Kristopher J. Rowland

Highly efficient excitation and detection of whispering gallery modes in a dye-doped microsphere using a microstructured optical fiber

A technique for the excitation of whispering gallery modes (WGMs) has been demonstrated using a dye-doped microsphere positioned onto the tip of a suspended core microstructured optical fiber. With this configuration, we have shown that both the excitation and collection efficiency of the WGMs modulated fluorescence spectra of the dye are greatly improved compared to a more conventional excitation scheme; an overall efficiency increase by a factor of 200 is demonstrated. It is also shown that positioning the resonator onto the fiber tip does not impact its sensitivity, providing a compact and robust architecture for applications such as localized in-vivo/vitro biosensing.

Bandgaps and antiresonances in integrated-ARROWs and Bragg fibers; a simple model

We consider the spectral properties of dielectric waveguides with low refractive index cores and binary layered claddings, such as Bragg fibers and integrated-ARROWs. We show that the full, nontrivial, 2-D spectrum of Bloch bands (hence bandgaps) of such claddings correspond, in structure and topology, to the dispersion properties of both constituent layer types; quantitatively demonstrating an intimate relationship between the bandgap and antiresonance guidance mechanisms. The dispersion functions of these layers, and the interactions thereof, thus form what we coin the Stratified Planar Anti-Resonant Reflecting OpticalWaveguide (SPARROW) model, capable of quantitative, analytic, descriptions of many nontrivial bandgap and antiresonance properties. The SPARROW model is useful for the spectral analysis and design of Bragg fibers and integrated-ARROWs with cores of arbitrary refractive index (equal to or less than the lowest cladding index). Both waveguide types are of interest for sensing and microfluidic applications due to their natural ability to guide light within low-index cores, permitting low-loss guidance within a large range of gases and liquids. A liquid-core Bragg fiber is discussed as an example, demonstrating the applicability of the SPARROW model to realistic and important waveguide designs.

Fluorescent polymer coated capillaries as optofluidic refractometric sensors

A capillary microresonator platform for refractometric sensing is demonstrated by coating the interior of thick-walled silica capillaries with a sub-wavelength layer of high refractive index, dye-doped polymer. No intermediate processing, such as etching or tapering, of the capillary is required. Side illumination and detection of the polymer layer reveals a fluorescence spectrum that is periodically modulated by whispering gallery mode resonances within the layer. Using a Fourier technique to calculate the spectral resonance shifts, the fabricated capillary resonators exhibited refractometric sensitivities up to approximately 30 nm/RIU upon flowing aqueous glucose through them. These sensors could be readily integrated with existing biological and chemical separation platforms such as capillary electrophoresis and gas chromatography where such thick walled capillaries are routinely used with polymer coatings. A review of the modelling required to calculate whispering gallery eigenmodes of such inverted cylindrical resonators is also presented.

Enhancing the radiation efficiency of dye doped whispering gallery mode microresonators

We present a novel form of a Whispering Gallery Mode (WGM) sensor that exploits dye doped polystyrene microspheres, as active resonators, positioned onto the tip of a Microstructured Optical Fiber (MOF) as a means of overcoming the limited Q-factors for small resonators. We show that it is possible to substantially enhance the fluorescence emission of selected WGMs of the microspheres, resulting in an increase of the signal-to-noise ratio of the modes and of the effective Q-factor. This is done by positioning the resonator into one of the holes of a suspended core MOF and matching the resonator diameter with the hole diameter where it sits, effectively breaking the symmetry of the environment surrounding the sphere. Furthermore we demonstrate that using this experimental configuration, the lasing efficiency of the dye-doped microspheres is also significantly enhanced, which also contributes to an enhancement in the observed Q-factor.

Novel Low-Loss Bandgaps in All-Silica Bragg Fibers

We demonstrate that higher order bandgaps in all-silica Bragg fibers can have modes with four orders of magnitude lower confinement loss than those using the fundamental bandgap. A scheme for exploiting the higher order gaps for any specific wavelength via a global scaling of the fiber geometry is proposed. This approach provides lower losses than by reducing the confinement loss of the fundamental gap by scaling the core. Using a variety of modeling techniques, we have examined the band structure and guidance of idealized air-core all-silica Bragg fibers. It is demonstrated that the higher order, low loss, bandgaps analyzed here are uniquely accessible to single-material Bragg fibers, and are fundamentally different from the higher order gaps typically associated with depressed-index Bragg fibers such as the ldquoOmniguiderdquo fibers. Further analysis suggests that some of the key features of the guided modes of Bragg fibers can be understood by considering the properties of single hollow-core homogeneous dielectric waveguides (ldquoboreholesrdquo).

Bragg waveguides with low-index liquid cores.

The spectral properties of light confined to low-index media by binary layered structures is discussed. A novel phase-based model with a simple analytical form is derived for the approximation of the center of arbitrary bandgaps of binary layered structures operating at arbitrary effective indices. An analytical approximation to the sensitivity of the bandgap center to changes in the core refractive index is thus derived. Experimentally, significant shifting of the fundamental bandgap of a hollow-core Bragg fiber with a large cladding layer refractive index contrast is demonstrated by filling the core with liquids of various refractive indices. Confirmation of these results against theory is shown, including the new analytical model, highlighting the importance of considering material dispersion. The work demonstrates the broad and sensitive tunability of Bragg structures and includes discussions on refractive index sensing.

Single-ring hollow core optical fibers made by glass billet extrusion for Raman sensing.

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.

Excitation and lasing of whispering gallery modes in dye doped microspheres at the tip of a microstructured optical fiber and application for a sensitive dip sensor architecture

A new concept for exciting whispering gallery modes (WGMs) using small core microstructured optical fibers (MOFs) is presented. Here a 10 μm spherical dye doped micro resonator was positioned onto the tip of the MOF, and the application of this device for refractive index sensing applications is presented. With this configuration, both the excitation and collection efficiency of the WGM modulated fluorescence spectra of the dye are found to be greatly improved compared to the more traditional excitation scheme in which the resonator is attached to a glass slide and excited using a confocal microscope. This novel MOF-tip configuration provides a more compact and robust architecture for in vivo/vitro biosensing applications. It is also shown that the same architecture can be used to operate the dye doped resonator beyond its lasing threshold, resulting in improved performances.

Whispering gallery mode and surface plasmon resonance based refractometric sensors

We present four refractometric sensing platforms based on whispering gallery mode (WGM) modulated fluorescence spectra from active microsphere or capillary microresonators and scattered surface plasmon (SP) spectra from rough thin metal coatings and metal nanoparticles in waveguide and fibre dip configurations, respectively. Redshifts of the detected spectra upon increasing sample refractive index are demonstrated for each platform; label-free bio-sensing is also demonstrated.

A fiber tip label free biological sensing platform for in vivo applications

The novel platform presented in this paper is design to answer the unmet challenge of real time label free in-vivo sensing by bringing together a refractive index transducing mechanism based on Whispering Gallery Modes in dye doped microspheres combined with Microstructured Optical Fibers. In addition to providing the provides the remote excitation and collection of the WGM signal, the fibre also allows easily manipulation of the microresonator and the use this sensor in a dip sensing architecture, alleviating the need for a complex microfluidic interface. Here, we demonstrate the ability of this sensing platform to be operated above its lasing threshold, enabling enhanced performance.