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

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

Block Diagram of EDFA

Amplification Mechanism

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.

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.

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.

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

Maximum possible gain

ss

pp

PP

G

1

• Fibre NA = 0.16

• Fibre length = 9 m

• 200 mW of pump power @ 980 nm

INPUT-OUTPUT Characteristics

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

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

Different configurations of EDFA

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.

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)

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