Optical fibre nanowire sensors and applications

Optical fibre nanowires (OFN) have recently attracted increased attention because of their numerous applications in sensing and particle manipulation [1] and their extraordinary optical and mechanical properties, which include, amongst others:biocompatibility: OFNs show good compatibility with cells/ biological matter as they are made from silica.configurability/flexibility: OFN are manufactured stretching optical fibres, thus they maintain their original size at the extremities (fig. 1), allowing for prompt connection to any fiberised source/detector. robustness: OFNs are extremely strong and have a conventional fibre pigtail at their extremity, thus can be handled with tools typical of the macroscopic world.large evanescent fields: a considerable fraction of the transmitted power can propagate outside the OFN physical boundary when the OFN size is small, allowing for enhanced sensing and optical manipulation. strong confinement: when OFNs are nanostructured and metal coated, light can be confined to 50-100 nm spot sizes, allowing for nanosensing and single nanoparticle trapping/manipulation

Comparative study of the effective single mode operational bandwidth in sub-wavelength optical wires and conventional single-mode fibers

We present the first experimental comparison of effective single mode operation bandwidth in sub-wavelength optical wires (SOWs) and conventional single-mode fibers (SMFs). The full transmission spectrum, half-turn bend loss and mode field diameter were measured and compared for a variety of SMFs of different cut-off wavelength and a SOW. The SOW was shown to offer an enormously broadband single-mode operation bandwidth with a larger mode field area than the SMFs. Applications of SOWs include fiber lasers, sensors, photolithography and optical coherence tomography amongst other

Optical properties of a low-loss polarization maintaining microfiber

A polarization preserving single-mode microfiber was successfully fabricated by a flame brushing method. A polarization extinction ratio of 16dB is typically maintained through the device with excess loss of 0.2dB

SEM characterization of mm-long nanowires

The fabrication of optical fibre nanowires has recently attracted much attention [1-5]. Nanowires longer than 110mm [2] and with diameters smaller than 20 nm [5] have now been fabricated using a top-down approach. Because of the extraordinarily large ratio between length and diameter (>100000), the characterization of optical fibre nanowires requires instrumentation capable of measuring lengths over a range spanning more than five orders of magnitude. In our experiments dimensional characterization along the nanowire has been performed using an SEM and calibrated references. The samples are first attached to conductive carbon pads to avoid electrostatic build-up. Charging makes accurate metrology difficult because the electron beam can be deflected by the induced electric field on the sample. Fig. 1 illustrates a nanowire with a radius r=30nm wrapped around a microfibre with r=2µm. Variations in radius of another nanowire along its length are shown in fig. 2

Polarisation effects in optical microcoil resonators

Optical microcoil resonators (OMRs) fabricated by wrapping a microfibre around a rod to allow evanescent coupling between adjacent turns as in Fig 1. (a) have recently attracted much interest due to their high Q-factor and large extinction ratios resonances, low input and output coupling losses, large evanescent field and compactness [1,2], with applications such as sensing [3] and signal processing [4]. However, theoretical models published so far have neglected polarisation effects, and hence in order to develop a more detailed understanding we have modelled the OMR with polarisation-dependent coupled mode equations in the linear [5] and nonlinear regimes

Focused ion beam engraved phase-shifted Bragg grating microcavity resonator

A cavity with minimal-volume confinement was created in a microfiber engraving a high-contrast phase-shifted Bragg grating by focused ion beam. While waveguiding by the air/silica boundary provides a diffraction-limited 2D confinement, the grating introduces the third degree of confinement. Theoretical simulations verified the phase-shifted cavity confinement and showed a reasonable agreement with the experimental demonstration. This cavity can find a variety of applications ranging from sensing to quantum dynamic experiments

All-fiber fused directional coupler for highly efficient spatial mode conversion

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP01 mode is coupled into the higher order mode (LP11, LP21, LP02) in the other arm, over a broadband wavelength range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry

Conical and bi-conical high-Q optical nanofiber microcoil resonator

The Q-factor of the optical nanowire microcoil resonator is calculated and compared for different geometries. The results suggest that the Q-factor is very sensitive to the coupling conditions and high-Q resonators can be obtained more easily when the geometry of the nanowire microcoil resonator or its coupling contour has a bi-conical profile

Fast-response high-temperature microfiber coupler tip thermometer

A compact temperature sensor based on a broadband microfiber coupler tip is demonstrated. The thermometer dynamic range spans from room temperature to 1511°C with a response time of tens of ms. This is the highest temperature measured with a silica optical fiber device. A resolution of 0.66°C was achieved for a coupler tip diameter of ~12.56 µm. Better resolution can be achieved with smaller size microfiber coupler tips