We report the fabrication of the first extruded hollow core optical fiber with a single ring of cladding holes, and its use in a chemical sensing application. These single suspended ring structures show antiresonance reflection optical waveguiding (ARROW) features in the visible part of the spectrum. The impact of preform pressurization on the geometry of these fibers is determined by the size of the different hole types in the preform. The fibers are used to perform Raman sensing of methanol, demonstrating their potential for future fiber sensing applications.
© 2016 Optical Society of America
Hollow core optical fibers have certain key advantages over conventional index-guiding optical fibers. They demonstrate a large overlap between the guided mode and the hollow core region, can handle high power laser beams as light is guided in air instead of glass, and they can extend light transmission outside the transparency limits of the fiber material itself [1–3]. The most widespread category of such fibers is hollow core photonic band gap fibers (HC-PBGF), where the guiding mechanism relies on a large number of sub-wavelength features periodically arranged around the hollow core to create a photonic band gap that guides light in the hollow core, making them relatively costly and difficult to fabricate by capillary stacking [4, 5]. Antiresonance guiding fibers, where optical guiding is achieved by Fabry-Perot reflection off the walls of a waveguide offer an alternative guidance mechanism; the simplest case of these - a thin capillary suspended inside an outer thicker jacket, have emerged as a category of fibers that combines the advantages of HC-PPBGFs  with greatly simplified fabrication. These simplified hollow core fibers are currently being made using the stack and draw technique, with the size of the different holes controlled by differential pressurization of the inner and outer capillaries. This results in geometries such as the single suspended ring [7, 8], or negative radius of curvature hollow cores structures which are suitable for sensing applications .
In this work we demonstrate the first single ring hollow core optical fibers based on a preform made by billet extrusion. Billet extrusion is a relatively simple, automated fabrication technique that allows us to produce fibers with features not easily attainable by capillary stacking . Single ring hollow core fibers were made from lead-silicate glass (F2, Schott) using increasing values of equal pressurization for all preform holes, resulting in preferential inflation of the slightly larger outer holes. The fibers are designed to support guidance in the visible part of the spectrum and we demonstrate their use for Raman spectroscopy of methanol. Our results highlight that an extruded single ring hollow core optical fiber has the potential to be used for sensing applications.
2.1 Fiber fabrication and loss measurements
Single ring hollow core structures consist of a hexagonal core glass ring that is suspended through 6 struts to an outer glass jacket, as shown in Fig. 1. The preform for these structures was fabricated using the glass billet extrusion technique . Lead-silicate glass (F2, Schott) was heated to 580 °C, and a force of 20 kN was applied with a ram to force the softened material through a steel die that was fabricated such that the output face of the die formed the inverse of the desired final structure. Due to restriction on the maximum output face diameter and minimum size of die features that allow glass flow, the ratio of strut and core glass ring thickness to preform outer diameter (OD) was limited to 2.2%, which is larger than the desired ratio of <1%. Therefore, we designed the internal structure of the die in such a way that the glass flow within the die enabled thinning of the struts and core glass ring relative to the preform OD, leading to a preform with OD of 10.3 mm, jacket wall thickness of 1.9 mm, core diameter of 2.4 mm, core glass ring thickness of 75 μm and strut thickness of 35 μm, as shown in Fig. 1(a). The ratio of core glass ring and strut thickness to preform OD is 0.7 and 0.3% respectively.
This preform was mounted in the drawing tower furnace, connected to the single pressurization line and was directly drawn into a fiber while increasing amounts of pressure were applied in a step-wise manner to control the relative thickness of the core glass ring relative to the outer diameter. The furnace temperature during fiber drawing was 950°C and the internal pressures used were 0, 4, 10 and 20 mbar; to maintain a constant outer diameter of 250 μm the draw speeds used at each pressure step were 2.1, 2.4, 3.9 and 8.1 m/min respectively. Scanning electron microscope (SEM) images of the final fabricated fiber structure cross-sections are shown in Fig. 1 for different internal pressurization conditions. The impact of the preform pressure on the optical fiber structure, and in particular the core size, the outer hole diameter, the core glass ring thickness and the outer jacket thickness, is summarized in Table 1. The maximum internal pressure used was 20 mbar, above which the outer jacket of the fiber broke apart.
The changes in the cross-sectional geometry of the fibers can be understood by considering the effect that pressure has on the size of the different holes of the fibers (central core hole versus outer 6 holes) during fiber drawing. For glass capillaries with a single hole, mathematical models have been developed that describe the impact of pressure on the hole diameter [11–14]. Kostecki, et al.  have shown that the models for capillary drawing can be employed for fibers with non-circular holes by describing the hole size as the diameter of a circle that has the same circumference as the non-circular hole.
We used this method to measure the diameter of the core hole and the average diameter of the outer holes for the preform and fibers. To investigate the impact of pressure, we determined the relative hole size as the ratio of hole diameter to outer diameter (Table 1). Figure 2 shows the change in relative hole diameter in the fibers compared to the preform for the two different type of holes. When no preform pressure is applied, both the core hole and the outer holes show slight hole closure. With increasing preform pressure, the outer holes inflate while the core hole retains the size or for 20 mbar preform pressure even decreases in size. This behavior demonstrates that the ~20% larger size of the outer holes compared to the core hole in the preform leads to a preferential inflation of the outer holes, which limits the inflation that can be achieved for the core hole. An inflation of both the core hole and the outer holes is predicted to decrease the core glass ring thickness. The restriction of the core hole inflation due to smaller size relative to the outer holes limited the decrease of the core glass ring thickness from 2.0 µm for no pressurization to 0.95 µm for 20 mbar pressurization. The impact of hole size on the degree of inflation shows that the size of the core hole in the preform needs to be equal or larger than the outer holes to control core hole inflation and core glass ring thickness in the fiber through preform pressurization.
We note here that, for silica-based hollow core optical fibers made by capillary stacking, an additional parameter that is often used to control the relative features sizes is differential pressurization of different subsets of holes, i.e. inner holes vs. outer set of holes, to maintain the correct features size during fiber drawing, for example in photonic bandgap and Kagome optical fibers [15, 16]. In the case of the single ring hollow core optical fibers presented here balancing the size of the core hole and outer holes was done at the preform die design stage, were the area of the central core and the surrounding holes was the same. Due to the effects of glass flow during preform extrusion the resulting geometry was slightly altered, with larger outer hole area that resulted in preferential inflation of the outer holes during fiber drawing under a single inflation pressure. These results highlight a different approach to controlling the geometry of hollow core optical fibers, using preform design rather than differential pressurization to tailor the final fiber structure.
In our experiments we found that 10 mbar of internal pressurization, shown in Fig. 1(d), resulted in the right combination of core glass ring thickness and outer jacket thickness as further increasing the pressure resulted in an extremely brittle fiber. This fiber had an outer diameter of 250 μm, with the hexagonal single ring core being 52 μm across (side-to-side distance). The average core ring thickness was 1.45 μm, supported by six struts (820 nm average thickness).
Fiber loss measurements were performed using the cutback method with a supercontinuum source (NKT Photonics SuperK Compact). The transmitted light was free-space coupled into an optical spectrum analyzer (ANDO AQ6315E). The measured optical loss spectrum for the fiber made at 10 mbar preform pressure is shown in Fig. 3. The measured optical loss for an unfilled fiber at the excitation wavelength of 488 nm was measured to be 21 dB/m, as opposed to 34 dB/m predicted by theory for a thick wall capillary with the same diameter as the fiber core diameter, showing the reduced optical loss of the single ring hollow core optical fiber at certain wavelength ranges. These results suggest that the thin walls of the fiber core are responsible for the waveguiding mechanism, likely due to antiresonance reflection optical waveguiding (ARROW) effects , although the measured fiber loss spectrum deviates from this ideal scenario due to the, as well as variations in the thickness of the core glass ring – in this case a standard deviation of 0.15 μm around the mean value of 1.45 μm (13% variation) [16, 18].
2.2 Sensing experiments
The fiber made using 10 mbar preform pressure was used for the Raman sensing experiments. The experimental setup used in the Raman sensing part of this work is shown in Fig. 4. 100 mW light from a 488 nm CW laser (Toptica iBeam SMART) was reflected on a long-pass Raman filter (Semrock 488 nm long-pass RazorEdge ultrasteep) and focused using a 40 × microscope objective; while a lower magnification objective would have been a better match between the optical mode diameter and the focused beam size, trials showed that a 40x objective resulted in the largest amount of coupled light into the optical fiber and was therefore used in the sensing experiments. Light from the fiber was collected in backscattering geometry through the same objective and was collected by a multimode patch cable connected to a cooled-CCD spectrometer (Horiba Jobin Yvon iHR320). To enable complete filling of the 20 cm length of fiber, and avoid air voids that could affect light coupling both ends of the fiber were inserted into end cap sample chambers. A cover slip (0.1 mm thickness) was used as the optical window of the sample chamber, and the sealed assembly was subsequently filled with methanol, resulting in all holes of the fiber structure being filled. This ensured both ends of the filled fiber were in contact with the cover slip, thus avoiding the formation of bubbles that can disrupt the filling and measurement process.
Initially the sealed fiber assembly was filled with methanol, and the pump beam focused into the free liquid volume inside the glass seal surrounding the fiber, as shown in Fig. 5(a). The collected spectrum, shown in Fig. 5(b), shows a contribution from the front glass window of the setup, visible as a series of broad peaks below 1500 cm−1 . At longer wavenumbers, the spectrum is dominated by the strong Raman methanol peaks  at 2832 and 2940 cm−1, as well as the broader peak around 3330 cm−1. When the pump light is primarily coupled into the outer cladding of the fiber, the spectrum contains a large contribution from the glass Raman background, visible as a series of peaks below 1500 cm−1, with a strong peak at 1000 cm−1 from the F2 glass , while the methanol signature disappears. The glass contribution is significantly reduced when light is coupled into the hollow core of the fiber, while the methanol signal is stronger than the signal collected from the bulk methanol solution using the same optical configuration but focusing in the liquid outside the fiber. We note here that our fiber is not designed to operate in the index-guiding regime, a method used for Raman sensing in hollow core optical fibers in the literature , and therefore provides a different approach to liquid filled hollow core fiber Raman sensing. These results are promising for extruded single ring hollow core optical fibers for Raman sensing.
In this work single ring hollow core optical fibers were fabricated using the extrusion technique for the first time. Special design of the internal die structure led to the core glass ring thickness of the preform being smaller than the corresponding die feature at the die exit. For our preform with slightly larger outer holes compared to the core hole, increasing pressurization results in progressively larger outer holes and a thinner outer jacket that ultimately limits the mechanical stability of the fibers and the core glass ring thickness. The fabricated single ring hollow core fibers have lower optical losses than the equivalent thick wall capillaries in selected spectral windows. Optical loss in these fibers can be reduced by further refinement of the fabrication technique in order to reduce the thickness of the core ring thickness in the preform as well as any variations in that thickness, while reducing the size of the nodes. For a single inflation pressure as used here, the dimensions and design of the extrusion die need to be tailored to achieve equal size for the core hole and outer holes in the preform to ensure balanced inflation of both types of holes. We have demonstrated the use of these single ring hollow core optical fibers for Raman sensing of methanol, a promising first step in using extruded single ring hollow core optical fibers for chemical sensing applications.
Georgios Tsiminis and Kristopher Rowland acknowledge financial support from Super Science Fellowships provided by the Australian Research Council. Tanya Monro acknowledges support from an ARC Georgina Sweet Laureate Fellowship. The authors would like to thank Alastair Dowler for fiber fabrication. This work was performed at the OptoFab node of the Australian National Fabrication Facility utilizing Commonwealth and South Australia State Government funding.
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