13-08-2012, 11:31 AM
Photonics and Optical Communication
[attachment=31232]
Motivation
The introduction of optical amplifiers lead to a tremendous increase of the
bandwidth-distance product of optical communication systems. The first optical
amplifiers were introduced in the mid 1990’s.
Optical amplifiers are „modulation transparent“ and “speed transparent”. This
means that the modulation scheme can be changed without replacing the
optical amplifiers. Furthermore, the operating speed of the system is not limited
by the optical amplifiers. Only the power budget of the optical amplifiers has to
be adapted if the number of channels is changed.
Optical amplifiers are photonic devices like optical sources and optical
detectors. In particular optical amplifiers share a lot of similarities with laser
structures. A received optical signal, within a certain spectral range, gets
optically amplified. Optical amplification meaning that the signals are alloptically
amplified. The optical amplifier amplifies an entire spectral range, so
that several channels can be amplified at the same time. This is one of the
major advantages of optical amplifiers.
In the following the operating principle and the implementations of different
optical amplifiers will be discussed.
Classification of optical amplifiers
Optical amplifiers can be classified in three major groups. The two most
important groups are the semiconductor optical laser type amplifiers (SOA)
and the fiber-type amplifiers. Examples of fiber-based amplifiers are erbium
doped optical amplifiers (EDFA) and praseodymium-doped fiber
amplifiers (FDFA). Furthermore, amplifiers exist, which use the effect of
Raman and Brillouin scattering. However, the operating principle of Raman
and Brillouin based optical amplifiers will not be discussed in this course.
In general, power is needed to carry out the optical amplification, which can be
provided by electrical or optical means. Independent of the provided power the
amplification process is a pure optical amplification process.
Semiconductor Optical Amplifiers
The semiconductor optical amplifier is based on a conventional semiconductor
laser structure. Light, which is coupled in a cavity of a semiconductor laser is
amplified due to stimulated emission. Semiconductor optical amplifiers have the
advantage that they can operate at wavelengths of 850nm, 1310nm and
1550nm as semiconductor laser structures are available for these wavelengths.
Semiconductor optical amplifiers are quite different from fiber amplifiers.
Semiconductor optical amplifiers have the clear disadvantage that they can
amplify only at a single (wavelength) channel. In order to amplify several
channels several amplifiers are needed.
Erbium-doped fiber amplifiers (EDFA)
The erbium ions in the in the erbium doped silica fiber can be excited at a
number of different energy levels according to the following wavelengths,
514nm, 532nm, 667nm, 800nm, 980, and 1480nm. Obviously, the highest
energy level can be excited by light at a wavelength of 514nm. The electronic
level at 514nm is an energetically unstable state so that the electrons will drop
to a lower energetic state.
The electrons will drop down to the lowest energy state at 1480nm, because
the energy states at 532nm, 667nm, 800nm and 980nm are all unstable states.
These transitions are non-radiative transitions. Only the transition from the
lowest energetic state at 1480nm to the ground state is a radiative transition.
The transition from 1480nm to the ground level leads to the emission of light at
1520-1620nm.
It is of course not necessary to excite the erbium-doped silica fiber at 514nm
and go through all four transitions down to 1480nm. It is sufficient to excite the
doped fiber at a wavelength of 980nm or 1480nm. The optical amplifiers which
work in the wavelength range of 1528nm to 1561nm is called an C-band EDFA
and the optical amplifiers which perform best in the wavelength range of
1561nm to 1610mnm are called L-band EDFA.
The bit rates in optical communication systems are very high so that the
average lifetime of an ion at 1480nm (10ms) is sufficiently long to obtain
stimulated emission. The optical power emitted by the laser diode is coupled in
the erbium doped silica fiber by a fiber coupler.
[attachment=31232]
Motivation
The introduction of optical amplifiers lead to a tremendous increase of the
bandwidth-distance product of optical communication systems. The first optical
amplifiers were introduced in the mid 1990’s.
Optical amplifiers are „modulation transparent“ and “speed transparent”. This
means that the modulation scheme can be changed without replacing the
optical amplifiers. Furthermore, the operating speed of the system is not limited
by the optical amplifiers. Only the power budget of the optical amplifiers has to
be adapted if the number of channels is changed.
Optical amplifiers are photonic devices like optical sources and optical
detectors. In particular optical amplifiers share a lot of similarities with laser
structures. A received optical signal, within a certain spectral range, gets
optically amplified. Optical amplification meaning that the signals are alloptically
amplified. The optical amplifier amplifies an entire spectral range, so
that several channels can be amplified at the same time. This is one of the
major advantages of optical amplifiers.
In the following the operating principle and the implementations of different
optical amplifiers will be discussed.
Classification of optical amplifiers
Optical amplifiers can be classified in three major groups. The two most
important groups are the semiconductor optical laser type amplifiers (SOA)
and the fiber-type amplifiers. Examples of fiber-based amplifiers are erbium
doped optical amplifiers (EDFA) and praseodymium-doped fiber
amplifiers (FDFA). Furthermore, amplifiers exist, which use the effect of
Raman and Brillouin scattering. However, the operating principle of Raman
and Brillouin based optical amplifiers will not be discussed in this course.
In general, power is needed to carry out the optical amplification, which can be
provided by electrical or optical means. Independent of the provided power the
amplification process is a pure optical amplification process.
Semiconductor Optical Amplifiers
The semiconductor optical amplifier is based on a conventional semiconductor
laser structure. Light, which is coupled in a cavity of a semiconductor laser is
amplified due to stimulated emission. Semiconductor optical amplifiers have the
advantage that they can operate at wavelengths of 850nm, 1310nm and
1550nm as semiconductor laser structures are available for these wavelengths.
Semiconductor optical amplifiers are quite different from fiber amplifiers.
Semiconductor optical amplifiers have the clear disadvantage that they can
amplify only at a single (wavelength) channel. In order to amplify several
channels several amplifiers are needed.
Erbium-doped fiber amplifiers (EDFA)
The erbium ions in the in the erbium doped silica fiber can be excited at a
number of different energy levels according to the following wavelengths,
514nm, 532nm, 667nm, 800nm, 980, and 1480nm. Obviously, the highest
energy level can be excited by light at a wavelength of 514nm. The electronic
level at 514nm is an energetically unstable state so that the electrons will drop
to a lower energetic state.
The electrons will drop down to the lowest energy state at 1480nm, because
the energy states at 532nm, 667nm, 800nm and 980nm are all unstable states.
These transitions are non-radiative transitions. Only the transition from the
lowest energetic state at 1480nm to the ground state is a radiative transition.
The transition from 1480nm to the ground level leads to the emission of light at
1520-1620nm.
It is of course not necessary to excite the erbium-doped silica fiber at 514nm
and go through all four transitions down to 1480nm. It is sufficient to excite the
doped fiber at a wavelength of 980nm or 1480nm. The optical amplifiers which
work in the wavelength range of 1528nm to 1561nm is called an C-band EDFA
and the optical amplifiers which perform best in the wavelength range of
1561nm to 1610mnm are called L-band EDFA.
The bit rates in optical communication systems are very high so that the
average lifetime of an ion at 1480nm (10ms) is sufficiently long to obtain
stimulated emission. The optical power emitted by the laser diode is coupled in
the erbium doped silica fiber by a fiber coupler.