Distributed water sensing using plastic optical fibres

Culshaw, Brian and Flockhart, Gordon and MacLean, Alistair and McCormack, John (2011) Distributed water sensing using plastic optical fibres. In: POF 2011, The 20th International Conference on Plastic Optical Fibres, 2011-09-14 - 2011-09-16.

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Water sensing systems using distributed detection techniques through which the presence or absence of water may be monitored and located along the length of an optical fibre have numerous potential applications in, for example, district heating systems and the water distribution network. A number of configurations based on silica fibres have been demonstrated in the past but there is understandable user caution since in all these configurations the water needs to be in close contact with the fibre itself. Consequently any minor damage to coatings – and these are typically exposed – allows water seepage to the surface of the silica and consequent vulnerability to catastrophic degradation. In this paper we assess the prospects which a plastic fibre base system may offer as a prospective distributed monitoring system for water and – potentially – for other aqueous parameters such as pH and ionic concentration which could be addressed using different interface detection materials. The principal benefit of plastic optical fibres lies in their ruggedness when exposed to water. The basic principle which we shall exploit is based upon configuring a cable structure in which the parameter to be monitored – here water – causes an interface material to swell. In turn this swelling material forces the optical fibre into contact with a mircobend inducing structure causing detectable optical loss. Varying the swelling material can assist in optimising the response to either particular aqueous solutions and/or to limitations imposed by other parameters such as temperature. Based upon our previous experience with silica fibre we investigated graded index POF as the optical transmission medium and assessed its sensitivity to microbend induced losses, both as a function of the period of the microbend applied and of the amplitude of the peak to peak displacements introduced by the microbender from the mutual axis of the fibre. Full details of these results will be presented in the paper but in summary we found that significantly greater displacements are required for POF and for the silica fibres and consequently the interface swelling material needs to offer significantly greater (by at least an order of magnitude) swelling capability in response to the aqueous trigger than we found with the silica precursor. These observations have significant implications for the design of a sensor system and we have produced a preliminary demonstration which certainly responds to the aqueous trigger but because of the different characteristics of the fibre response the sensor responds much more slowly and in the early configurations also exhibits more hysteresis. However for many applications for which a one off trigger is all that is required the convenience of the POF format and the inherent construction and architecture simplicity offer significant compensating benefits. Again more details will be presented in the final paper.