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EDFA (Erbium doped fiber amplifier) falls on one of the finest invention in this millennium that has remarkably transformed the fiber optic industry.Without EDFA, we would not be talking about terabit communication today. The commercial introduction of EDFA in the 1990s conquered the regenerator technology and opened doors to the WDM technology.
The EDFA, unlike the repeater, is transparent to the number of channels, bit-rate, protocol, and modulation formats thus revolutionizing the backhaul network (esp. for transoceanic distance) and uplifting the optical communication field and community at large.
In the crux of the EDFA technology is the rare earth Erbium doped fiber. The working principle of an EDFA is exactly the same as the principle of a laser, i.e., based on "stimulated emission". A laser with 980nm and/or 1480nm is pumped (co-propagating or/ and counter-propagating, ref. Figure 1-3) into the Erbium doped section of fiber and gets the Erbium ions jump to the excited state.
When the optical signal (photons @1550nm region) is coupled in, the excited ions fall back to the lower-energy state by radiating photons with the same wavelength, phase and direction as that of the optical signal. EDFA is capable of amplification of a ~24nm range of signal falling in 1550nm region.
Fig.1 EDFA with co-propagating pump
Fig.2 EDFA with counter-propagating pump
Fig.3 EDFA with co-and counter-propagating pump
An EDFA exhibits a gain tilt over the wavelengths. In a multichannel system, this effect would be accumulated over chained amplifiers such that the gain bandwidth would then be narrowed. A gain flattening filter (GFF) is therefore used when the EDFA is used as an in-line amplifier for a multi-channel scenario. The transfer function of this filter is inverse of the amplifier function resulting in flattened output optical spectrum over amplified wavelength range.
An EDFA has two modes of operation: Automatic Gain Control (AGC) and Automatic Power Control (APC). The AGC mode is favored for a multi-channel WDM system where the amplifier gain needs to be kept constant, regardless of the number of newly lit channels. Conversely, the APC mode keeps the output power constant and is mostly used in cases where the output power value plays an important role and where the channel count is low.