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This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for military forward operating base security in degraded visual environments. A simple end-to-end model of such an imager is described, including a simple scene model based on transformations applied to visible and infrared imagery, optical aberrations, focal plane sampling, scan conversion, receiver performance and image processing algorithms. The use of such a model as a design tool is discussed, especially with regard to optimizing scan conversion and image processing algorithms. The expected performance of the latest imager design is predicted.

This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for base security in degraded visual environments. The discussion starts with the selection of the optimum frequency band. The trade-offs between requirements on detection, recognition and identification ranges and optical aperture are discussed with reference to the Johnson Criteria. It is shown that these requirements also affect image sampling, receiver numbers and noise temperature, frame rate, field of view, focusing requirements and mechanisms, and tolerance budgets. The effect of image quality degradation is evaluated and a single testable metric is derived that best describes the effects of degradation on meeting the requirements. The discussion is extended to tolerance budgeting constraints if significant degradation is to be avoided, including surface roughness, receiver position errors and scan conversion errors. Although the reflective twist-polarization imager design proposed is potentially relatively low cost and high performance, there is a significant problem with obscuration of the beam by the receiver array. Methods of modeling this accurately and thus designing for best performance are given.

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.

Optically broadband interferometric techniques have been developed into cost effective and practical systems that enable the discrimination of strain and temperature in fibre optic sensors. Recent developments of temperature-insensitive strain and shape-sensing fibre-optic sensors are described. A variety of approaches are discussed that have led to recent development of multicore, temperature-insensitive vector bend sensors.

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.

An adaptive optics system usually has three basic elements, a wavefront sensor, a deformable element, and a feedback scheme. Typically these components are a Shack-Hartmann sensor, a bimorph or segmented mirror, and a DSP solution for performing the necessary calculations. These components are expensive, and give rise to a complex optical and computational system. In this paper a novel implementation of an adaptive optics system will be discussed. The wavefront sensor is based on an IMP grating to measure the curvature of the incoming light. This sensor has been found to be robust to scintillation, so is applicable to horizontal propagation paths. An OKO technologies deformable mirror is used, and the feedback loop calculations run on a standard Pentium III computer using Windows 2000. Results from recent trials of the system correcting for errors over various horizontal propagation lengths will be shown. Additionally results using this system for laser beam propagation will also be discussed.

We describe the measurements of curvature made using Bragg gratings written in separate cores of a multicore optical fibre. The gratings act as independent, but isothermal, strain gauges. Thus the difference in Bragg wavelength between the gratings provides a measurement of local curvature.

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 Green's 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.

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.

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.

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].

The first measurements of curvature made using Bragg gratings written in separate cores of a multicore optical fibre are described. The gratings act as independent, but isothermal, strain gauges. The difference in Bragg wavelength between the gratings provides a temperature-independent measurement of the local curvature.