Industrial Fiber Optics IF-FOM, Руководство оператора

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Measuring Attenuation in Optical Fiber
Overview
Fiber optic cables offer many advantages over copper cables, including lighter weight,
higher information carrying capacity, and lower signal loss. Eventually as light travels
through a fi ber some optical signal loss or attenuation does occur. The two main causes
of optical attenuation are scattering and absorption. Scattering is the redistribution of a
light beam from a single direction into many or all directions. In an optical fi ber scattering
is the result of imperfections in density and composition that are natural by-products of
manufacturing. Light rays hitting these imperfections are scattered, reducing the power in
the intended direction. Absorption is a process where impurities in the fi ber absorb optical
energy and dissipate it as heat. (Impurities occur during the manufacturing process when
unwanted chemical ions and other substances are introduced into the fi ber.)
Measuring attenuation in optical fi bers is important because it allows fi ber optic system
designers to predict the power which will be lost through a fi ber optic cable link. For long-
distance transmission, multiple optical links are used with repeaters to replace the power
lost to attenuation. Repeaters amplify weak signals and restore the power to a level high
enough to send the signal down the next link. By knowing the attenuation and other losses
in fi ber optic cable links, a designer can budget the correct quantity of expensive
repeater systems.
Procedure for measuring attenuation — Cutback method
Two ways to measure attenuation in fi ber cables are the cutback method and comparison to
a known reference cable. With the cutback method, optical power is launched into a cable
and the power coming out the end is measured. The cable is cut back by a known length
and the increase in output power is measured. The two readings are used with the cutback
length to calculate the loss per unit length of the cable.
The other approach is to launch power into a known reference cable and measure the
output. It is then replaced by the cable under test and the readings are compared.
The cutback method sacrifi ces some cable length but doesn’t require a reference cable be-
cause the measurement is relative. It is also less prone to error caused by changes in launch
conditions because the optical source is never disconnected. You can measure attenuation
with the Fiber Optic Test Set using the built in LED or an external optical source to launch
energy into the fi ber.
It is important that the cable being tested is long enough to get a good change in reading
when it is cut back. For plastic cable, a starting length of 10 meters is recommended.