Optical fibre pressure and strain sensors for Smart structures

Abstract

The paper will describe a number of optical fibre sensor concepts suitable for measurement of isotropic pressure or anisotropic strain. These sensors are capable of measuring pressure in hydraulic actuation systems and/or strain in structures. Three basic types of sensor have been researched: in-fibre gratings, micro-miniature Fabry-Perot devices and sensors based on optical delay measurement within conventional hire. An intrinsic feature of all the options is that they are hermetically-sealed against the ingress of water, and are therefore more likely to be practical for real engineering applications. The in-fibre grating sensor is a highly attractive option for single point sensing over a short gauge length. The sensor is a length of fibre with a periodic variation in refractive index along its length. This index variation is produced by focusing two converging ultraviolet laser beams onto the fibre. Work at the ORC has recently resulted in the production of high-performance gratings, with up to 100% reflectivity if desired. The gratings can also be written into conventional monomode fibres, in some cases with a single "shot" from a pulsed laser. Hollow glass microspheres have been investigated for their feasibility as pressure sensors. These spheres can be bonded, (in an on-axis position), to the end of a monomode fibre and, without need for additional optics, form a miniature low-finesse Fabry-Perot sensing head. This has a reflected spectrum which vanes periodically with optical frequency. The frequencies of peak reflectivities are dependent on pressure. The spheres are of similar dimensions to the optical fibre (~ 100µm) and hence should have an extremely rapid response and high mechanical resonant frequency. Several longer-gauge length sensors for pressure or strain monitoring are being investigated. One method involves a transversal filter concept. (A two-path optical fibre system, where signals are first split then recombined, the resulting signal then being dependent on the relative delays between the paths). A second long-gauge-length sensor involves the optical concept of white-light interferometry. The presentation will describe these methods to attendees who may not be optical specialists and will explain the applications (advantages and disadvantages) of optical sensors for structural monitoring.

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