A simple fibre optic sensor for measurement of vibrational frequencies

A non-contact non-interferometric fibre optic vibration sensor is described which is constructed from a 50/125 mu m multimode communication grade directional coupler, a modulated 820 nm LED and a PIN photodiode module. The sensing fibre is one arm of the directional coupler. The sensor is designed to detect small proximity variations between an unprepared reflective surface and the end of the sensing fibre. Vibrations of amplitude 1 mu m for mirrored surfaces and 4 mu m for unpolished aluminium can be detected for a sensor located approximately 50 mu m from the vibrating surface. The sensor output is reasonably linear for vibration amplitudes up to about 0.2 mm. The sensor uses synchronous detection circuitry based on common, low cost integrated circuits to improve the output signal-to-noise ratio by a factor of ten. The design of the vibration sensor is described in sufficient detail for it to be easily reconstructed at minimum expense.