Towards microstructured optical fibre sensors: surface analysis of silanised lead silicate glass

While protocols to surface functionalise silica glass platforms are well-established, the surface coating of other glass types have received limited attention. Here we fully characterise the surface attachment of a fluoroionophore on extruded lead silicate glass slides and demonstrate these slides as a model for investigating the surface chemistry in a microstructured optical fibre (MOF). This model system allows the utilization of multiple, complementary surface-sensitive techniques that cannot be used within the internal surface of the fibre structure. In characterising the fluoroionophore attachment, we observe that the fluorescence intensity from fluorescence imaging, the atomic nitrogen percentage measured by X-ray photoelectron spectroscopy (XPS), the carbonyl bond component (287.5 eV) in the XPS high resolution carbon spectrum, and Principal Component Analysis (PCA) of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) data can be used to provide relative quantification of the concentration of an attached fluoroionophore. We also show the first use of ToF-SIMS imaging and depth profiling of the Pb content within a glass substrate to provide information on the coverage provided by the coating and the relative thickness of an organic coating. Combined, these techniques provide a comprehensive picture of the coated glass surface that facilitates fibre sensor development.Herbert T. C. Foo, Heike Ebendorff-Heidepriem, Christopher J. Sumby, Tanya M. Monr

Extrusion of complex preforms for microstructured optical fibers

We report a significant advance in preform extrusion and die design, which has allowed for the first time the fabrication of complex structured preforms using soft glass and polymer billets. Structural preform distortions are minimized by adjustment of the material flow within the die. The low propagation loss of an extruded complex bismuth glass fiber demonstrates the potential of this advanced extrusion technique for the fabrication of novel soft glass and polymer microstructured fiber designs.Heike Ebendorff-Heidepriem and Tanya M. Monr

Surface analysis and treatment of extruded fluoride phosphate glass preforms for optical fiber fabrication

First published: 20 April 2016Fabrication of fluoride phosphate glass optical fibers using the extrusion method for preform fabrication has been studied using the commercial Schott N-FK51A glass. The extrusion step was found to create a surface layer of differing composition from the bulk glass material, leading to defects drawn down onto the optical fiber surface during fiber fabrication, resulting in high loss and fragile fibers. Similar phenomena have also been observed in other fluoride-based glasses. Removal of this surface layer from preforms prior to fiber drawing was shown to improve optical fiber loss from >5 dB/m to 0.5–1.0 dB/m. The removal of this surface layer is therefore necessary to produce low-loss fluoride phosphate optical fibers.Christopher A. G. Kalnins, Nigel A. Spooner, Tanya M. Monro, and Heike Ebendorff-Heideprie

High temperature sensing with single material silica optical fibers

We present recent developments in high temperature sensing using single material silica optical fibers. By using a single material fiber, in this case a suspended-core fiber, we avoid effects due to dopant diffusion at high temperature. This allows the measurement of temperatures up to the dilatometric softening temperature at approximately 1300°C. We demonstrate and compare high temperature sensing in two configurations. The first exploits a small section of single material fiber spliced onto a length of conventional single mode fiber, which operates through multimode interference. The second utilizes a type II fiber Bragg grating written via femtosecond laser ablation.Stephen C. Warren-Smith, Linh V. Nguyen, Heike Ebendorff-Heidepriem, and Tanya M. Monr

Ultra-simplified single-step fabrication of microstructured optical fiber

Manufacturing optical fibers with a microstructured cross-section relies on the production of a fiber preform in a multiple-stage procedure, and drawing of the preform to fiber. These processes encompass the use of several dedicated and sophisticated equipment, including a fiber drawing tower. Here we demonstrate the use of a commercial table-top low-cost filament extruder to produce optical fibers with complex microstructure in a single step - from the pellets of the optical material directly to the final fiber. The process does not include the use of an optical fiber drawing tower and is time, electrical power, and floor space efficient. Different fiber geometries (hexagonal-lattice solid core, suspended core and hollow core) were successfully fabricated and their geometries evaluated. Air guidance in a wavelength range where the fiber material is opaque was shown in the hollow core fiber.Cristiano M.B. Cordeiro, Alson K.L. Ng, Heike Ebendorff-Heideprie

Modal interferometric refractive index sensing in microstructured exposed core fibres

Optical fibre-based sensors measuring refractive index shift in bodily fluids and tissues are versatile and accurate probes of physiological processes. Here, we suggest a refractive index sensor based on a microstructured exposed-core fibre (ECF). By considering a high refractive index coating of the exposed core, our modelling demonstrates the splitting of the guided mode into a surface sensing mode and a mode that is isolated from the surface. With the isolated mode acting as a reference arm, this two-mode one-fibre solution provides for robust interferometric sensing with a sensitivity of up to 60, 000 rad/RIU-cm, which is suitable for sensing subtle physiological processes within hard-to-reach places inside living organisms, such as the spinal cord, ovarian tract and blood vessels.Ivan S. Maksymov, Heike Ebendorff-Heidepriem, and Andrew D. Greentre

Resist-free nanoimprinting on optical fibers for plasmonic optrodes

Nanostructure patterning on optical fibers enables miniaturized optrodes for photonic and plasmonic applications. Here we report a direct nanoimprint technique to produce high-quality nanostructure arrays on optical fiber endfaces. It has only one single step: imprinting optical fiber tips against a mold with nanostructures at the elevated temperature. This new method abandons resist used in traditional fiber-imprinting methods. Hundreds of fibers can be shaped simultaneously with one mold within minutes. The imprinted nanostructure arrays on optical fibers are transformed into plasmonic optrodes through metal deposition. Variation of imprint depths and mold patterns allows tailoring of the plasmonic resonances of these nanostructure arrays for high-performance refractometric sensing and on-fiber polarization. The sensitivity of 690 nm/RIU and figure of merit of 50 are both among the highest values for similar plasmonic nanostructure arrays. This resist-free nanoimprint paves the way towards a low-cost and high-throughput realization of plasmonic optrodes and their wide applications.Peipei Jia, Depeng Kong, Heike Ebendorff-Heideprie

Gravitational extension of a fluid cylinder with internal structure

First published online 3 February 2016Motivated by the fabrication of microstructured optical fibres, a model is presented for the extension under gravity of a slender fluid cylinder with internal structure. It is shown that the general problem decouples into a two-dimensional surface-tension-driven Stokes flow that governs the transverse shape and an axial problem that depends upon the transverse flow. The problem and its solution differ from those obtained for fibre drawing, because the problem is unsteady and the fibre tension depends on axial position. Solutions both with and without surface tension are developed and compared, which show that the relative importance of surface tension depends upon both the parameter values and the geometry under consideration. The model is compared with experimental data and is shown to be in good agreement. These results also show that surface-tension effects are essential to accurately describing the cross-sectional shape.Hayden Tronnolone, Yvonne M. Stokes, Herbert Tze Cheung Foo and Heike Ebendorff-Heideprie

Fabrication of imaging microstructured optical fibers

We demonstrate the fabrication of multi-core (imaging) microstructured optical fiber via soft-glass extrusion through a 3D printed die. The combination of 3D metal printing and extrusion allows for unprecedented control of the optical fiber geometry. We have exploited this to demonstrate a 100 pixel rectangular array imaging microstructured fiber. Due to the high refractive index of the glass used (n = 1.62), such a fiber can theoretically have a pixel pitch of less than 2 μm. This opens opportunities for ultra-small, high-resolution imaging fibers fabricated from diverse glass types.Stephen C. Warren-Smith, Alastair Dowler, Heike Ebendorff-Heideprie

Enhanced radiation dosimetry of fluoride phosphate glass optical fibres by terbium (III) doping

Published 1 Nov 2016Fluoride phosphate (FP) glass fibres have been developed for radiation dosimetry based on the mechanism of optically stimulated luminescence (OSL). Doping with Tb³⁺ ions improved the materials sensitivity; for samples melted in oxidising conditions, OSL intensity was increased from 8.8 × 10⁵ cnts/g/Gy for undoped glass to 812.3 × 10⁵ cnts/g/Gy for Tb³⁺-doped glass. The radiation sensor performance of Tb³⁺-doped glass fibres under both beta and X-ray irradiation demonstrated the capability of the fibres for radiation dosimetry applications.Christopher A. G. Kalnins, Heike Ebendorff-Heidepriem, Nigel A. Spooner and Tanya M. Monr