Optical Communication - Erbium Doped Fiber Amplifier (EDFA)

1. Why Erbium?
• Erbium has several important properties that make it an
excellent choice for an optical amplifier
• Erbium ions (Er3+) have quantum levels that allows them to be
stimulated to emit in the 1540nm band, which is the band that
has the least power loss in most silica-based fiber.
• Erbium's quantum levels also allow it to be excited by a signal
at either 980nm or 1480nm, both of which silica-based fiber
can carry without great losses

2. Origin of EDFA (Who, When and Where)
• Prof.David Payne and team
• Published the research paper in the
year 1987
• At the University of Southampton,
UK

3. Block Diagram of EDFA

4. Amplification Mechanism

5. Amplification Mechanism
• The possible pump wavelengths are 980 & 1480 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. These excited ions decay
(relax) 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. Hence, they are
characterized by a very long fluorescence time of about 10
ms.

6. Amplification Mechanism
• 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. 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
process5. This-decay phenomenon is known as spontaneous
emission and adds to the amplifier noise.

7. Amplification Mechanism
• Two more types of transitions occur when a flux of signal
photons that have energies corresponding to the band gap
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 new
photon of the same energy, wavevector and polarization as
the incoming signal photon.

8. LOSS-GAIN Characteristics
• Energy level diagram of
erbium ions in silica fibers
along with the absorbtion
and gain spectra of an
EDFA whose core was
codoped with germania to
increase the refractive
index

9. Maximum possible gain
ss
pp
P
P
G


1

10. • Fibre NA = 0.16
• Fibre length = 9 m
• 200 mW of pump power @ 980 nm
INPUT-OUTPUT Characteristics

11. Changes in Gain W.R.T. Pump Power and
Amplifier Length

12. 980 nm vs 1480 nm pumping EDFAs
• 1480 nm pumps
• Higher noise
• Need higher drive current - heat
dissipation required - expensive
• Smaller GE
• Large tolerance in pump
wavelength ~ 20 nm
980 nm pump
• Low noise
• Wasted energy because electrons
must relax unproductively
• Higher GE
• Narrow absorption band ~ 2 nm

13. Different configurations of EDFA

14. Different configurations of EDFA
• The pump light is usually injected from the same direction as
the signal flow. This is known an co-directional pumping. It is
also possible to inject the pump power in the opposite
direction to the signal flow, which is known as counter
directional pumping. One can employ either a single pump
source or a dual pump schemes.
• Counter Directional pumping allows higher gains, but co
directional pumping gives better performance.

15. Advantages
• Commercially available in C-band & L-band
• Insensitivity to light polarization state
• High gain
• Low noise figure: 4.5 dB to 6dB
• No distortion at high bit rates
• Simultaneous amplification of wavelength division multiplexed signals
• Immunity to cross talk among wavelength multiplexed channels
• Do not require high speed electronics
• Independent of bit rate (Bit rate transparency)

16. Drawbacks
• Pump laser necessary
• Need to use a gain equalizer for multistage amplification
• Difficult to integrate with other components
• Dropping channels can give rise to errors in surviving channels