After calculating a link's power budget (using the equation
described in Calculating Power Budget for Fiber-Optic Cable), you
can calculate the power margin (P_{M}), which represents the amount of power available after subtracting
attenuation or link loss (LL) from the power budget (P_{B}). A worst-case estimate of P_{M} assumes maximum LL:

P_{M} = P_{B} –
LL

A P_{M} greater than zero indicates
that the power budget is sufficient to operate the receiver.

Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion, connectors, splices, and fiber attenuation. Table 36 lists an estimated amount of loss for the 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 36: Estimated Values for Factors Causing Link Loss

The following example uses the estimated values in Table 36 to calculate link loss (LL) for a 2 km-long multimode link with a power budget
(P_{B}) of 13 dB:

- Fiber attenuation for 2 km @ 1.0 dB/km= 2 dB
- Loss for five connectors @ 0.5 dB per connector = 5(0.5 dB) = 2.5 dB
- Loss for two splices @ 0.5 dB per splice =2(0.5 dB) = 1 dB
- Higher-order loss = 0.5 dB
- Clock recovery module = 1 dB

The power margin (P_{M}) is calculated
as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 2 km (1.0 dB/km)
– 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB [HOL] –
1 dB [CRM]

P_{M} = 13 dB – 2 dB – 2.5
dB – 1 dB – 0.5 dB – 1 dB

P_{M} = 6 dB

The following sample calculation for an 8 km-long single-mode
link with a power budget (P_{B}) of 13 dB
uses the estimated values from Table 36 to calculate link loss (LL) as the sum of fiber
attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss for
seven connectors (0.5 dB per connector, or 3.5 dB). The power
margin (P_{M}) is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 8 km (0.5 dB/km)
– 7 (0.5 dB)

P_{M} = 13 dB – 4 dB – 3.5
dB

P_{M} = 5.5 dB

In both examples, the calculated power margin is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.

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