370
9 Analog Optical Fiber Channels
In this case, the carrier-to-noise ratio is directly proportional to the square of the
received optical power, so that for each 1-dB variation in received optical power, C/N
will change by 2 dB.
For well-designed photodiodes, the dark current is small compared with the shot
(quantum) noise for intermediate optical signal levels at the receiver. Thus, at intermediate power levels the shot noise term of the photodiode will dominate the system
noise. In this case,
C
N
limit 2
=
1
2
m
2
R ¯
P
2q F(M)B e
(9.12)
so that the carrier-to-noise ratio will vary by 1-dB for every 1-dB change in the
received optical power.
If the laser has a high RIN value so that the reflection noise dominates over other
noise terms, then the carrier-to-noise ratio becomes
C
N
limit 3
=
1
2
m
2
RINB e
(9.13)
which is a constant. In this case, the performance cannot be improved unless the
modulation index is increased.
Example 9.1 As an example of the limiting conditions, consider a link with a laser
transmitter and a pin photodiode receiver having the following characteristics:
Transmitter
Receiver
m = 0.25
R = 0.6 A/W
RIN = − 143 dB/Hz
B e = 10 MHz
P c = 0 dBm
i D = 10 nA
R eq = 750
F t = 3 dB
where P c is the optical power coupled into the fiber. To see the effects of the different
noise terms on the carrier-to-noise ratio, Fig. 9.3 shows a plot of C/N as a function
of the optical power level at the receiver. For high levels of received power the
source noise dominates to give a constant C/N. At intermediate levels, the quantum
noise (shot noise) is the main contributor, with a 1-dB drop in C/N for every 1-dB
decrease in received optical power. For low light levels, the thermal noise of the
receiver is the limiting noise term, yielding a 2-dB rolloff in C/N for each 1-dB drop
in received optical power. It is important to note that the limiting factors can vary
significantly depending on the transmitter and receiver characteristics. For example,
for low-impedance amplifiers the thermal noise of the receiver can be the dominating
performance limiter for all practical link lengths (see Problem 9.1).
9 Analog Optical Fiber Channels
In this case, the carrier-to-noise ratio is directly proportional to the square of the
received optical power, so that for each 1-dB variation in received optical power, C/N
will change by 2 dB.
For well-designed photodiodes, the dark current is small compared with the shot
(quantum) noise for intermediate optical signal levels at the receiver. Thus, at intermediate power levels the shot noise term of the photodiode will dominate the system
noise. In this case,
C
N
limit 2
=
1
2
m
2
R ¯
P
2q F(M)B e
(9.12)
so that the carrier-to-noise ratio will vary by 1-dB for every 1-dB change in the
received optical power.
If the laser has a high RIN value so that the reflection noise dominates over other
noise terms, then the carrier-to-noise ratio becomes
C
N
limit 3
=
1
2
m
2
RINB e
(9.13)
which is a constant. In this case, the performance cannot be improved unless the
modulation index is increased.
Example 9.1 As an example of the limiting conditions, consider a link with a laser
transmitter and a pin photodiode receiver having the following characteristics:
Transmitter
Receiver
m = 0.25
R = 0.6 A/W
RIN = − 143 dB/Hz
B e = 10 MHz
P c = 0 dBm
i D = 10 nA
R eq = 750
F t = 3 dB
where P c is the optical power coupled into the fiber. To see the effects of the different
noise terms on the carrier-to-noise ratio, Fig. 9.3 shows a plot of C/N as a function
of the optical power level at the receiver. For high levels of received power the
source noise dominates to give a constant C/N. At intermediate levels, the quantum
noise (shot noise) is the main contributor, with a 1-dB drop in C/N for every 1-dB
decrease in received optical power. For low light levels, the thermal noise of the
receiver is the limiting noise term, yielding a 2-dB rolloff in C/N for each 1-dB drop
in received optical power. It is important to note that the limiting factors can vary
significantly depending on the transmitter and receiver characteristics. For example,
for low-impedance amplifiers the thermal noise of the receiver can be the dominating
performance limiter for all practical link lengths (see Problem 9.1).