An Extrinsic Optical Fibre Sensor Employing A Partially Reflecting Mirror

A novel extrinsic optical fibre displacement sensor is described which intensity modulates an incoherent light source. Light is transmitted along multimode graded index fibre into a GRIN-rod lens coupler system. The transmission and reflection properties of this system are fully categorized. Linear movement of a partially reflecting mirror parallel to the axis of the lens coupler forms the basis of the measurement technique. This device exhibits several important features which makes it suitable for use in a process control environment. Firstly, the measurement is largely independent of the modal power distribution present at the sensor. Secondly, it can be made independent of any fluctuations in the source output power using only a single transmit and return fibre. Finally, the mechanism has a dynamic range of several millimetres which makes it eminently suitable for connection to a Bourdon tube thus providing measurement of both pressure and temperature. The characteristics of a prototype sensor in which a Bourdon tube provides conversion of the measurand to displacement of the mirror are reported, together with the requirements of the associated telemetry system. In addition a series of pressure measurements against detected optical power showing a linear relationship are presented which demonstrate the ability of the device to give an accurate measurement of both pressure and temperature.