Dual Wavelength Intensity Modulated Optical Fibre Sensor System

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Abstract
A novel dual wavelength intensity modulated optical fibre displacement sensor system is described which employs a single Graded Index (GRIN) rod lensed fibre together with an interference filter and moving mirror element in the sensing head. The dual wavelength approach essentially utilises two spectral slices for a single LED transmission spectrum to provide the measurand and reference signals. In this way the sensor system is fully referenced for all major common-mode variations that may be present within the system components. The terminal transceiver unit is also discussed together with a specially designed LED coupler and demultiplexer device which enables the system to operate with a single fibre connection. The performance characteristics of the prototype dual wavelength sensor system are reported showing a linear displacement range over some 20 mm. In addition the coupling of the sensing head to a Bourdon tube is described which provides accurate pressure measurement over a range 3-15 psi. Finally an initial investigation of the immunity of the system to common-mode variations is presented using the mechanism of fibre bend loss.

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