Stephen C. Warren-Smith

Suspended nanowires: Fabrication, design and characterization of fibers with nanoscale cores

We report a new approach for the fabrication of nanowires: the direct drawing of optical fibers with air suspended nanoscale cores. The fibers were made from lead silicate glass using the extrusion technique for preform and jacket tube fabrication. Fibers with core diameters in the range of 420-720 nm and practical outer diameters of 110-200 microm were produced, the smallest core sizes produced to date within optical fibers without tapering. We explored the impact of the core size on the effective mode area and propagation loss of these suspended nanowires relative to circular nanowires reported to date. As for circular nanowires, the propagation loss of these suspended nanowires is dominated by surface roughness induced scattering.

Enhancement of fluorescence-based sensing using microstructured optical fibres

We develop a generic model of excitation and fluorescence recapturing within filled microstructured optical fibres (MOFs) with arbitrary structure and demonstrate that the light-matter overlap alone does not determine the optimal fibre choice. Fibre designs with sub-wavelength features and high-index glasses exhibit localised regions of high intensity, and we show that these regions can lead to approximately two orders of magnitude enhancement of fluorescence recapturing. Here we show how this regime can be exploited for sensing and demonstrate experimentally in-fibre excitation and fluorescence recapturing within a filled, solid-core MOF.

Sensing in suspended-core optical fibers

Introducing nanostructures and coatings into optical fibers creates new ways of interacting light with materials, and powerful new platforms for sensing. Advances in the design, fabrication and application of fibers to chemical and biological sensing will be described.

Antibody immobilization within glass microstructured fibers: a route to sensitive and selective biosensors

Glass microstructured optical fibers have been rendered biologically active for the first time via the immobilization of antibodies within the holes in the fiber cross-section. This has been done by introducing coating layers to the internal surfaces of soft glass fibers. The detection of proteins that bind to these antibodies has been demonstrated experimentally within this system via the use of fluorescence labeling. The approach combines the sensitivity resulting from the long interaction lengths of filled fibers with the selectivity provided by the use of antibodies.

Exposed-core microstructured optical fibers for real-time fluorescence sensing

New methods for fabricating glass exposed-core microstructured optical fiber are demonstrated. The fiber designs consist of an optical fiber with a suspended micron-scale core that is partially exposed to the external environment, which is particularly useful for sensing. These fibers allow for strong evanescent field interactions with the surrounding media due to the small core size, while also providing the potential for real-time and distributed measurements. The experimental performance of an exposed-core fiber is compared to an equivalent microstructured fiber with an enclosed (protected) core in terms of their performance as evanescent field sensors. We demonstrate that the exposed-core fiber can provide a significantly improved measurement response time.

Fluorescence-Based Aluminum Ion Sensing Using a Surface-Functionalized Microstructured Optical Fiber

The first microstructured optical fiber-based sensor platform for aluminum ions using a surface-attached derivative of lumogallion (3), a known fluorescence-based indicator, has been fabricated. These fibers allow for strong evanescent field interactions with the surrounding media because of the small core size while also providing the potential for real-time and distributed measurements. The fluorescence response to aluminum ions was first demonstrated by applying the procedure to glass slides. This was achieved through the covalent attachment of the fluorophore to a polyelectrolyte-coated glass surface and then to the internal holes of a suspended-core microstructured optical fiber to give an effective aluminum sensor. Whereas the sensor platform reported is fabricated for aluminum, the approach is versatile, with applicability to the detection of other ions.

Theoretical study of liquid-immersed exposed-core microstructured optical fibers for sensing

The absorption and fluorescence sensing properties of liquid-immersed exposed-core microstructured optical fibers are explored for the regime where these structures act as supported nanowires with direct access to the sensing environment. For absorption-based sensing we demonstrate that the amount of power propagating in the sensing region of the exposed-core fiber can compete with that of traditional MOFs. For fluorescence-based sensing, we see that in addition to the enhanced fluorescence capture efficiency already predicted for small-core, high refractive index contrast fibers, an improvement of up to 29% can be gained by using liquid-immersed exposed-core fibers. Additionally, calculation of the losses associated with interfaces between filled and unfilled sections predict significant benefit in using high refractive index substrate glasses for liquid-immersed exposed-core fiber sensing. This work demonstrates that, for fiber dimensions of interest, the exposed-core fiber is an attractive new sensor technology.

Enhanced fluorescence sensing using microstructured optical fibers: a comparison of forward and backward collection modes

A general model of excitation and fluorescence recapturing by the forward and backward modes of filled microstructured optical fibers (MOFs) is presented. We also present experimental results for both backward and forward fluorescence recapturing within a MOF as a function of fiber length and demonstrate a good qualitative agreement between the numerical model and experimental results. We demonstrate higher efficiency of fluorescence recapturing into backward modes in comparison with that of forward modes.

Exposed core microstructured optical fiber Bragg gratings: refractive index sensing.

Bragg gratings have been written in exposed-core microstructured optical fibers for the first time using a femtosecond laser. Second and third order gratings have been written and both show strong reflectivity at 1550 nm, with bandwidths as narrow as 60 pm. Due to the penetration of the guided field outside the fiber the Bragg reflections are sensitive to the external refractive index. As different modes have different sensitivities to refractive index but the same temperature sensitivity the sensor can provide temperature-compensated refractive index measurements. Since these Bragg gratings have been formed by physical ablation, these devices can also be used for high temperature sensing, demonstrated here up to 800°C. The fibers have been spliced to single mode fiber for improved handling and integration with commercial interrogation units.

Silica exposed-core microstructured optical fibers

We report the fabrication of silica microstructured optical fibers with the core exposed along the whole length, and characterize the stability of these new fibers when exposed to some typical sensing and storage environments. We show the fiber loss to be the best achieved to date for exposed-core fibers, while the deterioration in the transmission properties is up to ∼2 orders of magnitude better than for the previously reported exposed-core fibers produced in soft glass. This opens up new opportunities for optical fiber sensors requiring long term and/or harsh environmental applications while providing real time analysis anywhere along the fibers length.