Page 222 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

216 Advanced Topics

wavelength-related information. So a different detector and electronic
channel are required for each wavelength. Electronic amplifiers must
be specially designed for the modulation rate that they are intended
to amplify.
The optical amplifier was exactly the right solution to launch the
new age of high-capacity optical communications using wavelength-
division multiplexing. There is now much research activity directed at
achieving all-optical signal processing: transmission, amplification,
adding dropping channels, dispersion compensation, and even signal
retiming and reshaping. It is an ambitious but worthy goal.
The optical amplifier has revolutionized the architecture of optical
communications systems. Its impact was first seen in undersea fiber
optic cables. Instead of having to bury complex electronics under the
ocean, one now installs erbium-doped optical amplifiers. The ampli-
fiers automatically handle whatever combination of wavelengths and
modulation rates that the operator wishes to feed in at the transmis-
sion station. These can be changed at will with no effect on the cable
performance.
Optical gain will also introduce noise. Spontaneous emission still
occurs in the background. Without a resonator, it is no longer possi-
ble to single out a specific mode. These spontaneous photons get am-
plified along with the signal. This is called amplified spontaneous
emission (ASE). When the gain is greater than 20 dB, this form of
noise dominates, and since it is proportional to the signal, further
levels of gain do not really improve the signal-to-noise ratio (SNR)
further. An excellent analysis of this noise has been given by Yariv
(see Bibliography). The basic elements of this analysis are given
below. The signal-to-noise ratio of the amplified signal is expressed
as

SNR output =
Amplified signal power
(9.22)
Shot noise power + Amplified spontaneous emission power

To detect the amplified signal power at the output, you would use a
photodiode; the photocurrent has been given in Eq. 3.25a. The output
power is proportional to the square of this photocurrent:

qGS input 2
2
i output = (9.23)
hf
where G is the gain of the amplifier, and S input is the signal entering the
amplifier. The shot noise power is proportional to the square root of the

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