Transverse Strain and Shape Monitoring using Novel Multicore Fibres
Manuel Silva-Lopez, Amanda Fender, Jim Barton, Bill MacPherson
Optical fibre Bragg gratings are one of the most promising optical fibre sensors for a range of physical measurements. In particular fibre Bragg gratings have seen widespread application for strain and temperature measurement in structural monitoring in aerospace, marine and civil engineering applications. Fibre Bragg gratings have also been successfully demonstrated for many other applications. They have many properties which make them attractive over other conventional sensors, such as the dielectric structure makes them insensitive to electromagnetic interference. Also their small physical size, flexibility and strength allow the optical fibres to be embedded within structures, most commonly glass- or carbon-fibre reinforced composite materials. The main operating advantage of optical fibre Bragg grating sensors is the measurand is directly encoded in the wavelength of the grating allowing absolute measurements to be made. This principal advantage gives rise to probably the most important advantage of all, the capability to multiplex many sensors onto a single fibre cable.
Transverse strain sensing
Fibre Bragg grating sensors are an active area of research world-wide and the technology has been extensively developed. However, most work on strain sensing concentrates on measuring the axial component along the length of the fibre. However when fibre Bragg grating sensors are embedded in various materials there strain field is often complex and strain from different directions affect the grating. The measurement of transverse strain, i.e. strain orthogonal to the fibre axis is therefore important for embedded applications. This is important for structural monitoring in aerospace, marine, and civil engineering applications. There has also been significant research on temperature compensation for grating sensors. Many examples involve the use of a compound or dual sensor to measure two parameters from which axial strain and temperature can be determined. Some of these techniques increase the physical dimensions of the fibre, thereby complicating the process of embedding fibres in structures.
This project aims to investigate the development and use of novel grating structures in collaboration with the Photonics Research Group at Aston University to make quasi-distributed measurements of transverse strain and avoiding unwanted temperature sensitivity. Multi-parameter sensors will be investigated based on fibre grating structure and also novel fibre geometries will also be investigated.
A novel multicore fibre has been used to study the sensitivity to the transverse strain. The fibre cross section is illustrated in figure 1, the overall diameter is 125 mm and it has four cores arranged in a square matrix with a core spacing of 50 mm. Fibre Bragg gratings were written in each one of the four cores by our colleagues at Aston University. In general when a cylinder is compressed between two flat plates a stress field appears through the cross section, figure 2 shows the contour of this field. Therefore every core in the multicore
fibre samples a different amount of stress, depending of its
position, and the orientation of the load can be inferred from a
measurement of the strain induced by the applied stress.
Figure 1 : Cross section of multicore fibre Figure 2 : Model of response to transverse load
Bend (curvature) sensing
In many physical structures, particularly those in which size and weight are critical, the monitoring of strain and bending to make real-time shape measurements offers several performance advantages, beyond that of simply avoiding failure. In acoustic and electromagnetic antennae, monitoring deformation allows shape changes to be compensated in subsequent signal processing. Deformation measurements can also be used as inputs to active damping and shape control systems to control shape directly, or to locate the position of the end of a flexible member. Bend measurement can be achieved by using two strain gauges at known distances from the neutral plane of the bending; temperature cross-sensitivity can be avoided by keeping the two strain gauges at the same temperature. The bend or curvature sensitivity is directly proportional to the separation of the two strain gauges. For embedded sensors the size of the sensor is of importance and is ideally as small as possible.
To achieve small size bend sensors this project aims to investigate multi-core optical fibre sensors. This work aims to continue to develop previous work in the using multi-core optical fibre for 2-axes bend measurement. This has been previously demonstrated using interferometric sensors and more recently fibre Bragg gratings have been successfully inscribed into multi-core fibre for single axes bend measurement. This project aims to continue this work investigating different fibre grating structures in multi-core optical fibre as high sensitivity optical strain gauges to achieve a small scale bend sensor with intrinsic temperature insensitivity.
In a recent experiment, the multicore fibre containing four gratings, was placed in front of the output of a low-speed wind tunnel. The system was able to measure the wind speed and the direction of the wind via curvature measurement using the multicore fibre. This work may have applications in testing the aerodynamic forces over small-scale objects and in curvature-shape monitoring using multiplexing schemes.
- M. Silva-Lopez, C. Li, W.N. MacPherson, A.J. Moore, J.S. Barton, J.D.C. Jones, D. Zhao, L. Zhang, I. Bennion, Differential birefringence in Bragg gratings in multicore fiber under transverse stress, Optics Letters Vol. 29, No. 19, 2225-2227, 2004
- "Transverse loading of multicore fibre gratings", Manuel Silva-Lopez, Donghui Zhao, Cheng Li, William MacPherson, Gordon Flockhart, Andrew J. Moore, James Barton, Julian Jones, Lin Zhang, Ian Bennion, Second European Workshop on Optical Fiber Sensors (EWOFS'2004), Santander, (Spain, 2004, Paper S1-18) edited by Jose Miguel Lopez-Higuera and Brian Culshaw, Proc. of SPIE Vol. 5502, (SPIE, Bellingham, WA) pp.96-99, 2004 (ISBN 0-8194-5434-6)
- C R Furness, W N MacPherson, M Silva-Lopez, J S Barton, A J Moore, J D C Jones, D Zhao, L Zhang, I Bennion, "Deflection of Multicore Optical Fibre Under Aerodynamic Loading", Photon '04, Glasgow, 6-9 September 2004
- M J Gander, W N MacPherson, R McBride, J D C Jones, L Zhang, I Bennion, P M Blanchard, J G Burnett and A H Greenaway ‘Bend measurement using Bragg gratings in multicore fibre’ Electron Letts 36 (2000) 120-1
- M J Gander, D Macrae, E A C Galliot, R McBride, J D C Jones, P M Blanchard, J G Burnett and A H Greenaway 'Two-axis bend measurement using multicore optical fibre' Opt Comm 182 (2000) 115-121