Page 194 - Optical Communications Essentials

Optical Amplifiers

184 Chapter Eleven

1 in Fig. 11.7. 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.
Another possible pump wavelength is 1480nm. 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. 11.7. 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.
Two more types of transitions occur when a flux of signal photons that have
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 (direction of travel), and polarization as the incoming signal photon.
The widths of the metastable and ground-state levels allow high levels of stimu-
lated emissions to occur in the 1530- to 1560-nm range. Beyond 1560nm, the
gain decreases steadily until it reaches 0dB (unity gain) at 1616nm.

11.4.2. EDFA configurations
An EDFA consists of an erbium-doped fiber, one or more pump lasers, a passive
wavelength coupler, optical isolators, and tap couplers, as shown in Fig. 11.8.
The wavelength-selective coupler (WSC) handles either 980/1550-nm or
1480/1550-nm wavelength combinations to couple both the pump and signal
optical powers efficiently into the fiber amplifier. The tap couplers are wave-
length-insensitive with typical splitting ratios ranging from 99:1 to 95:5. They
generally are used on both sides of the amplifier to compare the incoming sig-
nal with the amplified output. The optical isolators prevent the amplified signal
from reflecting into the device, where it could increase the amplifier noise and
decrease its efficiency.

11.4.3. EDFA pump lasers
The erbium-doped fiber in the C-band is pumped optically by 980- and/or 1480-nm
pump lasers. As shown in Fig. 11.8, the pump light usually is injected from the
same direction as the signal flow. This is known as codirectional pumping. It is
also possible to inject the pump power in the opposite direction to the signal flow,

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.