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14 Basic Measurement and Monitoring Techniques
where β R (y) is the loss coefficient for the reverse-scattered light. Because the modes
in the fiber excited by the backscattered light can be different from those launched
in the forward direction, the parameter β R (y) may be different from β(y).
Substituting Eqs. (14.7) and (14.8) into Eq. (14.9) yields
P D (x) = S P(0)ex p
−
2α(x)x
10 log e
(14.10)
where the average attenuation coefficient α(x) of the forward and reverse losses is
defined as
α(x) =
1
2x
x
0
[α(y) + α R (y)]dy
(14.11)
Using this equation, the average attenuation coefficient can be found from a data
trace such as the one shown in Fig. 14.13. For example, the average attenuation
between two points x 1 and x 2 , where x 1 > x 2 , is
α = −
10
log P D (x 2 ) − log P D (x 1 )
2(x 2 − x 1 )
(14.12)
Example 14.4 An OTDR is used to measure the attenuation of a long length of
fiber. If the optical power level measured by the OTDR at the 8-km point is 0.5 of
the measured value at the 3-km point, what is the fiber attenuation?
Solution Eq. (14.12) can be expressed as
α =
10 log
P D (x2)
P D (x1)
2(x 2 − x 1 )
=
10 log 0.5
2(8 − 3)
= 0.3 dB/km
Drill Problem 14.3 An OTDR is used to measure the attenuation of a 300-m
long plastic optical fiber. If the optical power level measured at the 300-m
point is 0.025 of the power measured at the 100-m point, show that the fiber
attenuation is 0.040 dB/m or 40 dB/km.
14.6.3 OTDR Dead Zone
The concept of a dead zone is another important OTDR specification. Dead zone is
the distance over which the photodetector in an OTDR is saturated momentarily after