1. An optical communications link is to be built using fiber rated for 5000 MHz-
ID: 3709198 • Letter: 1
Question
1. An optical communications link is to be built using fiber rated for 5000 MHz-km. The light source is a laser diode producing 0.25 mW. The fiber has losses of 0.4 dB/km and is available in 2 km lengths. It can be spliced with a loss of 0.2 dB per splice, and there will be a 5 dB loss in the connectors throughout the system. The receiver used has a sensitivity of -30 dBm. The source rise time is 2 ns, and the receiver rise time is 4 ns.
a) Prepare a loss budget for a link 20 km long, and calculate the system margin in dB.
b) What would be the maximum bandwidth that could be used with the system in
Explanation / Answer
Solution:
a)
Transmitted power = 0.25 mW
= -6.02 dBn
Fiber length = 2Km each
splice loss = 0.2 dB/splice
fiber loss= 0.4dB/km
connection loss = 5dB
Total loss = fiber loss * 20 + (10-1) * splice loss + connection loss
= 0.4 * 20 + 9 * 0.2 + 5
= 14.8 dB
Total fiber loss = 0.4*20= 8 dB
Total splice loss = 9 splice * 0.2 dB.splice
= 1.8 dB
System margin= Pt - losses - (Pr)sensitivity
= -6.02 - 14.8 + 30
System margin = 9.2 dB
b)
source rise time= 2ns
Receiver rise time = 4ns
minimum pulse width = 4 ns
Maximum bandwidth = 1/pulse width
= 1/ 4 *10^-9
= 250 MHz
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