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Well, if you really didn’t know, I am sorry to break it to you… Yes - Nothing in life is perfect. EDFA just like the movie ‘the good, the bad and the ugly’ has these characters (-istics).It is therefore very important that we take care while designing a network with EDFAs. If there is only a single EDFA in the link, then these effects will not be as drastic as in a link with multiple cascaded EDFAs targeting longer distances.
The good: This is why the EDFA is so famous for; it overcomes the distance limit imposed by the dreadful attenuation. It is because of this good characteristic; high capacity WDM transmission over ultra-long haul has been possible. Figure 1 depicts a typical scenario of a WDM transmission over multiple spans with cascaded EDFAs. Depending on the signal or DSP, dispersion needs to be compensated along the link as well.
Fig.1 EDFA for WDM system
The bad: While amplifying signal, an EDFA adds something called an amplified spontaneous emission or ASE noise linearly to the signal leading to amplitude noise. The ASE noise spectrum is typically constant across the signal spectrum, i.e., independent of the signal, the amount of noise added by the EDFA is always constant. Optical-signal-to noise ration or OSNR is used to measure the strength of noise in the system. The OSNR gets lowered as the signal passes through successive EDFAs as shown in Figure 2.
Fig.2 Cascaded EDFA leading to lower OSNR output
The ugly: The non-linear interaction between the signal and the ASE noise from the EDFA results in a phase noise called Gordon–Mollenauer (GM) noise. Actually, the amplitude noise is converted to phase noise through self-phase modulation. Since this is phase noise, it severely impacts a phase modulated optical system, e.g. DPSK, DQPSK etc. The ugly part of GM is that it cannot be fully compensated because of the statistical nature of the noise.