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 1°C.
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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