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17 Jun 2002
Fiber Bragg grating sensor networks are measuring the stresses in the hulls of two navy ships.
From Opto & Laser Europe magazine.
Norway's navy has installed fiber Bragg grating (FBG) sensor networks on two of its ships. The networks measure stress in the ships' hulls, allowing the vessels to tackle rough seas with greater confidence (Meas. Sci. Tech. 13 471).
Karianne Pran and colleagues at the Norwegian Defence Research Establishment in Kjeller designed and built the FBG systems, which feature 50 sensors and use superluminescent diode sources emitting at 1550 nm.
Pran told OLE that a novel design allows more sensors to be put into each optical channel. "We have systematically assigned overlapping wavelength intervals for the gratings, based on finite-element modelling of the hull. The gratings never occupy the same wavelength at the same time, even if they sometimes attain the same wavelengths."
Polarimetric optical sensors have been used in other hull sensors, but Pran says that these showed too much drift in sensor signal, and that compensating the strain measurement for temperature effects proved difficult. "Using FBGs in an application like this is much easier, especially because the measurands are wavelength-encoded - which makes the system immune to variations in optical power."
The networks are installed in patrol boat HNoMS Skjold and mine counter-measure vessel HNoMS Orkla, which operate in rough seas along the Norwegian coast. Using the monitoring system, the ships can now tackle seas close to their design limits. The system could also increase the lifetimes of civilian ships by allowing their crews to monitor and limit the stress on the hull.
Pran and colleagues used FBGs supplied by Innovative Fibres of Canada, a tunable Fabry-Perot filter from Micron Optics, US, and a superluminescent diode from OptoSpeed, Switzerland. A new packaging design has improved sensor lifetime and upgraded electronics has improved low-frequency performance.
Pran and a colleague have set up a company, Light Structures, to commercialize the work.
Author
Michael Hatcher is technology editor of Opto & Laser Europe magazine.
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