11.6 Fiber Link Applications
463
Drill Problem 11.5 Consider an optical transmission path containing N
cascaded optical amplifiers each having a 20-dB gain. (a) If the fiber has a
loss of 0.2 dB/km, what is the span between optical amplifiers if there are no
other system impairments? (b) How many amplifiers are needed for a 900-km
link? (c) What is the noise penalty factor over the total path?
[Answers: (a) 100 km; (b) 8 amplifiers; (c) 6.6 dB].
11.6.3 Optical Amplifier as a Preamplifier
An optical amplifier can be used as a preamplifier to improve the sensitivity of directdetection receivers that are limited by thermal noise [19]. First, assume the receiver
noise is represented by the electrical power level N. Let S min be the minimum value
of the electrical signal power S that is required for the receiver to perform with a
specific acceptable bit-error rate. The acceptable signal-to-noise ratio then is S min /N.
If an optical preamplifier with gain G is used, the electrical received signal power is
G
2 S
and the signal-to-noise ratio is
S
N
preamp
=
G
2 S
N + N
(11.44)
where the noise term N
is the spontaneous emission from the optical preamplifier
that gets converted by the photodiode in the receiver to an additional background
noise. If S
min is the new minimum detectable electrical signal level needed to maintain
the same signal-to-noise ratio, then it is necessary to have
G
2 S
min
N + N =
S min
N
(11.45)
For an optical preamplifier to enhance the received signal level, one must have
S
min < S min , so that
S min
S
min
=
G
2 N
N + N > 1
(11.46)
The ratio S min to S
min is the improvement of minimum detectable signal or detector
sensitivity.
Example 11.8 Consider an EDFA used as an optical preamplifier. Assume that the
receiver noise N is due to thermal noise and that the noise N
’ introduced by the
463
Drill Problem 11.5 Consider an optical transmission path containing N
cascaded optical amplifiers each having a 20-dB gain. (a) If the fiber has a
loss of 0.2 dB/km, what is the span between optical amplifiers if there are no
other system impairments? (b) How many amplifiers are needed for a 900-km
link? (c) What is the noise penalty factor over the total path?
[Answers: (a) 100 km; (b) 8 amplifiers; (c) 6.6 dB].
11.6.3 Optical Amplifier as a Preamplifier
An optical amplifier can be used as a preamplifier to improve the sensitivity of directdetection receivers that are limited by thermal noise [19]. First, assume the receiver
noise is represented by the electrical power level N. Let S min be the minimum value
of the electrical signal power S that is required for the receiver to perform with a
specific acceptable bit-error rate. The acceptable signal-to-noise ratio then is S min /N.
If an optical preamplifier with gain G is used, the electrical received signal power is
G
2 S
and the signal-to-noise ratio is
S
N
preamp
=
G
2 S
N + N
(11.44)
where the noise term N
is the spontaneous emission from the optical preamplifier
that gets converted by the photodiode in the receiver to an additional background
noise. If S
min is the new minimum detectable electrical signal level needed to maintain
the same signal-to-noise ratio, then it is necessary to have
G
2 S
min
N + N =
S min
N
(11.45)
For an optical preamplifier to enhance the received signal level, one must have
S
min < S min , so that
S min
S
min
=
G
2 N
N + N > 1
(11.46)
The ratio S min to S
min is the improvement of minimum detectable signal or detector
sensitivity.
Example 11.8 Consider an EDFA used as an optical preamplifier. Assume that the
receiver noise N is due to thermal noise and that the noise N
’ introduced by the