Fiber Optic Instrumentation Networks

About the fiber optic instrumentation networks

Little if any saving arises out of installing fiber optic links in instrumentation networks; indeed there may be a cost penalty. However, there are great advantages in terms of the links' immunity to contamination of the signals carried. Immunity to current surges caused by stray electromagnetic fields also affords protection to computers in the network, as such surges can cause software corruption.

As with fiber optic sensors, the cost of short fiber optic links in instrumentation networks is dominated by the cost of the terminating transducers. However, as the length of the link becomes greater, the cost of the fiber optic cable becomes more significant. The cheapness of plastic cables is attractive for instrumentation networks but they cannot generally be used over distances greater than about 30m because signal attenuation is very high.

One disadvantage of fiber optics compared with electrical conductors in networks is that light connections at the ends of the cable are much more costly than electrical connections. Branches are impossible with glass and silica fibers, though a technique is available for creating branches in plastic cables.

Current research is looking at ways of distributing a range of discrete sensors measuring different variables along a fiber optic cable. Alternatively, sensors of the same type, which are located at various points along a cable, are being investigated as a means of providing distributed sensing of a single measured variable. For example, the use of a 2km long cable to measure the temperature distribution along its entire length has been demonstrated, measuring temperature at 400 separate points to a resolution of 1C.

Various types of branching network and multiplexing schemes have been proposed, some of which have been implemented as described in Grattan (1989). Wavelength division multiplexing is particularly well suited to fiber optic applications and the technique is now becoming well established. A single fiber is capable of propagating a large number of different wavelengths without cross-interference, and multiplexing thus allows a large number of distributed sensors to be addressed. A single optical light source is often sufficient for this, particularly if the modulated parameter is not light intensity.

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