Fibre sensing : specifying components and systems

Culshaw, B. and Habel, W. (2004) Fibre sensing : specifying components and systems. In: Symposium on Optical Fiber Measurements, 2004-09-28 - 2004-09-30. (

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Optical fibres have been used in sensing for almost 40 years"'. The first ideas emerged even before the concept of low loss, low dispersion fibres appeared on the horizon. Fibre sensing now has evolved in a very different context from fibre communications. The communications industry is relatively homogeneous especially at the transmission level where fibres have undoubtedly contributed critically to the revolution in the way in which society functions. Communications is also the industry of large corporate interests and to function it relies heavily on interchangeable product and therefore well defined standards and protocols. The sensing industry is markedly different. This is the domain of the specialist and the small company servicing niche needs. The niches come with their own specific demands. Operating at the bottom of an oil well is radically different from operating in deep space but temperatures and strains are to he measured in both. Some impression of this diversity can he gleaned from the observation that within the UK estimates of the number of companies involved in sensing and measurement using all technologies start at around 1,000 and continue ever upwards - very much greater than the numbers involved in the communications business which of itself has a total turnover far exceeding that in sensing and measurement. These contrasts have a profound effect on the way in which the two industries are operated, standardised and regulated. These observations are essential background to any discussion on standards and specifications in the context of optical fibre sensors. This paper will endeavour to bring together the issues conceming standards and specification development within the industry. This is a fragmented task. The final customer is interested in measurement capability, reliability and cost. The manufacturer's task is to optimise these trade-offs within the constraints imposed through - typically ~ a small manufacturing enterprise. The manufacturer assembles his sensor system from components and often these components have been fabricated with an entirely different market in mind and are therefore less than perfectly suited to the need in question. There is always a technical risk implicit in using such components. The fragmentation is important. Chemical, biochemical, biomedical and physical measurands are all addressed using fibre based optical transducers. All operate in very different environments. Consequently, to enable some focus to this discussion, we shall concentrate here on issues concerning fibre sensors to measure physical measurands: strain, pressure, acceleration, temperature, direction and rotation are the most important of these. The main body of the paper starts with a basic description of the fibre sensor itself to highlight the critical interfaces and the areas in which standards and specification issues are foremost. There is actually relatively little standardisation material at the component level which directly supports the fibre sensor industry. There are some critical areas where recognised measurement processes and procedures would he extremely welcome. We briefly discuss the evolution of one of the most focused standards endeavours - that of fibre gyroscopes which is we believe unique in Optical Fibre Sensing in having been considered by a US based (IEEE) Standards Committee. We also mention some IEC activity. Finally this leads into a discussion on the standard processes which would he desirable, the standards which are needed and how these needs may be approached. To exemplify this we consider the specific case of the fibre Bragg grating.