Non-contact vibration measurement based on an extrinsic Fabry–Perot interferometer implemented using arrays of single-mode fibres

This paper presents a novel non-contact vibration-monitoring technique based on transient measurements from a Fabry–Perot interferometric displacement sensor implemented using single-mode fibre. Two different extrinsic sensor configurations are demonstrated using one-fibre and two-fibre arrays of 4/125 µm single-mode fibre butted with a gradient-index lens. The design concept, mathematical modelling and results are demonstrated. The sensor configurations exploit fringe discrimination over multiple orders. A working range up to 14 µm is demonstrated for the measurement of vibration amplitude at 1 kHz excitation using the one-fibre configuration. Similar tests using the two-fibre configuration indicated a displacement measurement up to 65 µm and vibration measurement up to 7 µm at 2.5 kHz. The two-fibre configuration is also assessed in conjunction with an absolute scheme for the measurement of vibration using dual-wavelength signal processing. In this scheme, two wavelength signals are captured from the sensing interferometer to provide unambiguous measurement of vibration direction.