318
8 Digital Optical Fiber Links
Table 8.2 Example for a
tabular form for keeping track
of component contributions to
an optical-link rise-time
budget
Component
Rise time
Rise-time budget
Allowed rise-time
budget
t sys = 0.7/B NRZ =
0.28 ns
Laser transmitter
25 ps
GVD in fiber
12 ps
Receiver rise time
0.14 ns
System rise time
(Eq. 8.17)
0.14 ns
form. This can be illustrated by way of an example for the SONET OC-48 (2.5 Gb/s)
link that was looked at in Example 8.2.
Example 8.4 Assume that the laser diode together with its drive circuit has a rise
time of 0.025 ns (25 ps). Take a 1550-nm laser diode spectral width of 0.1 nm and an
average dispersion of 2 ps/(nm km) for the fiber. These parameter values then yield
a GVD-related rise-time degradation of 12 ps (0.012 ns) over a 60-km long optical
cable. Assuming the InGaAs-APD-based receiver has a 2.5-GHz bandwidth, then
from Eq. (8.5) the receiver rise time is 0.14 ns. Using Eq. (8.17) to add up the various
contributions results in a total rise time of 0.14 ns.
Table 8.2 lists the components in column 1 and the associated rise times in column
2. Column 3 gives the allowed system rise-time budget of 0.28 ns for a 2.5-Gb/s NRZ
data stream at the top. This is found from the expression 0.7/B NRZ where B NRZ is
the bit rate for the NRZ signal. The calculated system rise time of 0.14 ns is shown
at the bottom. The system rise time, in this case, is dominated by the receiver and is
well within the required limits.
Drill Problem 8.3 A single-mode optical fiber link that uses a 1310-nm Fabry–
Perot laser diode is designed to operate over a 6-km distance in a campus
network. Suppose that the components of the link operating over this distance
have the following rise time values:
1. The laser transmitter rise time = 0.30 ns.
2. A pin photodiode receiver rise time is 0.14 ns.
3. The dispersion in the optical fiber is 2.0 ps/(nm km).
4. The laser diode has a 2.0-nm spectral width.
(a) Show that the system rise time is 0.33 ns.
(b) Show that the maximum bit rate for an NRZ signal is B NRZ =
2.1 Gb/s.
8 Digital Optical Fiber Links
Table 8.2 Example for a
tabular form for keeping track
of component contributions to
an optical-link rise-time
budget
Component
Rise time
Rise-time budget
Allowed rise-time
budget
t sys = 0.7/B NRZ =
0.28 ns
Laser transmitter
25 ps
GVD in fiber
12 ps
Receiver rise time
0.14 ns
System rise time
(Eq. 8.17)
0.14 ns
form. This can be illustrated by way of an example for the SONET OC-48 (2.5 Gb/s)
link that was looked at in Example 8.2.
Example 8.4 Assume that the laser diode together with its drive circuit has a rise
time of 0.025 ns (25 ps). Take a 1550-nm laser diode spectral width of 0.1 nm and an
average dispersion of 2 ps/(nm km) for the fiber. These parameter values then yield
a GVD-related rise-time degradation of 12 ps (0.012 ns) over a 60-km long optical
cable. Assuming the InGaAs-APD-based receiver has a 2.5-GHz bandwidth, then
from Eq. (8.5) the receiver rise time is 0.14 ns. Using Eq. (8.17) to add up the various
contributions results in a total rise time of 0.14 ns.
Table 8.2 lists the components in column 1 and the associated rise times in column
2. Column 3 gives the allowed system rise-time budget of 0.28 ns for a 2.5-Gb/s NRZ
data stream at the top. This is found from the expression 0.7/B NRZ where B NRZ is
the bit rate for the NRZ signal. The calculated system rise time of 0.14 ns is shown
at the bottom. The system rise time, in this case, is dominated by the receiver and is
well within the required limits.
Drill Problem 8.3 A single-mode optical fiber link that uses a 1310-nm Fabry–
Perot laser diode is designed to operate over a 6-km distance in a campus
network. Suppose that the components of the link operating over this distance
have the following rise time values:
1. The laser transmitter rise time = 0.30 ns.
2. A pin photodiode receiver rise time is 0.14 ns.
3. The dispersion in the optical fiber is 2.0 ps/(nm km).
4. The laser diode has a 2.0-nm spectral width.
(a) Show that the system rise time is 0.33 ns.
(b) Show that the maximum bit rate for an NRZ signal is B NRZ =
2.1 Gb/s.