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multimode and single-mode transmission. An efficient optical data link must transmit enough light to
overcome attenuation.
Dispersion is the spreading of the signal over time. The following two types of dispersion can affect signal
transmission through an optical data link:
Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds of
light rays.
Modal dispersion, which is the spreading of the signal over time caused by the different propagation
modes in the fiber.
For multimode transmission, modal dispersion, rather than chromatic dispersion or attenuation, usually
limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a factor.
However, at higher bit rates and over longer distances, chromatic dispersion limits the maximum link length.
An efficient optical data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specifications. In addition, the total dispersion must be within the limits
specified for the type of link in Telcordia Technologies document GR-253-CORE (Section 4.3) and
International Telecommunications Union (ITU) document G.957.
When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty in
the power budget. The optical power budget must allow for the sum of component attenuation, power
penalties (including those from dispersion), and a safety margin for unexpected losses.
Calculating the Fiber-Optic Cable Power Budget for an NFX350 Device
Calculate the link's power budget when planning fiber-optic cable layout and distances to ensure that
fiber-optic connections have sufficient power for correct operation. The power budget is the maximum
amount of power the link can transmit. When you calculate the power budget, you use a worst-case
analysis to provide a margin of error, even though all the parts of an actual system do not operate at the
worst-case levels.
To calculate the worst-case estimate for fiber-optic cable power budget (P
1. Determine values for the link's minimum transmitter power (P
For example, here, (P
(dBm).
P
= –15 dBm
T
P
= –28 dBm
R
) and (P
) are measured in decibels, and decibels are referenced to 1 milliwatt
T
R
B
) for the link:
) and minimum receiver sensitivity (P
T
54
).
R
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