Fiber Optic Essentials

176 Fiber Optic Essentials

Reflections show OTDR

pulse width and resolution

Slope of trace is

fiber attenuation

Connectors show both

loss and reflection

Splice loss

(usually not

reflective)

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 ±04 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 [7].

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,