122
3 Optical Signal Attenuation and Dispersion
where D PMD , which is measured in ps/
√
km, is the average PMD parameter. Typical
values of D PMD range from 0.03 to 1.3 ps/
√
km depending on the type of cable
installation. Buried cable typically does not encounter significant variations in the
surrounding environments, so the PMD value tends to be low and relatively stable.
However, aerial cables experience larger fluctuations in the value of PMD, which
result in both gradual and rapid stress variations in the fiber due to temperature
fluctuations or from sudden movements of the fiber due to wind.
To keep the probability of errors due to PMD low, a standard limit on the maximum
tolerable value of τ PMD ranges between 10 and 20% of a bit duration. Thus τ PMD
should be no more than 10–20 ps for 10 Gb/s data rates and 3 ps at 40 Gb/s. For
example, taking the lower tolerance limit, this means that for a 10 Gb/s link that has
20 spans of 80 km each, the PMD of the fiber must be less than 0.2 ps/
√
km.
Example 3.13 Consider a 1600 km fiber link on which data is being sent at a bit
rate B = 10 Gb/s. Assume that the maximum tolerable delay due to PMD is 10% of
a bit period, so that τ PMD < 0.1/B = 0.1/(10 × 10
9 /s) = 10
−11 s = 10 ps. Thus for
this link, from Eq. (3.45) the PMD must satisfy the condition
D P M D < 0.1τ P M D /
√
L = 10 ps/
√
1600 km = 0.25 ps/
√
km
Drill Problem 3.8 Consider a 1600 km fiber link on which data is being sent
at a bit rate B = 40 Gb/s. Assume that the maximum tolerable delay due to
PMD is 20% of a bit period. Show that the tolerable PMD of the transmission
fiber should <0.13 ps/
√
km.
3.3 Design and Characteristics of SMFs
This section addresses the basic design and operational characteristics of single-mode
fibers. These characteristics include index-profile configurations used to produce
different fiber types, the concept of cutoff wavelength, signal dispersion designations
and calculations, the definition of mode-field diameter, and signal loss due to fiber
bending.
3 Optical Signal Attenuation and Dispersion
where D PMD , which is measured in ps/
√
km, is the average PMD parameter. Typical
values of D PMD range from 0.03 to 1.3 ps/
√
km depending on the type of cable
installation. Buried cable typically does not encounter significant variations in the
surrounding environments, so the PMD value tends to be low and relatively stable.
However, aerial cables experience larger fluctuations in the value of PMD, which
result in both gradual and rapid stress variations in the fiber due to temperature
fluctuations or from sudden movements of the fiber due to wind.
To keep the probability of errors due to PMD low, a standard limit on the maximum
tolerable value of τ PMD ranges between 10 and 20% of a bit duration. Thus τ PMD
should be no more than 10–20 ps for 10 Gb/s data rates and 3 ps at 40 Gb/s. For
example, taking the lower tolerance limit, this means that for a 10 Gb/s link that has
20 spans of 80 km each, the PMD of the fiber must be less than 0.2 ps/
√
km.
Example 3.13 Consider a 1600 km fiber link on which data is being sent at a bit
rate B = 10 Gb/s. Assume that the maximum tolerable delay due to PMD is 10% of
a bit period, so that τ PMD < 0.1/B = 0.1/(10 × 10
9 /s) = 10
−11 s = 10 ps. Thus for
this link, from Eq. (3.45) the PMD must satisfy the condition
D P M D < 0.1τ P M D /
√
L = 10 ps/
√
1600 km = 0.25 ps/
√
km
Drill Problem 3.8 Consider a 1600 km fiber link on which data is being sent
at a bit rate B = 40 Gb/s. Assume that the maximum tolerable delay due to
PMD is 20% of a bit period. Show that the tolerable PMD of the transmission
fiber should <0.13 ps/
√
km.
3.3 Design and Characteristics of SMFs
This section addresses the basic design and operational characteristics of single-mode
fibers. These characteristics include index-profile configurations used to produce
different fiber types, the concept of cutoff wavelength, signal dispersion designations
and calculations, the definition of mode-field diameter, and signal loss due to fiber
bending.