176 Fiber Optic Essentials
Reflections show OTDR
pulse width and resolution
Slope of trace is
Connectors show both
loss and reflection
Figure 8.5 Information in the OTDR display.
enough backscattered light to process. This means the OTDR cannot
distinguish most patchcords in a LAN cable plant or any events close to
the OTDR itself. For example, when using an OTDR to nondestructively
test a failed cable, it was determined that it broke near the connector.
But since the connector was too close to the OTDR to be precise where
the fracture was, the cable had to be destructively tested (opened up) to
discover the flaw. Also, since the OTDR does not see the connectors on
each end of the cables, it underestimates the actual cable plant loss.
Other uncertainties have a variety of causes. The variations in backscat-
ter of fiber can cause major changes in the backscattered light, making
splice or connector measurements uncertain by as much as ±04 dB. In
multimode, cable the higher order modes are not filled by the OTDR laser,
which makes it underestimate the loss of LED sources. Highly reflective
events, such as connectors, can cause “ghosts” that are confusing.
However, the ability to non-destructively test fibers which are remote,
buried, and otherwise inaccessible can be extremely valuable .
8.4.3 BIT ERROR RATE TEST (BERT)
While it is important that light reaches one end of the fiber to the other,
a digital signal is composed of ones and zeros, and the fundamental
measurement of the quality of a data communications system is found
in the probability that the transmitted bits are correctly received. The Bit
Error Ratio (BER) is the ratio of the number of bits received incorrectly
compared to the number of bits transmitted.
In Figure 8.6 we see a BER test block diagram, where on one side of
the device or system being tested is a pattern generator, and on the other
side is an error detector. The pattern generator produces test patterns,