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How to Calculate Power Margin for Fiber-Optic Cables
After calculating a link's power budget, you can calculate the power margin (P
amount of power available after subtracting attenuation or link loss (LL) from the power budget (P
worst-case estimate of P
P
= P
– LL
M
B
P
greater than zero indicates that the power budget is sufficient to operate the receiver.
M
Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion,
connectors, splices, and fiber attenuation.
factors used in the following sample calculations. For information about the actual amount of signal loss
caused by equipment and other factors, refer to vendor documentation.
Table 31: Estimated Values for Factors Causing Link Loss
Link-Loss Factor
Higher-order mode losses
Modal and chromatic dispersion
Faulty connector
Splice
Fiber attenuation
The following sample calculation for a 2-km-long multimode link with a power budget (P
the estimated values from
attenuation (2 km @ 1 dB/km, or 2 dB) and loss for five connectors (0.5 dB per connector, or 2.5 dB) and
two splices (0.5 dB per splice, or 1 dB) as well as higher-order mode losses (0.5 dB). The power margin
(P
) is calculated as follows:
M
P
= P
– LL
M
B
assumes maximum LL:
M
Table 31 on page 69
Estimated Link-Loss Value
Single mode—None
Multimode—0.5 dB
Single mode—None
Multimode—None, if product of bandwidth and distance is less than 500 MHz-
km
0.5 dB
0.5 dB
Single mode—0.5 dB/km
Multimode—1 dB/km
Table 31 on page
69. This example calculates link loss (LL) as the sum of fiber
), which represents the
M
lists an estimated amount of loss for the
69
). A
B
) of 13 dB uses
B
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