Page 373 - Fiber Optic Communications Fund

354 Fiber Optic Communications

Example 8.2
In a 10-Gb/s unrepeatered fiber-optic system based on PSK, rectangular NRZ pulses are transmitted with a
peak power of 5 dBm. The fiber loss is 50 dB. The gain and n of the pre-amplifier used at the receiver are
sp
30 dB and 1.5, respectively. R = 0.9 A/W. Find the error probability if the receiver is (a) a balanced homodyne
or (b) a balanced heterodyne. Ignore thermal noise. Repeat this example if the signal is OOK with the same
peak power.

Solution:
Signal calculation
Launch peak power = P (dBm)= 5dBm
0
Fiber loss(dB) = 50 dB
Preamplifier gain, G(dB) = 30 dB
The peak power of the received signal is
P (dBm)= P (dBm)− fiber loss(dB) + G(dB)
r 0
= 5dBm − 50 dB + 30 dB
=−15 dBm, (8.145)

P = 10 P r (dBm)∕10 mW = 3.16 × 10 −5 W. (8.146)
r
The bit interval is
1 −10
T = = 10 s. (8.147)
b
10 × 10 9
The energy of bit ‘1’ is
E = P T
1
r b
= 3.16 × 10 −5 × 10 −10 J
= 3.16 × 10 −15 J. (8.148)

For PSK, the average energy E = E .
1
av
Noise calculation
The PSD of ASE is given by Eq. (6.17),
ASE = n hf(G − 1), (8.149)
sp
n = 1.5, (8.150)
sp
h = 6.626 × 10 −34 Js, (8.151)
c 3 × 10 8
f = = = 193.54 THz, (8.152)
1550 × 10 −9
G = 10 G(dB)∕10 = 1000, (8.153)