From DWDM

4.4.4 Optical Gain Equalizers

In DWDM transmission, optical gain equalization is required because the DWDM signals eventually become unequal in power. This is because the gain characteristic of amplifiers and filters is not flat, because optical cross-connects do not have the same loss characteristics for all channels, or because dispersion is not the same for all channels (see Fig. 4.23), and so on. In addition, as dropping and adding wavelengths takes place, the wavelengths added most likely will not have the same amplitude as those passing through. The end result is that all DWDM channels in the fiber do not arrive at the same optical strength at the receiver. Optical gain equalization improves the signal-to-noise ratio, and thus it enhances the performance of optical amplifiers and allows for longer fiber spans between amplifiers. Therefore, gain equalization is a key function in long-haul applications.

Optical gain equalizers monitor each wavelength channel and selectively make amplitude adjustments on each channel to flatten the optical power spectrum within a fraction of a decibel. They may be static or dynamic.

Static equalizers consist of filters with specific gain profile that counteract the gain variability of channels in the DWDM mix. Such static equalizers, although...


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