Module 15 : ERBIUM DOPED FIBER AMPLIFIERS (EDFA)
Lecture : ERBIUM DOPED FIBER AMPLIFIERS (EDFA)
  \includegraphics[width=\textwidth]{edfa5}
 

Pump can also work directly at 1480 nm to excite the ions directly to the metastable level $ m$. In this case, rapid relaxation would occur to the lowest sub-level within the group of levels $ m$from which laser action would take place. Though pumping with 1480 nm is used and has an optical power conversion efficiency which is higher than that for 980 nm pumping, the latter is preferred because of the following advantages it has over 1480 nm pumping.

  • It provides a wider separation between the laser wavelength and pump wavelength.
  • 980 nm pumping gives less noise.
  • Unlike 1480 nm pumping, 980 nm pumping cannot stimulate back transition to the ground state.

The excited ions make transition to the ground state, either by

(i)
Spontaneous Emission :
  Spontaneously emitted photons bear no phase relationship with one another. It can take place in the entire bandwidth of transition and can even travel backwards. These, therefore, contribute to noise. A fraction of the spontaneously emitted photons will be emitted in a direction which is within the numerical aperture of the fiber and will, therefore, be captured and guided. Such photons, in turn, may interact with rare earth ions and be amplified as well. The Amplified Spontaneous Emission (ASE) is a major source of noise in the system. For low signal power, ASE is approximately independent of the signal but at high signal power, ASE reduces population inversion faster than the pumping rate.
(ii)
 Stimulated Emission :
 

The stimulated emission takes place in the wavelength window 1520 to 1560 nm if a data signal with the corresponding wavelength is fed into the fiber. The newly generated photons by stimulated emission are responsible for amplification of signal as it travels along the fiber.

We know that population inversion cannot be achieved in a two level system. However, 1480 nm pumping directly to the metastable level can give rise to population inversion because the levels in case of Er $ ^{3+}$ are actually sets of closely

1