Remote optical excitation and sensing of structural vibration

This thesis is concerned with non-contact techniques for optical excitation of mechanical vibrations in structures and with the use of non-contact fibre optic interferometers for the measurement of parameters which characterise these mechanical vibrations. Taken together, the excitation and sensing systems provide a method for fully remote non-destructive testing (NDT) of structures, which can sense changes in any parameter which affects the vibrations. The technique is applicable to a wide range of structures but is particularly suitable for use with delicate structures or structures for which the use of conventional excitation and sensing techniques would load the structure and modify the parameters being measured. The ultimate usefulness of this NDT technique depends on the accuracy with which important vibrational parameters such as resonant frequency and damping can be measured. In this thesis, transverse mechanical vibrations are thermoelastically excited in structures using sinusoidally modulated 800 nm laser diodes with peak-to-peak powers of the order of 25 mW. The vibrations are sensed using a modified form of a fibre optic Mach-Zehnder interferometer, which uses a 2 mW He-Ne laser source. The interferometer senses changes in optical path between the partially reflecting unpolished surface of the vibrating structure and the end of an optical fibre placed at a distance of 100 -150 mm from the surface. The measurements have been made using a very lightly damped structure (a high Q cantilever) under three separate environmental noise conditions. Data was collected for the resonant frequency and also for the related loss-dependent parameters of half-power bandwidth and damping factor for the first 5 vibrational modes. A number of ways of making these measurements were tried and the advantages and disadvantages of each were identified. The thesis identifies the accuracy with which each of the vibrational mode parameters can be determined and the origin of the effects, which are responsible for the ultimate limitation on this accuracy