Page 275 - Optical Communications Essentials

Optical Link Design

Optical Link Design 265

16.1.2. Link margin
Link margin (also called a loss margin or a system margin) is an optical-power
safety factor for link design. This involves adding extra decibels to the power
requirements to compensate for possible unforeseen link degradation factors.
These degradations could arise from factors such as a dimming of the light
source over time, aging of other components in the link, the possibility that cer-
tain splices or connectors in the actual link have a higher loss than anticipated,
or additional losses occurring when a cable is repaired.
ITU-T Recommendation G.957 specifies that a link margin ranging from 3.0
to 4.8dB should be allowed between the transmitter and the receiver to offset
possible equipment degradation. In an actual system, designers typically add a
link margin of 3 to 10dB depending on the performance requirements of the
application, the number of possible repairs, and the system cost.

16.1.3. Power penalties
Certain operational factors in a link usually contribute to signal impairment.
Among these factors are modal noise, chromatic dispersion, polarization mode
dispersion, reflection noise in the link, low extinction ratios in the laser, or fre-
quency chirping. When any of these dispersion or nonlinear effects contribute
to signal impairment, there is a reduction in the signal-to-noise ratio (SNR) of
the system from the ideal case. This reduction in SNR is known as the power
penalty for that effect, which generally is expressed in decibels.

■ Modal noise arises when one is using multimode fiber and a laser source.
When one is using a laser diode source, initially the optical output excites only
a few lower-order modes at the beginning of a multimode fiber. As the light
travels along the fiber, its power gets coupled back and forth among various
propagating modes. This leads to a power fluctuation at the fiber end, which
degrades the signal-to-noise ratio, thereby leading to a power penalty. The use
of a light-emitting diode (LED) greatly reduces modal noise since an LED has
a wide spectral output range and therefore couples power into the various pos-
sible modes more uniformly.
■ Chromatic dispersion is described in Sec. 15.2. The power penalty arising
from chromatic dispersion (CD) can be calculated from

2
P CD 5 log [1 (4BLD∆λ) ] (16.1)
where B is the bit rate in gigabits per second, L is the fiber length in kilo-
meters, D is the chromatic dispersion in ps/(nm km), and ∆λ is the spectral
width of the source in nanometers. To keep the power penalty less than 0.5 dB,
a well-designed system should have the quantity BLD∆λ 0.1.
■ Polarization mode dispersion (PMD) arises in single-mode fibers because the
two fundamental orthogonal polarization modes in a fiber travel at slightly
different speeds owing to fiber birefringence, as described in Sec. 15.4. As

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