The effect of environmental acoustic noise on optical fibre based velocity and vibration sensor systems

Interferometric fibre optic sensors are increasingly finding applications 'in the field'. Examples are provided by the use of fibre-based laser Doppler velocimetry (LDV) systems for wind tunnels and by laser vibrometry systems used for example in the monitoring of machine tool wear. Both the wind tunnel and the factory floor are harsh environments for interferometric sensors. In particular the pick-up of acoustic noise from the air or from vibrating surfaces is a potentially serious problem, broadening and distorting the signal spectrum. A new broadening mechanism, called environmental noise broadening, is discussed and the magnitude of this effect is estimated for typical LDV systems. For flow velocity measurements in wind tunnels, this broadening leads to an overestimate of the turbulence intensity which is more serious for low-speed flows. The statistical formalism introduced here is applicable to any type of fibre-linked extrinsic interferometric sensor.