Measurement of temperature and strain over a wide range using fibre optic laser-based sensor systems

Fibre optic laser-based sensor systems show considerable advantages over passive sensors using spontaneous emission, due to the higher optical power delivered by the sensor and the narrow line width associated with the measurand. In recent years, the use of Fibre Bragg Grating (FBG)-based laser systems has enabled compact systems to be developed and recent work by the authors and others has shown the conditions under which high temperature operation of such fibre lasers can occur. As a result, a number of sensor schemes are possible, and this work will report on results taken on the measurement of both temperature and strain over the range from 22 to 500[degrees]C and 0 to 1200 respectively. The systems involved in such measurements use fibre lasers configured with a conventional uniform FBG coupled to a chirped grating to form the laser cavity, enclosed by a length of Er[sup]3+-doped fibre as the gain medium and pumped by 1480 nm laser source. In this way components derived from the optical communications field can readily be applied to optical instrumentation and sensing. The perturbation of the uniform grating by the measurand is used as the primary mechanism in these sensor systems and the availability of chirped gratings with a particularly broad bandwith, coupled with specially fabricated photosensitive fibre into which the FBGs are written has enabled a uniquely large measurement range of temperature, together with the determination of strain, to be achieved. This then offers the possibilities of measurements in situations which otherwise are difficult, e.g. furnace linings and aero-engines. The potential for multiplexing using such a system, to enable a broader range of probes to be utilized will be discussed.