-1(dominantly overlaid by the 1388 cm-1 peaks of 12C16O2) are marked in Fig. 6(b), which demonstrates the ability of our system to discriminate between different concentrations of multi-component gases for isotopes.
Fig. 6. (a) Fiber-enhanced Raman spectrum of ambient CO2 at 5 m fiber length (integration time: 5 s, the input laser power is 400 mW). (b) The Raman spectrum of ambient CO2 Raman spectrum (integration time: 300 s, the input laser power is 1.8 W).
The natural relative concentrations of the individual isotopes are 98.89% for 12C and 1.11% for 13C [33]. The υ+[13] and υ+[12] peaks are selected as indicators for calculating the LOD of 13C16O2 and 12C16O2, respectively. LOD is the gas concentration at which the Raman signal intensity is twice the intensity of the baseline noise. The spectral data underwent five rounds of averaging, resulting in a baseline noise of 88, while the intensities of υ+[13] and υ+[12] are 1488 and 57757, respectively. Consequently, the signal-to-noise ratios (SNR) are 16.85 and 654.10. The concentration of CO2 in the air is about 400 ppm. Based on the isotopic concentration ratios, 13C16O2 and 12C16O2 are approximately 4 ppm and 396 ppm, respectively. Considering the previously obtained SNR, the calculated LOD for 13C16O2 is 0.5 ppm, and the LOD for 12C16O2 is 1.20 ppm, achieved with an integration time of 300s and a laser power of 1.8 W. Different LODs for the isotopes represent the different Raman activity. The reciprocal ratio between 13C16O2 and 12C16O2 can be considered a representation of their Raman activity ratio. In other words, the Raman activity of 13C16O2 is 2.4 times that of 12C16O2, which is consistent with the previous research [34].
4. Conclusion
In this paper, the enhanced Raman spectra of AR-HCF of different lengths were quantitatively characterized using the Raman signals of nitrogen and oxygen in the air as reference. The results show that the Raman signal yield has a linear relationship with the fiber length. Based on the reported fiber loss, the optimal length of AR-HCF in the system can be extrapolated to be 11 m, indicating that the performance of FERS can be further improved. We also demonstrate the suppression capacity of the AR-HCF for unwanted silica background at different lengths by fitting the decay curve of the silica Raman signal, and it shows that a longer fiber length is needed to suppress the silica signal completely. In addition, to solve the problem that the short-length AR-HCF silicon Raman signal is not completely attenuated, a small core diameter multimode fiber acts as a pinhole to filter the signal, thus removing the silica Raman and lowering the baseline, allowing the system to achieve better multi-component gas sensing performance with longer integration times. The high sensitivity of the enhanced gas sensing system using a 5-meter-long AR-HCF was able to study several weakly vibrating Raman bands with a LOD of 0.5 ppm for 13C16O2 and 1.2 ppm for 12C16O2, indicating that AR-HCF has significant potential for on-line monitoring of multiple gases from trace to high percentage ranges. Based on the pursuit of AR-HCF with lower losses, we expect AR-HCF-enhanced Raman systems with longer optimal lengths and lower LODs to emerge in the future.
Funding
National Key Research and Development Program of China (2022YFB2404300); National Natural Science Foundation of China (62025505).
Disclosures
The authors declare no confilicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon request.
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