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Fiber Fundamentals

Nov 21, 2011 10:00 PM, By Bennett Liles

A refresher course on the latest in fiber-optic technology.


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CHECKING AND TESTING

Fiber-optic cable is not perfect. With all its advantages in induced noise rejection, immunity to inductive eavesdropping, and being impervious to ground loops, it is especially vulnerable to contamination by dirt and other foreign matter at the junction points, as well as excessive bending. Optical power can be measured through a fiber-optic system with a light meter calibrated for the specific wavelengths used, and the condition of the cable can be checked with an instrument called an optical time-domain reflectometer (OTDR). This instrument contains a very precise timing mechanism that measures the length of time a signal takes to be sent down the fiber and return. Any imperfections or faults in the line can be located exactly by timing the pulse return and noting imperfections in the shape of the waveform.

The first step is to measure the output of the transmitter with the light meter and compare it to the specs. Then the light level at the receiving end of the fiber is measured and compared to the published receiver sensitivity figure. If the light level is above the receiver sensitivity, the system should perform correctly. If this is not the case, the receiver diode or the receiver circuitry is suspect.

The convergence of broadcast, AV, and IT along with the advantages of fiber and the cost and weight of copper have merged to bring fiber to the forefront for all three areas. The AV techs who understand fiber systems will lead the way as their industry more fully embraces fiber-optic systems.



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