James G. Burnett

Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre

We demonstrate an all-fibre curvature sensor that uses two-core photonic crystal fibre (PCF) as the sensing element. The PCF acts as a two-beam interferometer in which phase difference is a function of curvature in the plane containing the cores. A broadband source illuminates both cores, and the spectrum at a single point in the far-field interferogram is recorded. Applying a three-wavelength phase recovery algorithm to the data provides an unambiguous measurement of the interferometer phase, and hence curvature.

Two-dimensional bend sensing with a single, multi-core optical fibre

Measurement of two-dimensional bending in a structural element using intrinsic optical fibre strain gauges would normally require three sensors to be attached to, or embedded within, the structure. The same measurement can now be made using a single multi-core optical fibre, reducing deployment cost and increasing practicality. Fabrication of a novel three-core photonic crystal fibre is described. The ability of the fibre sensor to measure bend in two dimensions is demonstrated in the laboratory using interferometric interrogation at a single wavelength. Deployment of the sensor to measure the deformation of a bridge undergoing loading trials is described.

Measurement of bending in two dimensions using multicore optical fibre

We describe the use of a four-core optical fibre as the basis of a sensor capable of measuring the angle through which the fibre is bent in two dimensions. The intended application of the sensor is in measuring the shape of flexible structures.

Channeled spectrum interrogation of an all-fibre broadband interferometric differential strain sensor

Optical fibre interferometric strain sensors embedded into structures offer a very accurate and robust method for shape measurement [1]. Many schemes have been demonstrated in which strain and/or temperature in a structure are inferred from monochromatic optical phase delay [2].

Measurements of low-level atmospheric turbulence

Phase-diversity wavefront sensing has been implemented for the measurement of turbulence-distorted atmospheric wavefronts in applications of adaptive optics for essentially-horizontal propagation paths. The selected implementation of phase-diversity provides a wavefront sensor capable of estimating atmospheric distortions when observing extended scenes and provides a range-weighted sensing of the atmospheric distortions dependent on the angular region of the scene used for measurement. The data inversion, based on a Greens function analysis, is fast and robust enough for real-time implementation. For measurements of the atmospheric properties this wavefront sensor is being used with bright, compact sources to give high signal to noise measurements for integrated atmospheric effects along defined optical paths. The implementation used facilitates measurements of the atmospheric distortions along separate propagation paths. By simultaneous measurements along 3 separate paths a library of spatio-temporal atmospheric distortions and information about the isoplanicity of the distortions will be compiled for use in assessing applications of adaptive optics in horizontal propagation conditions. The principles of measurement, the details of implementation and some preliminary results will be described.

Optical propagation through low-level turbulence

Turbulence effects close to the air-ground interface may be expected to be non-Kolmogorov, even if that model is an adequate description of free-air turbulence effects. Direct measurements of the optical effects of propagation through the boundary layer are therefore required and are being undertaken as part of a program in which various potential applications of adaptive optics are being examined. The measurements are intended to characterize the spatio- temporal characteristics of optical wavefronts after propagation through the air-ground boundary layer. The objective in these measurements is to describe the level of performance that will be required in an adaptive system intended to mitigate the deleterious effects of atmospheric propagation on image formation and on other optical measurements. The principles of measurements and the preliminary results are presented.

Two-dimensional bend sensing with a single multiple-core optical fibre

The shape of a structure which is known to deform in one-dimension only can be measured using a single optical fibre with one or more sensing elements multiplexed down its length. However, to monitor the shape of a structure that can deform in three-dimensions the full vector strain, or strain field, is required. To achieve this a minimum of three fibres, embedded in a non-collinear geometry, is required.