"Introduction". In: Fiber-Optic Communication Systems

5.2. DESIGN GUIDELINES 193

Table 5.1 Power budget of a 0.85-μm lightwave system

Quantity Symbol Laser LED

Transmitter power P ̄tr 0dBm −13 dBm

Receiver sensitivity P ̄rec −42 dBm −42 dBm

System margin Ms 6dB 6dB

Available channel loss CL 36 dB 23 dB

Connector loss αcon 2dB 2dB

Fiber cable loss αf 3.5 dB/km 3.5 dB/km

Maximum fiber length L 9.7 km 6km

when an APD is used in place of ap–i–nphotodiode, the transmission distance can be
increased to 8 km even for an LED-based transmitter. Economic considerations would
then dictate the choice between the laser-based transmitters and APD receivers.

5.2.4 Rise-Time Budget

The purpose of therise-time budgetis to ensure that the system is able to operate prop-
erly at the intended bit rate. Even if the bandwidth of the individual system components
exceeds the bit rate, it is still possible that the total system may not be able to operate at
that bit rate. The concept of rise time is used to allocate the bandwidth among various
components. The rise timeTrof a linear system is defined as the time during which the
response increases from 10 to 90% of its final output value when the input is changed
abruptly. Figure 5.5 illustrates the concept graphically.
An inverse relationship exists between the bandwidth∆fand the rise timeTras-
sociated with a linear system. This relationship can be understood by considering a
simpleRCcircuit as an example of the linear system. When the input voltage across an
RCcircuit changes instantaneously from 0 toV 0 , the output voltage changes as

Vout(t)=V 0 [ 1 −exp(−t/RC)], (5.2.6)

whereRis the resistance andCis the capacitance of theRCcircuit. The rise time is
found to be given by
Tr=(ln 9)RC≈ 2. 2 RC. (5.2.7)

Figure 5.5: Rise timeTrassociated with a bandwidth-limited linear system.