Miniaturised gap sensor using fibre optic Fabry-Perot interferometry for structural health monitoring

A miniaturised structural health monitoring device has been developed capable of measuring the absolute distance between close parallel surfaces using Fabry-Perot interferometry with nm-scale sensitivity. This is achieved by fabricating turning mirrors on two opposite cores of a multi-core fibre to produce a probe with dimensions limited only be the fibre diameter. Two fabrication processes have been investigated: Focused ion beam milling, which has resulted in a sensor measurement accuracy, sensitivity and range of +/- 0.056 mu m, +/- 0.006 mu m and similar to 16000 mu m respectively; and ultrafast laser assisted etching of the cleaved fibre end, where a sensor measurement accuracy, sensitivity and range of +/- 0.065 mu m, +/- 0.006 mu m and similar to 7500 mu m have been demonstrated.