Page 284 - Optical Communications Essentials

Optical Link Design

274 Chapter Sixteen

As noted in Chap. 15, the pulse spreading t PMD resulting from polarization
mode dispersion is given by
t PMD D PMD fi b er l en g th (16.10)

where D PMD is the polarization mode dispersion measured in units of ps/ k m .
Let us now calculate the expected rise times for some of the link examples
that are given in Sec. 16.2.

16.3.2. Multimode LAN link rise time
First let us consider two examples of LAN links that use an LED and a VCSEL,
respectively.

Example of LED-Based LAN Link The following components are used in the 160-m
Fast Ethernet LAN link in Sec. 16.2.2:
■ An LED with a 2.0-ns rise time and a 75-nm spectral width
■ A pin photodiode receiver with a front-end bandwidth B RX 100MHz
■ A multimode fiber with D CD 20ps/(nm km) and B mod 400MHz km at 850nm

Using Eq. (16.7) yields t RX 3.5ns, Eq. (16.8) gives t CD 0.24ns, and from Eq. (16.9)
we have t mod 0.18ns. Then, ignoring the negligible PMD effects, the total rise
time is
2
2
2
2
t sys (t TX t mod t CD t RX ) 1/2
2
2 1/2
2
2
[(2.0) (0.18) (0.24) (3.5) ] 4.0ns
Since the Fast Ethernet signal uses an NRZ format, the rise time needs to be less than
0.7/(100Mbps) 7.0ns. Thus the rise-time criterion is well satisfied.
Analogous to power budget calculations, a convenient procedure for keeping track
of the various values in the rise time is to use a tabular or spreadsheet form. Table 16.5
shows an example of this for the above calculation.
Example of Maximum Length of Gigabit Ethernet Links Now suppose the LAN con-
sists of Gigabit Ethernet links that use VCSEL sources and 62.5-µm fibers. Also
assume that now the link length is 220m. In this case we have the following condi-
tions:
■ A VCSEL with a 0.1-ns rise time and a 1-nm spectral width
■ A pin photodiode receiver with a front-end bandwidth B RX 1000MHz
■ A multimode fiber with D CD 20ps/(nm km) and B mod 160MHz km at 850nm

Then t mod 0.60ns, t CD 0.01ns, and t RX 0.35ns, so that (again leaving out the
negligible PMD effects) the total rise time is
2
2
2
2
t sys (t TX t mod t CD t RX ) 1/2
2 1/2
2
2
2
[(0.10) (0.60) (0.01) (0.35) ] 0.70ns
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