380
9 Analog Optical Fiber Channels
The source noise is, with RIN = −135 dB/Hz = 3.162 × 10
−14 /Hz,
i
2
s
= RIN(RP)
2 B e = 5.69 × 10
−13 A
2
The shot noise is
i
2
shot
= 2q(RP + i D )B e = 9.5 × 10
−14 A
2
The thermal noise is
i
2
th
=
4K B T
R eq
F e = 8.25 × 10
−13 A
2
Thus the carrier-to-noise ratio is C/N = 75.6
or in dB: C/N = 10 log 75.6 = 18.8 dB.
9.5 When an APD is used, the carrier power and the shot noise change.
The carrier power is 0.5 (15 × 10
−5 A
2 ).
The shot noise is
i
2
shot
= 2q(R P + i D )M
2 F(M)B e = 4.76 × 10
−10 A
2
Thus the carrier-to-noise ratio is C/N = 236.3 or in dB, C/N = 23.7 dB.
9.6 (a) The modulation index is m =
32
i=1
(0.044)
2
1/2
= 0.25.
The received power is −10 dBm = 100 µW.
The rest of the answers for part (a) are the same as in Problem 9.4.
(b) The modulation index is m =
32
i=1
(0.07)
2
1/2
= 0.396.
The received power is −13 dBm = 50 µW.
The carrier power is 0.5(1.19 × 10
−5 A)
2
= 7.06 × 10
−11 A
2 .
The source noise is, with RIN = −135 dB/Hz = 3.162×10
−14 /Hz.
i
2
s
= RIN(RP)
2 B e = 1.42 × 10
−13 A
2
The shot noise is
i
2
shot
= 2q(RP + i D )B e = 4.8 × 10
−14 A
2
The thermal noise is
i
2
th
=
4kBT
R eq
F e = 8.25 × 10
−13 A
2
Thus the carrier-to-noise ratio is C/N = 69.6
or in dB: C/N = 10 log 69.6 = 18.4 dB.