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Recent Advancement of Anti-Resonant Hollow-Cadre Fibers for Sensing Applications

past ¹ , ^1,* , ^two , ⁱⁱⁱ , ³ , ⁴ , ^five , ^vi and ^7,*

¹

Hubei Primal Laboratory of Intelligent Wireless Communications, Hubei Engineering Research Center of Intelligent Internet of Things Technology, College of Electronics and Information Engineering, South-Central University for Nationalities, Wuhan 430074, China

^two

Key Laboratory of Optoelectronic Technology & Systems (Ministry of Didactics), Chongqing University, Chongqing 400044, China

³

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, People's republic of china

^four

Found for Infocomm Research, i Fusionopolis Way, #21-01 Connexis, Singapore 138632, Singapore

⁵

CNRS, University of Bordeaux, Bordeaux INP, ICMCB, UMR 5026, F-33600 Pessac, France

^six

Section of Electrical and Electronic Engineering, College of Engineering, Southern University of Science and Technology, Shenzhen 518055, China

⁷

School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

^*

Authors to whom correspondence should exist addressed.

Academic Editor: Carlos Marques

Received: 21 March 2021 / Revised: 14 Apr 2021 / Accepted: 15 April 2021 / Published: 19 April 2021

Abstract

Specialty fibers take enabled a wide range of sensing applications. Particularly, with the recent advocacy of anti-resonant effects, specialty fibers with hollow structures offering a unique sensing platform to achieve highly authentic and ultra-compact cobweb optic sensors with large measurement ranges. This review presents an overview of recent progress in anti-resonant hollow-core fibers for sensing applications. Both regular and irregular-shaped fibers and their performance in various sensing scenarios are summarized. Finally, the challenges and possible solutions are briefly presented with some perspectives toward the future development of anti-resonant hollow-core fibers for advanced sensing.

1. Introduction

Optical fiber, as a transmission medium, has enabled the world to move into a rapid and high-capacity advice age. Particularly, ultra-long altitude transmission has been made possible by ultralow propagation loss (~0.17 dB/km) of silica fiber [one]. Still, faster speed, larger capacity, and longer distance have get inevitable trends of advice requirements, particularly concerning 5G and 6G. Thus, a promising fiber is urgently needed to rival or exceed the conventional single-mode cobweb (SMF) in transmission operation. Fortunately, anti-resonant hollow-cadre fibers (AR-HCFs) have the potential to reach the aforementioned advice goals, due to their lowest attenuation, optical nonlinearity, and chromatic dispersion over a broad bandwidth [2]. Thanks to breakthroughs at the specialty fiber manufacturing level over the past few years, AR-HCFs take gradually become a enquiry hotspot. However, the manual loss of AR-HCFs over long distances (>fifty km) has not been addressed and this limits applications in novel optical cobweb communication systems. Therefore, most of the reported research work concerning specialty fibers with hollow structures is concentrated on transmission loss reduction.

Recently, AR-HCFs with variously irregular cladding structures have been designed and made to reduce transmission loss, and the hollow-cadre nested anti-resonant nodeless fiber (HC-NANF) is the nigh mutual [3]. Figure 1 shows the gradually decreasing manual loss of AR-HCFs in the past five years. In 2015, Walter Belardi from the University of Bathroom demonstrated the attenuation of hollow-core anti-resonant fiber (HC-ARF) of 175 dB/km at 480 nm [iv]. The twelvemonth after, a hollow-core revolver fiber with a double-capillary cogitating cladding was reported by A. F. Kosolapov et al., which reduced the manual loss to 75 dB/km at 1850 nm [v]. In 2018, Wang et al. proposed a hollow-core conjoined-tube negative-curvature cobweb with a low transmission loss of 2 dB/km at 1512 nm and a <16 dB/km bandwidth spanning across the O, E, S, C, L telecom bands (1302–1637 nm) [half-dozen]. After, based on HC-NANF, Thomas D. Bradley et al. achieved lower transmission losses of 1.iii  dB/km [vii] and 0.65 dB/km [8] respective to the transmission altitude of 0.five km and i.2 km, respectively. Quickly, they reported that using HC-NANF reached the currently lowest attenuation of 0.28 dB/km over the C and L bands [9]. The manual loss of HC-NANF is approaching the conventional SMF, which is located in the aforementioned guild of magnitude. Additionally, the nonlinearities of AR-HCFs are 3~4 orders of magnitude lower than SMF, resulting in ultralow dispersion. On the basis of ultralow transmission loss and chromatic dispersion, Lumenisity limited experimentally demonstrated that HC-NANF realized 10 Gbit dumbo wavelength segmentation multiplexing (DWDM) manual over x km links [x]. Nevertheless, AR-HCF-based novel fiber devices still have a long way to overcome the difficulties of ultra-long transmission altitude. Moreover, AR-HCFs as well face limitations of volume production and training engineering science. Though AR-HCFs accept unsolved issues in fiber communications, they accept shone in optical fiber sensing applications owing to their unique anti-resonant consequence and inline hollow-core platform.

This article illustrates a comprehensive review of the recent advancement of AR-HCFs for sensing applications. In the past five years, AR-HCFs take been widely employed in the sensing of various physical parameters, including temperature, strain, curvature, displacement, liquid level, and mechanical force, etc. All these measurands fully exploit the intrinsic advantages of anti-resonant effects that can generate specific resonant wavelengths over a broadband window. The sensitivity is and then acquired by monitoring the shift or intensity fluctuation of the specific resonant wavelength. Additionally, AR-HCF is also a promising candidate for optofluidic applications because the hollow structure tin can offer a platform for fluid–light interaction. Consequently, AR-HCFs have attracted more than attention for crucial applications in biomedical and biochemical fields, such as the detection of antibiotics, bisphenol, nitrous oxide, methyl hydride, and acetylene. Particularly, multiple gas detection will accept significant potential applications in the fields of petrochemical and environmental protection, since AR-HCFs tin can act as a cell for gas–light interaction over extended lengths. In view of the aforementioned superiorities, AR-HCFs could pave the way for highly accurate, ultracompact, and practical sensing applications. In this text, we present the state-of-the-art of AR-HCF-based sensing applications. Firstly, we innovate the basic principle and some typical structures of AR-HCFs. Department 3 then focuses on recent advancements in sensing applications. Finally, nosotros provide an outlook of evolution prospects and our related insights and viewpoints.

2. Principle and Various Structure of AR-HCFs

2.1. Principle of Lite Propagation in AR-HCFs

In general, AR-HCFs features irregular silica cladding and an air core, which induces a special light propagation path. Here, AR-HCFs obey the light-guiding mechanisms of the anti-resonant reflecting optical waveguide (Arrow) [11], rather than the principle of total internal reflection of SMF. Pointer exploits coherent reflections at the air–silica interface to effectively guide the forwards propagation light into the central hollow-cadre. To intuitively reverberate the light propagation in the AR-HCFs, the hollow-core capillary with regular silica cladding is selected equally the representative to describe the anti-resonant theory. As shown in Figure 2a, hollow-cadre capillary with a length L is sandwiched past the 2 segments of SMFs. The hollow-core capillary with regular ring cladding can be regarded as the FP crenel forth the radial direction. The light beams, I_one and I₂ , correspond the first reflection of their incident light at the air–silica and silica-air interface, respectively. Here, the incident light of I_two is the refract light generated past the initial incident lite. And then, the 2 beams I₁ and I₂ will form interference in the air core on the condition that the initial incident angle

$\theta$

meets the grazing incidence (

$\theta$

~ninety°). When the incident light of I₂ reaches the phase matching condition, information technology will leak out of the cladding. The leaked light is called resonant light, similarly, the reflected part is AR calorie-free. In short, AR-HCFs exploit coherent reflections from cladding to confine the guided light and propagate in the central air core. Thus, these light transmission mechanisms reveal the bones principle of the AR effect in HCFs. In addition, it is worth noting that the generation of the AR effect is limited by the length of the HCF. Plain, if L is short enough, the Fabry–Perot (FP) cavity formed by the two fusion splicing interfaces dominates the whole transmission spectrum, equally exhibited in Effigy 2b. With an increasing L, however, the FP effect will gradually be weak or disappear caused by the increasing space loss. At this moment, nigh all of the calorie-free follows the ARROW transmission mechanisms. It can exist seen that a critical length exists between FP and AR event. The critical length L_c corresponds to the centric transmission length of beam I₂ , information technology tin exist expressed equally follows [12]:

$$L_{\mathrm{c}}=\sqrt{n_0^2+{\mathrm{due\; north}}_2^{\mathrm{two}}-n_{\mathrm{ane}}^{\mathrm{ii}}}\left(\frac{r}{\sqrt{{\mathrm{north}}_{\mathrm{i}}^2-n_{\mathrm{two}}^{\mathrm{ii}}}}+\frac{2d}{\sqrt{n_{\mathrm{ane}}^{\mathrm{two}}-n_0^{\mathrm{ii}}}}\right)$$

(1)

where

${\mathrm{due\; north}}_0$

,

${\mathrm{northward}}_{\mathrm{i}}$

, and

$n_2$

represent the refractive index of air, fiber core of SMF, and cladding, respectively. r and d denote the radius of the air core and the thickness of the ring cladding, respectively. If the capillary length is longer than L_c , the AR effect will be excited in the whole process. Otherwise, the sandwich construction only induces the FP effect. Figure 2b shows that several resonant wavelengths with a periodic distribution that is located in the AR issue-based manual spectrum. According to the phase matching status, the resonant wavelength

${\lambda }_r$

can exist given as follows [xiii]:

$${\lambda }_r=\frac{\mathrm{ii}\left(d\sqrt{n_2^2-n_0^2}\right)}{\mathrm{one\; thousand}}$$

(2)

where one thousand is the resonance order. All of the aforementioned statements about AR-HCFs are based on the regular shape of the silica cladding, while most of the awarding scenes rely on an irregular construction to raise the sensitivity. In view of diversified sensing applications, the adjacent part will accordingly introduce the various construction of AR-HCFs.

2.2. Various Structures of AR-HCFs

In the past decade, AR-HCFs have experienced rapid development, mainly concentrated on structure improvement. The simplest structure is the hollow-core capillary exhibited in Figure 2a; thus, it is regarded equally fundamental to exploiting novel structures. The optimization progress tin be generally divided into 3 stages. Firstly, the negative curvature core [xiv] with a dense arrangement is introduced to environment the air core [15,sixteen,17,xviii], equally presented in Figure 3a. Then, several capillary tubes are designed to suspend on the silica cladding to class a negative curvature construction [xix,xx,21,22,23,24,25,26,27,28,29,30], equally shown in Effigy 3b. Finally, the suspended capillary tubes are optimized to be nested structures, equally displayed in Figure 3c,d. In this manner, the number of coherent reflections at the air–silica interface is increased and, thus, confinement loss is reduced, which is the latest HC-NANF [31,32].

All these AR-HCFs utilise the negative curvature effect in the surrounding region of the central core to reduce attenuation and promote longer propagation altitude, and so they are also referred to every bit negative curvature fibers (NCF). In other words, in most cases, AR-HCFs focus the performance enhancement on the optimization of negative curvature structure. In general, the performance of novel structures depends on pregnant indicator parameter improvements later on optimization. Appropriately, Table ane lists several key parameters of various AR-HCFs every bit contrasts.

Tabular array ane illustrates that the silica thickness is one or two orders of magnitude lower than the core bore. Here, the geometric size of the air core and silica cladding are in accordance with the AR theory in Department two.1. Meanwhile, it tin exist seen from Tabular array one that the transmission loss of various AR-HCFs is gradually decreasing with optimization stages budgeted the nested topological structure. Indeed, the latest HC-NANFs based on nested structures achieve low attenuation in wide bandwidth ranges, single-mode transmission in hollow-core regions, and relatively long-altitude propagation (~10 km) [x]. Currently, most sensing applications are limited by the same advantages, and, on the contrary, they require high loss structure modification, multimode coexistence, and micro or small size. Information technology is obvious from Figure 3 that the well-nigh pop AR-HCFs still concentrate on the type of suspended capillary tubes. On the one hand, they accept appropriate transmission loss values that can be satisfied in nigh sensing application scenarios [33,34,35,36,37,38,39]. On the other hand, their singled-out advantage is that they can offering a versatile multichannel and lab-on-a-fiber platform [forty,41,42,43,44,45]. Every bit a issue, the following sections will emphasize the novel and advanced sensing applications based on the most common AR-HCFs at present.

3. AR-HCF-Based Sensing Applications

Concerning the diversified sensing applications of AR-HCFs, not only tin they be divers as discrete specialty fiber sensing devices, but can too exist treated as a versatile platform for lab-on-a-fiber. The sometime is mainly used for common physical or chemic measurand detection, which is too the well-nigh reported field. The latter pays more attention to biomedical or online fluidic applications, which are relatively frontier research fields for now. All of the mentioned sensing awarding mechanisms can be broadly classified into two types, namely, special devices and versatile platforms. Thus, nosotros will discuss the AR-HCF-based sensing applications in the light of these classifications.

iii.i. Special Device-Based Sensing Applications

Similar to the Bragg wavelength for fiber Bragg grating (FBG) [46,47,48] and loss peak for long menstruum grating (LPG) [49,50], the resonant wavelength is a remarkable characterization for AR-HCFs. Appropriately, AR-HCFs can be besides regarded as one of the functional fiber sensing devices. As analyzed in Section ii.ane, the unique AR event can provide a series of resonant wavelengths with the periodic distribution. The measurand sensitivity can be acquired by interrogating the wavelength shift or intensity. In contempo years, most research works have reported sensing applications for generic parameters, including strain, displacement, curvature, static force per unit area, temperature, liquid level, and mechanical strength, et al. Moreover, some modified structures based on AR-HCFs can achieve the simultaneous measurement of dual or multiple parameters. Figure four provides a comprehensive review of various sensing applications based on unmarried- or double-layer AR effects.

Figure 4a shows that the inner capillary is coated with silver, then wraps the HCF [51]. This nigh common HCF agrees with the theory of Section two.1, so it will class an FP cavity in the cladding. The leaky mode of the HCF actually features the resonant wavelengths in the transmission spectrum. The initial leaky mode will be reflected dorsum into cladding with the capillary location movement because the covered part is transformed to the silica–silvery interface. Thus, the intensity of the resonant wavelength is sensitive to the deportation, and it finally obtains a high sensitivity of 0.578 dB/μm. Besides, equally shown in Effigy 4b, HCF is inserted into the polymethyl methacrylate (PMMA) hollow-cadre cobweb [52]. This is a double-layer AR structure since the refractive index of PMMA is between air and silica. The distinct difference between the unmarried- and double-layer AR structure is the costless spectrum range (FSR), and the latter is larger than the former owing to the longer optical path. The tensile strain will reduce the thickness of the PMMA resulted in the optical path change, then the resonant wavelength will shift. Using this double-layer AR construction, it achieves the strain sensitivity of 27.9 pm/με. As shown in Figure 4c, the HCF is used for liquid level monitoring [53], and the sensing mechanism is similar to that in Figure 4a. The leaky mode intensity is influenced by the modify of the surrounding refractive alphabetize. More specifically, the contrast of resonant wavelength will increase or subtract with the liquid level rising or falling. On the basis of this simple HCF structure, a liquid-level sensitivity of 0.4 dB/mm has been realized. Furthermore, the temperature cross-sensitivity is but 0.004 mm/°C, which can be regarded as insensitive. It is apparent that all the sensing mechanisms from Effigy 4a to Effigy 4c are based on the external refractive index change. On the contrary, Figure 4d devotes to the inner refractive index variation to induce the wavelength shift [54]. As displayed in Equation (two), the resonant wavelength non just depends on the silica thickness and refractive index, but likewise the air core refractive index. It exploits femtosecond laser drilling on the ring cladding to form a microchannel, which keeps the air-core pressure level equivalent to the external environment. The manufactured office is placed into the airtight container, then the increasing pressure will induce the air core refractive index change. The gas pressure sensitivity reaches 3.592 nm/MPa by the wavelength demodulation in the range of 0 to 2 MPa. Information technology can be seen from Figure 4a to Effigy 4d, that they only cause a unmarried parameter change in the whole sensing process, and all of these structures transfer to the refractive index variation. While temperature is treated as a sensing variable, which can simultaneously induce parameter changes in multiple structures in the AR-HCFs. As shown in Figure 4e, HCF is applied to loftier-temperature measurement attributed to the AR consequence [55]. It is difficult to use common multimode interference for high-temperature sensing because the employed high lodge cladding modes have a nonlinearity variation with the temperature increasing. For the HCF, the AR effect is just dependent on the geometric size and cloth refractive alphabetize. Thereby, AR-HCFs have a linear human relationship with temperature in a large dynamic range. Effigy 4e shows a high-temperature sensitivity of 33.iv pm/°C with the working range from room temperature to 1000 °C. The sensitivities of all the same structures are determined by their own fabric properties. Effigy 4f enhances the curvature sensitivity assisted by coating the sensitive textile [56]. Later on the polydimethylsiloxane (PDMS) coated on the HCF, the mode confinement is reduced and bending losses are significantly higher. By the external structure modifications, information technology acquires the curvature sensitivity of −5.26 dB/grand^−ane, which is several times college than without PDMS coated. In addition, the structure modifications tin be likewise conducted by the special design of HCF. Figure 4g illustrates a novel cobweb with a single-hole twin eccentric core, which simultaneously has AR effect and inline Mach-Zehnder interference (IMMI) functions [57]. In this case, the dual or multiple parameters can be measured simultaneously based on different mechanisms and without crosstalk. A curvature sensitivity of −1.54 dB/m⁻¹ and the temperature sensitivity of 70.71 pm/°C is realized based on the AR consequence and IMMI, respectively. The listed seven examples reveal that the almost common HCF mainly rely on the AR effect for sensing at present.

In summary, AR effect-based HCFs have been widely used for all kinds of sensing applications, covering solids, gas, and liquid. Among the AR-HCFs, some have been used for practical applications. Nonetheless, the detection of nearly parameters is function of mechanisms research, and, thus, remains at a laboratory stage.

iii.2. Versatile Platform Based Sensing Applications

The HCFs in three.ane are relatively unproblematic structures without negative curvature topological configurations. This section introduces AR-HCFs with negative curvature structures. Here, the ascendant AR consequence, like the Figure 4 examples, will nevertheless exist in the irregular hollow-core region. The AR-HCFs in this section mainly exploit the multichannel structure for fluidic sensing. In dissimilarity to common HCFs, these AR-HCFs demand to fill various gases, liquids, or solid materials in the air aqueduct to enhance the calorie-free–matter interaction. This versatile platform tin can offering a promising road for compact, integral, and biocompatible all-fiber multifunctional optofluidic devices for in-situ applications.

As shown in Effigy 5a, light is guided inside an air cadre filled with the analyte gas, and the AR-HCF serves as a depression-book and robust absorption cell. Light amplification by stimulated emission of radiation-based gas sensors utilize borosilicate fiber because the assimilation wavelength of the monitoring nitrous oxide is located at v.26 µm. The feasibility of exploiting the 8-pigsty AR-HCF with 1.fifteen m length as a gas prison cell is verified, and the minimum detection limitation (MDL) of 20 ppbv with 70 s signal interrogation fourth dimension is achieved [58]. Similarly, Figure 5b replaces the viii-pigsty negative curvature with a nested nodeless structure for nitrous oxide sensing [59]. Compared with Effigy 5a, the whole length of AR-HCF is increased to 3.2 m, so the time and area of light–gas interaction is accordingly enhanced. Finally, the experimental results evidence that the response time shortens to 1s and the MDL decreases to ~five.4 ppbv. Effigy 5a and Figure 5b are used for single gas detection, which is a waste of resources. In practical applications, most occasions require simultaneous measurement of multi-component gas, for instance, gas micro-leakage in a transformer. Equally shown in Figure 5c, laser-based dual gas sensing of methane and carbon dioxide is achieved by a length of 1m silica AR-HCF. The absorption wavelengths of these two gasses are iii.334 µm and 1.574 µm, respectively, corresponding to the near- and mid-infrared spectral region [60]. That is, the monitoring crosstalk between methane and carbon dioxides can exist well avoided, which paves the way for multi-component gas detection. The experimental results show that the MDL of methane and carbon reaches 24 ppbv and 144 ppmv, respectively. From Figure 5a to Figure 5c, all the utilized AR-HCFs retain the single-mode transmission in the air core region. In order to larn lower MDL, Effigy 5d demonstrates a way-phase-deviation (MPD) photothermal spectroscopy for acetylene detection [61]. In a 4-m-long AR-HCF, it supports LP₀₁-similar and LP₁₁-similar manual in a broadband. The photothermal effect induces variations of the dual-mode refractive index. The MPD is sensitive to gas assimilation and insensitive against the external environment perturbation. The proof-of-concept realizes the MDL down to pptv. AR-HCFs are treated every bit microcells to provide a gas–light interaction platform in the aforementioned four examples. More importantly, AR-HCFs have a great potential for multi-component gas simultaneous detection owing to the wide transmission broadband in the air core region, about roofing all molecule absorption lines. In add-on to gas detection, AR-HCFs are also attracting attention for their potential in liquid sensing. The distinct difference of absorption spectroscopy measurements between gas and liquid is the different filling medium. As shown in Effigy 5e, the hollow-cadre region is filled with antibody, which volition cause the assimilation of the guided lite. Using a 1-grand-long AR-HCF achieves the detection of sulfamethoxazole (SMX) and sodium salicylate (SS) down to 0.1 µM (26 ppb) and 0.four µM (64 ppb), respectively [62]. Effigy 5f conducts a proof-of-concept experiment of Raman spectroscopy using ethanol, which provides an original idea for noninvasive biochemical analysis [63]. In contrast to Figure 5e–g principally proposes a low-loss micro-machining method for optofluidic applications [64]. They exploit the focused ion beam to manufactory the cladding of AR-HCFs, and no additional loss is generated in the whole process. One time the hole of the cladding is opened, it will shine new light on the inline optofluidic applications. This method is similar to femtosecond laser drilling on the ring cladding in Figure 4d. Both contribute to forming the inline channel. Figure 5h utilizes a 40-cm-long AR-HCF to reach the MDL of 1.69 pM for bisphenol A detection [65], which is the experimental verification for Figure 5f. The applications depicted in Figure 5a–h are all employed in fluidic detection, including gas and liquid. While in Figure 5i, HC-NANF is wound into a coil to exist integrated into a resonator fiber optic gyroscope [66]. This structure non merely breaks through the limitation of nonlinear influence, only as well considerably improves spatial mode purity. Information technology is a promising candidate for ceremonious aircraft navigation usage. In recent years, some AR-HCF-based optofluidic applications take been used for practical applications based on similar methods developed for microstructured fiber-based devices [67,68,69,lxx,71,72].

4. Prospects and Conclusions

Enabled by the specialty fiber manufacturing industry, AR-HCFs have shown peachy potential in optical fiber communication and sensing. AR-HCFs take very low transmission loss, optical nonlinearity, and chromatic dispersion over a broad bandwidth. They also take intrinsic advantages of high sensitivity, compact structures, and robust operation. All these remarkable advantages promote diversified sensing applications of AR-HCF. Every bit a functionalized device, it has been extensively used for common parameter sensing, including solid, gas, and liquid. Meanwhile, as a versatile platform, information technology sheds new low-cal on optofluidic fields, mainly including gas micro-leakage, multi-component gas, pharmaceutical substances, and tissue fluid detection. Specially, the versatile platform has made a significant contribution to providing a new route for photoacoustic spectroscopy (PAS) or photothermal spectroscopy (PTS). With the rapid development of micro-machining techniques, there take been substantial laboratory-stage research achievements, inspiring hope for practical applications. Furthermore, it is believed that AR-HCF will go the ultimate optical fiber once at that place has been a breakthrough in long-distance unmarried-fashion manual loss.

Writer Contributions

W.Northward. and 50.W. contributed to the idea. W.Northward. contributed to the writing of the manuscript. Due west.N., C.Y., Y.L., R.Ten., P.L., D.J.J.H., S.D., P.P.S. and Fifty.W. contributed to the reviewing and editing of the manuscript. C.Y. and L.Due west. supervised the project. All authors take read and agreed to the published version of the manuscript.

Funding

This work was supported past the Key Technology R&D Programme of Hubei province under Grant 2020BBB097, Singapore Ministry of Education Bookish Research Fund Tier two (MOE2019-T2-two-127 and T2EP50120-0005), A*STAR nether AME IRG (A2083c0062), the Singapore Ministry of Education Academic Enquiry Fund Tier i (RG90/xix and RG73/19) and the Singapore National Enquiry Foundation Competitive Enquiry Program (NRF-CRP18-2017-02). This work was also supported by South-Central University for Nationalities and Nanyang Technological Academy.

Data Availability Statement

Data sharing non applicable.

Conflicts of Interest

The authors declare no disharmonize of interest.

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Figure 1. Transmission loss of AR-HCFs in dissimilar years.

Figure ane. Transmission loss of AR-HCFs in different years.

Effigy 2. Schematic illustration of (a) optical pathways in the SMF–capillary–SMF structure and (b) transmission spectra as the increase of capillary length. [Reprinted/Adjusted] with permission from [12] © The Optical Society.

Figure 2. Schematic analogy of (a) optical pathways in the SMF–capillary–SMF structure and (b) transmission spectra as the increase of capillary length. [Reprinted/Adjusted] with permission from [12] © The Optical Society.

Figure 3. Scanning electron micrographs (SEMs) of AR-HCFs. (a) [Reprinted/Adapted] with permission from [16] © The Optical Society. (b) © 2017 IEEE. Reproduced with permission from Hayes et al., J. Lightwave Technol. 35, 437–442 (2017) [22]. (c) © 2017 IEEE. Reproduced with permission from Jose E. Antonio-Lopez et al., IEEE Photonics Conference (2017) [31]. (d) © 2020 IEEE. Reproduced with permission from Y. Hong, J. Lightwave Technol. 38, 2849–2857 (2020) [32].

Effigy 3. Scanning electron micrographs (SEMs) of AR-HCFs. (a) [Reprinted/Adapted] with permission from [16] © The Optical Gild. (b) © 2017 IEEE. Reproduced with permission from Hayes et al., J. Lightwave Technol. 35, 437–442 (2017) [22]. (c) © 2017 IEEE. Reproduced with permission from Jose Due east. Antonio-Lopez et al., IEEE Photonics Conference (2017) [31]. (d) © 2020 IEEE. Reproduced with permission from Y. Hong, J. Lightwave Technol. 38, 2849–2857 (2020) [32].

Figure iv. Special device-based sensing applications. (a) © 2016 IEEE. Reproduced with permission from R. Gao, J. Sel. Height Quant. 23, 5,600,106 (2016) [51]. (b) © 2017 IEEE. Reproduced with permission from R. Gao, IEEE Photonic. Tech. L. 29, 857-860 (2017) [52]. (c) © 2015 IEEE. Reproduced with permission from S. Liu, J. Lightwave Technol. 33, 5239-5243 (2016) [53]. (d) [Reprinted/Adapted] with permission from [54] © The Optical Social club. (eastward) © 2018 IEEE. Reproduced with permission from D. Liu, J. Lightwave Technol. 36, 1583-1590 (2018) [55]. (f) Reproduced with permission [56]. Copyright 2020 MDPI. (g) [Reprinted/Adapted] with permission from [57] © The Optical Gild.

Figure iv. Special device-based sensing applications. (a) © 2016 IEEE. Reproduced with permission from R. Gao, J. Sel. Peak Quant. 23, 5,600,106 (2016) [51]. (b) © 2017 IEEE. Reproduced with permission from R. Gao, IEEE Photonic. Tech. 50. 29, 857-860 (2017) [52]. (c) © 2015 IEEE. Reproduced with permission from South. Liu, J. Lightwave Technol. 33, 5239-5243 (2016) [53]. (d) [Reprinted/Adapted] with permission from [54] © The Optical Order. (e) © 2018 IEEE. Reproduced with permission from D. Liu, J. Lightwave Technol. 36, 1583-1590 (2018) [55]. (f) Reproduced with permission [56]. Copyright 2020 MDPI. (one thousand) [Reprinted/Adapted] with permission from [57] © The Optical Club.

Figure 5. Versatile platform based sensing applications. (a) [Reprinted/Adjusted] with permission from [58] © The Optical Club. (b) [Reprinted/Adapted] with permission from [59] © The Optical Society. (c) Reproduced with permission [60]. Copyright 2020 MDPI. (d) Reproduced with permission [61]. Copyright 2020, Springer Nature. (e) Reproduced with permission [62]. Copyright 2018 MDPI. (f) [Reprinted/Adjusted] with permission from [63] © The Optical Social club. (g) [Reprinted/Adapted] with permission from [64] © The Optical Guild. (h) Reproduced with permission [65]. Copyright 2020, Elsevier. (i) [Reprinted/Adapted] with permission from [66] © The Optical Society.

Figure 5. Versatile platform based sensing applications. (a) [Reprinted/Adapted] with permission from [58] © The Optical Society. (b) [Reprinted/Adjusted] with permission from [59] © The Optical Society. (c) Reproduced with permission [threescore]. Copyright 2020 MDPI. (d) Reproduced with permission [61]. Copyright 2020, Springer Nature. (e) Reproduced with permission [62]. Copyright 2018 MDPI. (f) [Reprinted/Adapted] with permission from [63] © The Optical Society. (1000) [Reprinted/Adapted] with permission from [64] © The Optical Lodge. (h) Reproduced with permission [65]. Copyright 2020, Elsevier. (i) [Reprinted/Adapted] with permission from [66] © The Optical Society.

Table 1. Parameters in various AR-HCFs.

Table ane. Parameters in various AR-HCFs.

Type	Core Bore	Silica Thickness	Transmission Loss	Reference
Densely arrangement	45.8 μm	0.51 μm	300 dB/km	[15]
	50 μm	0.28 μm	180 dB/km	[16]
	lx μm	1.4 μm	17 dB/km	[xviii]
Suspended capillary tubes	xxx μm	0.44 μm	180 dB/km	[nineteen]
	30 μm	0.83 μm	30 dB/km	[21]
	41 μm	0.545 μm	7.7 dB/km	[26]
Nested Suspended tubes	25 μm	2.iii μm	75 dB/km	[5]
	~33 μm	~0.78 μm	two dB/m	[31]
	~35 μm	~0.5 μm	6.6 dB/km	[32]

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