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CABA Home & Building Automation Quarterly, Summer, pp. 18-19, 2003-06-01
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New fibre-optic temperature sensing system shows promise for fire
detection
Liu, Z. G.
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New fibre-optic temperature sensing system shows promise for fire detection
Liu, Z.G.
NRCC-46397
A version of this document is published in / Une version de ce document se trouve dans: CABA Home & Building Automation Quarterly, Summer 2003, pp. 18-19
New fibre-optic temperature sensing system shows promise for fire detection By Dr. Zhigang Liu
Fire researchers at IRC, in collaboration with the University of Ottawa, are studying the feasibility of a new fibre-optic-based fire-detection system. The system aims to reduce false alarms and provide early and long-distance detection of fires in areas with restricted access or difficult ambient conditions, such as in aircraft, tunnels, underground railways and stations, telecommunications facilities, and nuclear and petrochemical plants. Fires that are not detected until they are fully developed are a serious problem and can result in extensive property damage and loss of life. False fire alarms are also a serious challenge in fire detection.
To overcome drawbacks associated with traditional fire detectors, the new distributed optical fibre detector uses a standard optical fibre as the sensing medium. It detects temperature change by measuring the Brillouin back scattered intensity and its frequency shift as the variation in temperature alters the cable’s refractive indices and geometric properties. Although Brillouin scattering yields signals that are approximately 105 to 106 times weaker than the incident light, they can be enhanced through a process known as
Stimulated Brillouin Scattering (SBS) by sending light from two lasers into opposite ends
of the sensing fibre. Through modulation of a pulsed laser, its amplification will occur at the expense of the second continuous wave (CW) laser. This amplification process allows the fibre length to be significantly increased to approach 100 km owing to improved signal-to-noise ratios resulting from non-linear amplification in the fibre. Unlike a conventional thermal detector that determines temperature change at a single location, new technology can measure the temperature at any and every point along the cables. The fibre-optic cable system is much more sensitive to temperature fluctuations than conventional heat detectors because of its low mass. This increased sensitivity means that such a system is able to detect fires earlier than is now possible, and even detect small fires. The technology also reduces the problem of false alarms because the system detects temperature changes by light and is immune to many kinds of
interference. In addition, the cable is strong, resilient and flexible, and can be directly placed near or inside protected facilities, providing greater accuracy in terms of locating the fire and determining its size.
IRC researchers have worked closely with the University of Ottawa to carry out both theoretical and experimental studies on the improvement of the acquisition time of the distributed fibre-optic system for temperature measurement. Their research showed that there is a clear relationship between the Brillouin power gain and the fibre temperature. By directly measuring the intensity of the Brillouin gain/loss signal versus time, rather than sweeping through a range of frequencies, the fibre-optic system can achieve real-time temperature measurement, while maintaining good spatial and temperature
resolutions and high signal-to-noise ratios. In addition, their research demonstrated that the Brillouin power gain increases with fibre temperature and is maximized when the fibre temperature at a unique fibre location is equal to a threshold temperature, Tth. This
maximum Brillouin power gain can be used to trigger an alarm signal for fire detection purposes. This can be achieved by turning the frequency difference between the pump and pulsed lasers to match the Brillouin frequency of the fibre at a known threshold temperature, Tth, based on specified fire detection conditions. Once the Brillouin power
gain becomes a maximum owing to the occurrence of the fire, an alarm signal would then be triggered immediately, indicating that the threshold temperature has been reached. These experiments have successfully used long optical fibres (11 and 22 km) for
monitoring real-time temperature variations. The maximum light gain signal appeared at the pre-selected threshold temperature.
The initial research has demonstrated the feasibility of using distributed optical fibre sensors for fire-detection applications. The next step is to study and evaluate the performance of this technology in real fire conditions such as those sometimes found in telecommunications facilities, aircraft and tunnels.
_______________________________-
Dr. Zhigang Liu is a researcher in the Fire Risk Management Program of the National Research Council’s Institute for Research in Construction.
Those interested in joining this research project or those with specific questions should contact Dr. Liu at (613) 990-5075, fax (613) 954-0483, or e-mail zhigang.liu@nrc.ca.
Fires can be difficult to identify and locate in restricted or inaccessible spaces, such as those sometimes found in telecommunications facilities.
