Erbium Doped Fiber Amplifiers (EDFA)


The above figure demonstrates the attenuation of optical fibers relative to wavelength. It can be seen that Rayleigh Scattering is more prevalent at higher frequencies. Rayleigh scattering occurs when minute changes in density or refractive index of optical fibers is present due to manufacturing processes. This tends to scatter either in the direction of propagation within the core or not. If it is not, this leads to increased attenuation. This accounts for 96% of attenuation in optical fibers. It can also be noted that lattice absorption varies wildly with the wavelength of light. From the graph, it is apparent that 1550 nm wavelength this value (and also Rayleigh Scattering) is quite low. It is for this reason that 1550 nm is a common wavelength of propagation with silica glass optical fibers. Although this wavelength allows for greater options in design, shorter wavelengths (such as 850 nm) are also used when distance of propagation is short. However, 1550 is the common wavelength due to the development of dispersion shifted fibers as well as something called the EDFA (Erbium doped fiber amplifier).

EDFAs operate around the 1550 nm region (1530 to 1610 nm) and work based on the principle of stimulated emission, in which a photon is emitted within a optical device when another photon causes electrons and holes to recombine. The stimulated emission creates a photon of the same size and in the same direction (coherent light). The EDFA acts as an amplifier, boosting the intensity of light with a heavily doped core (erbium doped). As discussed earlier, the lowest power loss for silica fibers tends to occur at 1550 nm, which is the wavelength that this stimulated emission occurs. The excitation, however, occurs at 980 or 1480 nm, which is shown to have high loss.

The advantages of the EDFA is high gain and availability to operate in the C and L bands of light, It is also not polarization dependent and has low distortion at high frequencies. The major disadvantage is the requirement of optical pumping.


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