450
11 Basics of Optical Amplifiers
manifold (a set of many levels) due to the effect known as Stark splitting. Furthermore,
thermal effects broaden each Stark level into an almost continuous band.
To understand the various energy transitions and photon emission ranges, consider
the following conditions:
• The pump band shown in the top left of Fig. 11.5 exists at a 1.27-eV separation
from the bottom of the
4 I 15/2 ground state. This energy corresponds to a 980-nm
wavelength.
• The top of the
4 I 13/2 metastable band (level D in Fig. 11.5) is separated from the
bottom of the
4 I 15/2 ground state band (level A in Fig. 11.5) by 0.841 eV. This
energy corresponds to a 1480-nm wavelength.
• The bottom of the
4 I 13/2 metastable band (level C in Fig. 11.5) is separated from
the bottom of the
4 I 15/2 ground state band (level A in Fig. 11.5) by 0.814 eV. This
energy corresponds to a 1530-nm wavelength.
• The bottom of the
4 I 13/2 metastable band (level C in Fig. 11.5) is separated from
the top of the
4 I 15/2 ground state band (level B in Fig. 11.5) by about 0.775 eV.
This energy corresponds to a 1600-nm wavelength.
This means that possible pump wavelengths are 980 and 1480 nm. The photons
emitted during transitions of electrons between possible energy levels in the
metastable band and ground-state band can range from 1530 to 1600 nm.
In normal operation, a pump laser emitting 980-nm photons is used to excite ions
from the ground state to the pump level, as shown by transition process 1 in Fig. 11.5.
These excited ions decay (relax) very quickly (in about 1 μs) from the pump band
to the metastable band, shown as transition process 2. During this decay, the excess
energy is released as phonons or, equivalently, mechanical vibrations in the fiber.
Within the metastable band, the electrons of the excited ions tend to populate the
lower end of the band. Here, they are characterized by a very long fluorescence time
of about 10 ms.
Another possible pump wavelength is 1480 nm. The energy of these pump photons
is very similar to the signal-photon energy, but slightly higher. The absorption of a
1480-nm pump photon excites an electron from the ground state directly to the
lightly populated top of the metastable level, as indicated by transition process 3
in Fig. 11.5. These electrons then tend to move down to the more populated lower
end of the metastable level (transition 4). Some of the ions sitting at the metastable
level can decay back to the ground state in the absence of an externally stimulating
photon flux, as shown by transition process 5. This decay phenomenon is known as
spontaneous emission and adds to the amplifier noise because it emits a background
noise photon.
Two more types of transitions occur when a flux of signal photons with energies
corresponding to the bandgap energy between the ground state and the metastable
level passes through the device. First, a small portion of the external photons will be
absorbed by ions in the ground state, which raises these ions to the metastable level, as
shown by transition process 6. Second, in the stimulated emission process (transition
process 7) a signal photon triggers an excited ion to drop to the ground state, thereby
emitting a new photon of the same energy, wave vector, and polarization as the
11 Basics of Optical Amplifiers
manifold (a set of many levels) due to the effect known as Stark splitting. Furthermore,
thermal effects broaden each Stark level into an almost continuous band.
To understand the various energy transitions and photon emission ranges, consider
the following conditions:
• The pump band shown in the top left of Fig. 11.5 exists at a 1.27-eV separation
from the bottom of the
4 I 15/2 ground state. This energy corresponds to a 980-nm
wavelength.
• The top of the
4 I 13/2 metastable band (level D in Fig. 11.5) is separated from the
bottom of the
4 I 15/2 ground state band (level A in Fig. 11.5) by 0.841 eV. This
energy corresponds to a 1480-nm wavelength.
• The bottom of the
4 I 13/2 metastable band (level C in Fig. 11.5) is separated from
the bottom of the
4 I 15/2 ground state band (level A in Fig. 11.5) by 0.814 eV. This
energy corresponds to a 1530-nm wavelength.
• The bottom of the
4 I 13/2 metastable band (level C in Fig. 11.5) is separated from
the top of the
4 I 15/2 ground state band (level B in Fig. 11.5) by about 0.775 eV.
This energy corresponds to a 1600-nm wavelength.
This means that possible pump wavelengths are 980 and 1480 nm. The photons
emitted during transitions of electrons between possible energy levels in the
metastable band and ground-state band can range from 1530 to 1600 nm.
In normal operation, a pump laser emitting 980-nm photons is used to excite ions
from the ground state to the pump level, as shown by transition process 1 in Fig. 11.5.
These excited ions decay (relax) very quickly (in about 1 μs) from the pump band
to the metastable band, shown as transition process 2. During this decay, the excess
energy is released as phonons or, equivalently, mechanical vibrations in the fiber.
Within the metastable band, the electrons of the excited ions tend to populate the
lower end of the band. Here, they are characterized by a very long fluorescence time
of about 10 ms.
Another possible pump wavelength is 1480 nm. The energy of these pump photons
is very similar to the signal-photon energy, but slightly higher. The absorption of a
1480-nm pump photon excites an electron from the ground state directly to the
lightly populated top of the metastable level, as indicated by transition process 3
in Fig. 11.5. These electrons then tend to move down to the more populated lower
end of the metastable level (transition 4). Some of the ions sitting at the metastable
level can decay back to the ground state in the absence of an externally stimulating
photon flux, as shown by transition process 5. This decay phenomenon is known as
spontaneous emission and adds to the amplifier noise because it emits a background
noise photon.
Two more types of transitions occur when a flux of signal photons with energies
corresponding to the bandgap energy between the ground state and the metastable
level passes through the device. First, a small portion of the external photons will be
absorbed by ions in the ground state, which raises these ions to the metastable level, as
shown by transition process 6. Second, in the stimulated emission process (transition
process 7) a signal photon triggers an excited ion to drop to the ground state, thereby
emitting a new photon of the same energy, wave vector, and polarization as the