John Holdsworth

Generation and propagation of gastric slow waves

1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle.

Gratings in Structured Optical Fibres

Grating writing in structured optical fibres and their properties and applications are reviewed. To date, most gratings have been written in a straightforward manner into structured fibres containing a photosensitive germanosilicate step-index core. However, gratings have also been written directly into single material, structured silica fibres and into air-clad cores using two and higher-photon processes with both UV and near IR pulsed (nanosecond-femtosecond) light. Given the intrinsic-added functionality possible within a structured optical fibre, structured fibre gratings offer further capabilities for sensors, diagnostics, lasers, and devices.

Improved field scanner incorporating parabolic optics. Part 1: Simulation

An unobstructed afocal scanning system design employing two off-axis parabolic reflectors as relay optics between two flat scan mirrors is proposed and investigated using OSLO optical software. It is found that, with a symmetric arrangement of the parabolic reflectors and appropriate selection of the first scan mirror rotational axis, the system provides linear scan lines at the image surface and excellent point spread function results in all scan positions. The design is functionally equivalent to a single-mirror scan engine and superior in every metric to a comparable dimension spherical mirror arrangement. This design is suited to two-dimensional laser scan engines and for confocal and two-photon microscopy in particular.

Spatio-spectral analysis of supercontinuum generation in higher order electromagnetic modes of photonic crystal fiber

The far-field spatial distributions of higher order electro-magnetic mode supercontinua were resolved spectrally and recorded. The supercontinua were created by precise control and direction of input pump energy offset axially from the photonic crystal fiber core. By processing the measured spectra, the spatial mode shape at each wavelength was determined. Discrete spectral features are associated with symmetrical spatial patterns arising from the host fiber geometry and suggest the electromagnetic mode pairing between the longer wavelength solitons and associated visible dispersive waves. Clear differences between supercontinua generated in fundamental and higher order electromagnetic modes exist. These data should inform theoretical studies as the solitons and the dispersive wave generated by fission may be matched by spatial orientation of the electromagnetic mode that both occupy.

Rotationally Variant Grating Writing in Photonic Crystal Fibres

The role of rotational alignment during grating writing is comprehensively examined by the measurement of core luminescence from a Ge-doped 12 ring photonic crystal fibre, computer simulation of transmission and, definitively, by inscription of gratings at three angles- 0°, 30° and the optimum luminescent angle for this fibre, 21.5°. All experi- mental results were in accordance with each other and in agreement with simulation. Control of the rotational alignment of the fibre is demonstrably critical for reproducible grating writing.

MEASUREMENT OF WEAK TRANSMITTANCES BY STOCHASTIC RESONANCE

We use stochastic resonance to measure weak transmittance amplitudes that are below the instrumental detection limit. Gaussian noise is added to the subthreshold (chopped) transmittance signal T(t) before detection by a crossing detector that uses a dc reference signal B>0. Without noise, no measurement is possible because T(t)<B. A fourfold improvement has been obtained in the detection limit, permitting the measurement of amplitudes that are as small as 0.25B.

Molecular versus crystallite PCBM diffusion in P3HT:PCBM blends

The diffusion of PCBM in P3HT:PCBM blend films has been investigated using multi-wavelength scanning absorption microscopy (MWSAM). By studying the depletion of PCBM in the vicinity of the phase segregated PCBM-rich regions that form upon thermal annealing, we are able to measure the diffusion constant and activation energy for PCBM diffusion in P3HT:PCBM blend films. The measured kinetic parameters are consistent with the diffusion of nanoscale PCBM crystallites rather than molecular PCBM. We show that the presence of two distinct diffusion processes in these blend materials provides an explanation for the large differences that have been reported for PCBM diffusion in P3HT:PCBM blends. This insight allows us to develop a unified model for PCBM mass transport in these materials.

A comparison of Delayed Self-Heterodyne Interference Measurement of Laser Linewidth Using Mach-Zehnder and Michelson Interferometers

Linewidth measurements of a distributed feedback (DFB) fibre laser are made using delayed self heterodyne interferometry (DHSI) with both Mach-Zehnder and Michelson interferometer configurations. Voigt fitting is used to extract and compare the Lorentzian and Gaussian linewidths and associated sources of noise. The respective measurements are wL (MZI) = (1.6 ± 0.2) kHz and wL (MI) = (1.4 ± 0.1) kHz. The Michelson with Faraday rotator mirrors gives a slightly narrower linewidth with significantly reduced error. This is explained by the unscrambling of polarisation drift using the Faraday rotator mirrors, confirmed by comparing with non-rotating standard gold coated fibre end mirrors.

Rare-earth doped eyewear provide for safer laser surgery

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Rotational dependence of laser light accessing photonic crystal fibre cores from the side

We investigate the fluorescence of an Er-doped aluminosilicate stepped index core with surrounding structured lattice when pumped transversely with the collimated output of a 980nm diode. A small rotational dependence is observed indicating the importance of cladding structure alignment during experiments that involve processing the core from the side, including Bragg grating writing.