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Abstract
Limited operating bandwidth originated from strong absorption of glass materials in the infrared (IR) spectral region has hindered the potential applications of microstructured optical waveguide (MOW)-based sensors. Here, we demonstrate multimode waveguide regime up to 6.5 µm for the hollow-core (HC) MOWs drawn from borosilicate soft glass. Effective light guidance in central HC (diameter ∼240 µm) was observed from 0.4 to 6.5 µm despite high waveguide losses (0.4 and 1 dB/cm in near- and mid-IR, respectively). Additional optimization of the waveguide structure can potentially extend its operating range and decrease transmission losses, offering an attractive alternative to tellurite and chalcogenide-based fibers. Featuring the transparency in mid-IR, HC MOWs are promising candidates for the creation of MOW-based sensors for chemical and biomedical applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Hollow-core microstructured optical waveguides (HC MOWs) represent a separate class of optical waveguides, in which confinement of transmitted light ensured by complex array of air-capillaries surrounding the central HC. In such structures, light guidance is achieved by two different methods, both, however, avoiding the need for total internal refraction [1]. First is two-dimensional photonic bandgap mechanism, which allows tight optical confinement and transmittance of the light inside HC and prevents its escape into the photonic-crystal cladding [1]. These structures refer to hollow-core photonic bandgap fibers (PBGFs) and lie within the scope of photonic-crystal fibers (PCFs). The central light-guiding HC surrounded by thin glass wall (50–150 nm) supports the transmission of a tiny wavelength band with low loss over the whole waveguide length [2,3]. Another method is based on anti-resonant reflecting optical waveguide structure. These are so-called hollow-core anti-resonant fibers (or inhibited-coupling fibers) [3]. They, in turn, do not allow the realization of photonic bandgap guidance because the glass wall thickness (typically hundreds of nm) is thicker than that required for broadband or multiband PBGFs. Nevertheless, featuring the relatively low loss operation over the broad wavelength range, hollow-core anti-resonant fibers can further enhance the capability of HC MOWs in various applications [4–6].
Although strong material absorption and high transmission losses of bulk silica prevent its usage in the mid-infrared (mid-IR) region beyond 2.5 µm, different research groups reported the low loss mid-IR guidance in silica-based HC MOWs with modified structures [7–14]. Yu et al. demonstrated silica hollow-core fiber with minimum attenuation of 34 dB/km at 3.05 µm. They confirmed the light confinement inside hollow-core and showed low bend loss [10]. Pryamikov et al. showed a waveguide regime in the silica hollow-core microstructured optical fiber (MOF) in spectral region of 3.5–5 µm [11]. Interestingly, but later this group changed the geometry of the core boundary and obtained light transmission up to 7.9 µm [12], where silica absorption is extremely high (∼109 dB/km) [15]. Wheeler et al. achieved the mid-IR guidance in hollow-core PCF with operating bandwidth between 3.1 and 3.7 µm, minimum attenuation of 0.05 dB/m at 3.33 µm, and low bend sensitivity of <0.25 dB per 5 cm diameter turn over 300 nm bandwidth [7]. Shepard et al. reported bandgap guidance above 3 µm in silica hollow-core PCF with minimum attenuation of 2.6 dB/m at 3.14 µm [16]. Lingsø et al. demonstrated hollow-core PCF with a transmission loss of 26 dB/km at 2.3 µm that is significantly lower than the loss of solid-core silica fibers at this wavelength range [8]. Interestingly enough, papers [10–12] referred to negative curvature optical fibers which are featured by the inverted curvature of the core wall that is believed to increase light localization inside hollow-core region [11,17].
Despite all benefits of ultraviolet-visible (UV-VIS) and near-IR spectroscopic methods, sensing in mid-IR wavelength range reveals unreachable horizons [18]. UV-VIS and near-IR absorption bands are usually non-specific, which leads to the need for spectral unmixing between overlapping absorption bands and, in turn, compromises the sensitivity of these methods. In contrast, the mid-IR spectral range contains fundamental absorption peaks from vibrational and rotational modes of molecules. Exciting molecules in the mid-IR (also known as the “fingerprint region”) allows to achieve high specificity and sensitivity of measurements necessary for identification of analyte compounds [18,19]. Moreover, mid-IR and Raman spectroscopic tools have been proven to differentiate between normal and diseased cancerous tissues by comparison of their spectra (e.g. lipid-to-water ratio) [20,21]. This makes the development of such mid-IR fiber-based endoscopic probes potentially useful for point-of-care medical devices for quick and label-free detection of boundaries between normal and cancerous tissues [22]. Additionally, mid-IR optoacoustic microscopy (OM) was recently adapted for highly sensitive and label-free metabolic imaging of carbohydrates, lipids, and proteins in live-cells [23] and allowed to resolve very low concentrations of lipids and proteins at laser powers of hundreds of microwatts. Implementing mid-IR OM in various areas of biosensing will inevitably require appropriate waveguiding systems.
Combination of mid-IR optical fibers and surface-enhanced Raman spectroscopy (SERS) methods has been attracting tremendous attention due to its potential for fast, precise, and easy sensing [24]. The enhancement factor associated with Raman scattering of adsorbed molecules on rough metal surfaces or nanostructures such as plasmonic-magnetic silica nanotubes, gold, or silver nanoparticles reaches the values of 1010 - 1011 [25]. Nevertheless, the use of hollow-core MOFs additionally increases SERS signal due to the high overlap between a guiding light mode and an analyte filling of fiber capillaries [26]. SERS-based immunoassay approach can be also used for the early detection of different biomarkers in body fluids [27,28].
Thus, there is a strong need for the development of optical fibers with effective transmission in the mid-IR region (2.5–25 µm) [29,30]. Being an alternative to the conventional fused silica-based MOWs, optical fibers fabricated from the other materials with higher transmittance in the mid-IR spectral region have received growing interest over the last decade [31]. Among the other compositions, one can highlight various polymer compounds as well as the group of soft glasses consisted of three subdivisions: chalcogenides, fluorides, and oxides [4,19,30,32]. Chalcogenide glass-based (As2S3) optical fibers are used for the light delivery in range of 1.5 - 10 µm, fluoride (or ZBLAN) glass fibers exhibit transmission up to 4 µm and polycrystalline (AgCl1-xBrx) fibers are best for 3 -18 µm [29].
However, despite established fiber drawing technology, some drawbacks cannot be overcome. Chalcogenide fibers are fragile and consist of toxic material that is undesirable for biomedical applications; they also show thermal instability and crystallization during heating [4]. ZBLAN fibers are brittle and slightly hygroscopic, while polycrystalline fibers are UV-sensitive and can be easily plastically deformed, inducing transmission losses originated from multiple scattering from the separated grain boundaries [30]. Although lower melting point of soft glasses (200 - 1000 °C) in comparison to bulk silica (1800 - 2300 °C) simplifies the process of fiber drawing, strong influence of temperature variation onto glass viscosity requires stable conditions during the whole fabrication process [4]. All these factors limit the successful fabrication of soft glass optical fibers in a controllable and repeatable way. Furthermore, mid-IR light-guidance has mostly been reported in solid-core soft glass-based fibers [4,33,34], while the fabrication of hollow-core waveguides is still challenging, unstable, and, in general, more sophisticated process.
Here, we reported for the first time to our knowledge, mid-IR light guidance up to 6.5 µm in borosilicate soft glass hollow-core MOWs. Waveguides demonstrated broadband transmission in the visible and mid-IR spectral regions from 400 nm to 6.5 µm with minimum loss of ∼0.4 and ∼1 dB/cm in the near- and mid-IR, respectively. Despite high transmission losses in the mid-IR region, proposed hollow-core waveguides are promising candidates for the creation of MOW-based sensors for chemical and biomedical applications. Mid-IR guidance and light propagation inside hollow-core is a unique combination and can be used for many applications such as high power and ultra-short pulse delivery, light – analyte interactions and terahertz sensing [11]. The robust structure, created by cladding capillaries surrounding the central hollow-core with a diameter of ∼240 µm, facilitates the integration of waveguide to various optofluidic tools [35]. Besides, in combination with advanced splicing techniques, various bendable endoscopic probes can be realized based on short sections of designed structure featuring both the mid-IR transmittance and flexibility [36,37]. Furthermore, the reported MOWs consisted of non-toxic compounds are an attractive alternative to tellurite and chalcogenide-based fibers.
2. Experiment details and set-up
The reported hollow-core MOWs were produced by a stack-and-draw method described in [38]. Three concentric capillary layers, surrounding central hollow-core and defining the overall waveguide structure, are illustrated in Fig. 1. Wall thickness of the capillaries forming the first cladding layer was ∼1.8 µm, the diameter of hollow-core was ∼240 µm, and the outer diameter was ∼600 µm. Such a waveguide structure supports multimode guidance [3]. MOW samples with length of 50 mm were cleaved before the measurements to eliminate the negative effect of the rough end faces on to coupling conditions. Properties of such waveguides have been discussed in detail in [1–3,6,39].
Fig. 1. Schematic of optical setups used for transmission spectra measurements of MOW samples. (a) Butt coupling setup allows the measurement in the wide spectral range from 400 nm to 18 µm. The auxiliary IR-fibers connected to MOW samples via special SMA/SMA terminators were utilized for both delivery of the broadband light source radiation and collection of the light transmitted through MOWs. (b) Free-space coupling is organized with the set of sapphire lenses enabling the transmission measurements up to 2.5 µm. Inserts are the scanning electron microscopy image of MOW cross-section (scale bar is 100 µm) and the schematic spectra illustrating the light guidance principle of investigated MOWs based on the model of Fabry-Perot resonator. DF stands for delivery fiber, CS – connection sleeve, CF – collecting fiber.
Borosilicate glass, used for fabrication of hollow-core MOWs, has the following chemical composition: SiO2 – 81%, B2O3 – 13%, Na2O + K2O – 4%, Al2O3 – 2%. Glass composition and its optical properties is similar to Schott Duran(R) glass [40].
To characterize optical transmission of MOWs, we employed two types of experimental setups (Fig. 1). The first was the butt-coupling setup (Fig. 1), based on the combination of broadband light sources and spectrometers, which allows to measure MOW transmittance up to 18 µm.
Measurements in the VIS wavelength region were conducted with compact CCD-spectrometer (200 - 1025 nm, FLAME-S-XR1, Ocean Optics, USA) and a halogen lamp (360 - 2600 nm, SLS201L/M, Thorlabs, USA). For the near-IR spectral domain, we switched to NIR spectrometer with 900–1700 nm wavelength range (NIRQuest512, Ocean Optics, USA). In both cases, silica-based auxiliary fiber cables (Art photonics GmbH, Germany) with 200/220 µm core/cladding diameters and 600/660 µm were utilized for the delivery of lamp illumination and the collection of light transmitted through MOW samples, respectively. To ensure the effective face-to-face connection between auxiliary fibers and MOW samples, we sealed the waveguide end faces into special SMA terminators and mounted into SMA/SMA connection sleeves, enabling their optical alignment.
Measurements in the mid-IR spectral range were conducted with a Fourier-transform infrared (FTIR) spectrometer, operating in the broad spectral domain from 1.45 to 20 µm (Matrix-F, Bruker, Germany). Chalcogenide IR-fibers 180/280 µm and polycrystalline IR-fibers 240/300 µm (both supplied by Art photonics GmbH, Germany) were used for the light delivery in the ranges 1.1 - 6.5 µm and 3 - 18 µm, respectively. Chalcogenide 500/550 µm and polycrystalline IR-fibers 900/1000 µm (both supplied by Art photonics GmbH, Germany) ensured the effective light collection.
Butt-coupling set-up (Fig. 1(a)) consisted of two auxiliary IR-fibers connected to the MOW sample through special SMA/SMA terminators. Due to the length of the MOW sample, transmission measurements were conducted. As a reference measurement, two IR-fibers were connected to each other with SMA/SMA terminator.
Free-space setup based on the set of sapphire lenses (Fig. 1(b)), allowed more efficient coupling to the waveguide hollow-core and better collection of transmitted light. However, the measuring interval in this case was limited by the transmittance range of these lenses and adequate signal-to-noise ratio. Optomechanical components helped to align the position of auxiliary fibers, lenses and MOW samples.
Transmission losses of investigated MOWs in free-space transmission set-up were estimated by the cutback technique. Several cutback measurements were performed on initial 50 mm long samples under fixed coupling conditions. Since the designed MOWs support multimode regime, the study was concentrated on the analysis of waveguide losses rather than on the efforts for modal filtering and elimination of higher order modes [16,41]. It is expected that the designed borosilicate soft glass–based MOWs show higher transmission losses comparing to previously reported IR-fibers, especially the ones targeting to signal processing and communication [7,14,16]. Nevertheless, we propose the realization of various spectroscopic systems featuring compact size that the waveguides losses of cm-long samples would not disturb the light guidance.
3. Results and discussion
3.1 Demonstration of mid-IR guidance
Figure 2 summarizes the results of transmission spectra measurements and loss analysis of investigated MOWs. In the Fig. 2, presented spectrum with shaded areas was summarized over 4 identical hollow-core MOWs. The broadband guidance was observed from 400 nm to 14 µm (where signal-to-noise ratio was adequate) with clearly defined optical resonances in the mid-IR spectral domain and minimum loss of ∼1 dB/cm (2-4 µm transmission window). Minimal losses in the near-IR region were observed in 0.8-1 µm transmission window and were equal to ∼0.6 dB/cm. Although the produced waveguides suffered from high material absorption beyond ∼7 µm, clearly defined optical resonances in the wide wavelength range of 400 nm – 6.5 µm can open previously unreachable horizons for the soft glass-based MOWs applications.
Fig. 2. Demonstration of broadband VIS – mid-IR light guidance. Blue curve states for the transmission spectrum of investigated MOWs, the shaded region illustrates the estimated loss interval summarizing the results of 4 MOWs. The red dotted line is the calculated spectrum referred to the positions of minima in MOWs transmission. The shaded areas illustrate summarized measurements of 4 identical hollow-core MOWs.
Using general transfer matrix method and taking into account the model of Fabry-Perot resonator [6], we calculated the resonance positions of the investigated hollow-core waveguides referring to the minima of transmission spectra.
The comparative analysis of the waveguide losses was performed on both butt-coupling and free-space optical setups. This demonstrated the importance of the coupling conditions on to the light confinement inside the hollow-core while propagation in cm-long MOWs (Fig. 3). Set of sapphire lenses ensured effective coupling to the hollow-core regions and formed the appropriate light cone with an angle less than the numerical aperture of the waveguides. Butt-coupling setup, in turn, was found to be less accurate, suffering from additional losses arising from the interfaces between investigated MOWs and auxiliary fibers. Nevertheless, it was possible to decrease transmittance losses of designed waveguide structures till 0.4 dB/cm at 1.05-1.33 µm transmission window using proper excitation optics (Fig. 1(b)). It was almost twice as low as the losses measured with butt-coupling technique (Fig. 1(a)).
Fig. 3. The comparison of attenuation measurements performed on butt-coupling (blue curve) and free-space (green curve) transmission setups. The shaded areas illustrate summarized measurements of 4 identical hollow-core MOWs.
3.2 MOW-based sensors for mid-IR application
There has been a growing interest in realization of the mid-IR guidance in hollow-core waveguide structures, enabling detection of various molecular compositions via their associated vibrational modes. Smaller size of hollow-core increases the overlap of the light mode with the analyte filling, allowing detection of small biomolecules at low analyte concentration, which is crucial for biomedical applications. Figure 4 indicates absorption bands of different molecules laying in the range of effective mid-IR transmission of investigated waveguides.
Fig. 4. Mid-IR transmission of MOW samples and the associated vibrational modes of different chemical bonds. Blue, yellow and red peaks correspond to water, lipids and proteins vibrational modes, respectively.
The presence of well-defined transmission minima and maxima and their periodicity offers the potential for selective filtering of different vibrational peaks that are part of all biological molecules [42]. An overlap of transmission minima and undesirable vibrational peak, which, for example, demonstrates the stronger absorption, can help to resolve other peaks located in the vicinity. Furthermore, different functionalization methods allow controllable tuning of optical properties shifting the positions of optical resonances [5,43].
Numerous research groups reported the importance of mid-IR and Raman spectroscopy for the detection of cancer tissues based on the correlation between their biochemical composition and malignancy. For instance, Puppels et al. showed that the water concentration in the squamous cell carcinoma is higher than in the surrounding healthy tissue and determined the water/lipid ratio using the high-wavenumber Raman spectroscopy tools (2500–3400 cm−1) [21]. Ji et al. used tissue biopsies to detect tumor infiltration, where Raman peaks of 2845 cm−1 (lipids) and 2930 cm−1 (proteins) had been used for stimulated Raman spectroscopy imaging [44]. Frank et al. were able to differentiate normal (more lipids) and malignant tissues (higher protein content) based on the ratio of amide I (1530 cm−1) and CH2 bending mode (1460 cm−1) [45]. Raman spectroscopy methods with excitation at 830 nm were also employed for principal component analysis and multivariate analysis for determination of normal, fibroadenoma, or infiltration duct carcinoma [46]. It was realized using the ratio of Amide I to CH2 bending mode as its discriminant parameter. Altogether, these findings suggest that IR and Raman spectroscopic tools open the potential for fast, precise, and minimally invasive in-vivo diagnosis of malignant tissues [42].
MOW transmittance in the mid-IR region can also be used for the applications in gas sensing. Fiber-enhanced Raman spectroscopic analysis (FERS), based on hollow-core photonic crystal fiber (HC-PCF), is a powerful technique for gas sensing of different compositions [47]. Hanf et al. [48], using HC-PCF with FERS, were able to analyze atmospheric gases CH4, CO2, N2O and showed the potential to analyze exhaled human breath for disease markers. Significant absorption of water vapor in the IR region makes MOW-based devices perfect candidates for real-time humidity measurements. Real-time carbon monoxide detector, exploiting the absorption of CO at 4.63 µm, can be also realized on the basis of MOW systems [49]. In principle, due to the presence of CH2 and CH3 absorption bands in the IR spectral-domain up to 6.5 µm, various hydrocarbon gases, such as methane (CH4), ethane (C2H6), etc., can be potentially detected by MOW-based sensors [50]. Table 1 summarizes different biological materials and associated vibrational modes.
Table 1. Different biological materials and their associated vibrational modes
3.3 Comparison with previous results
A comparison of our borosilicate soft glass-based hollow-core MOWs and previously reported works is given in the Table 2, emphasizing the microstructured and holly waveguides. HC and SC state for hollow- and solid-core, NC F means negative curvature fiber. Due to the waveguide structure and material composition, broadband mid-IR guidance has been demonstrated with soft-glass-based hollow-core MOWs. We have found very little works reporting the effective fabrication and IR guidance with such structures. Russell et al. [4] achieved photonic bandgap guidance with soft-glass hollow-core PCF in the range of 750–1050 nm. Other groups concentrated on soft glass solid-core PCFs and achieved the broadband transmittance up to 3 µm [33,34,62]. Dianov et al. succeeded in the production of silica negative curvature fibers and demonstrated mid-IR transmittance up to 7.9 µm [12]. Light guidance in silica fibers with negative curvature was also supported by theoretical calculations up to 6 µm [63]. Despite high material absorption in the IR spectral domain, few research groups received outstanding results in the fabrication of silica-based hollow-core fibers for mid-IR applications [7,9].
Table 2. Comparison of the optical performance of reported waveguides
4. Conclusion and outlook
In this work, we demonstrated mid-IR transmittance of borosilicate soft glass-based hollow-core MOWs. Produced structures were characterized in broad spectral range using two types of optical setups. Effective light guidance in the spectral region of 400 nm - 6.5 µm with the minimum loss of ∼0.6 dB/cm in the near-IR spectral domain (0.8-1 µm transmission window) and minimum loss ∼1 dB/cm in the mid-IR region (2-4 µm transmission window) was observed using the butt-coupling technique. However, proper coupling condition ensured by the set of sapphire lenses allowed to reduce the attenuation to ∼0.4 dB/cm in 1.05-1.33 µm transmission window. With benefits of mid-IR guidance, investigated hollow-core MOWs are promising candidates for the creation of MOW-based sensors including endoscopic probes for chemical and biomedical applications. Mid-IR guidance inside hollow-core is a unique combination and can be useful for many applications such as high power and ultra-short pulse delivery, light – analyte interactions and terahertz sensing. The robust structure, created by cladding capillaries surrounding the central hollow-core with a diameter of ∼240 µm, facilitates the integration of waveguide to various optofluidic tools. Furthermore, the proposed MOWs consisted of non-toxic compounds are an attractive alternative to tellurite and chalcogenide-based fibers.
Funding
Russian Foundation for Basic Research (18-29-08046, 19-32-90249).
Disclosures
The authors declare no competing financial interest.
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