What is DCM or DSCM or DCU? What are considerations while deploying DCM?

Chromatic dispersion compensation modules (DCM), also known as dispersion compensation units (DCU) or Dispersion slope compensation module (DSCM), can be added to an existing fiber link to compensate for high link dispersion totals.

These DCM are made of various spool lengths of dispersion compensating fiber (DCF) or Fiber Bragg Grating (FBG) and provide fixed compensation. 

In DCF based DCM, their negative chromatic dispersion characteristics compensate for the transmission fiber’s positive dispersion, while in FBG due to grating for shorter signal wavelengths to be reflected sooner and have less propagation delay through the unit. Longer signal wavelengths travel further into the fiber grating before they are reflected and therefore have more propagation delay through the unit. This is the exact opposite of fiber chromatic dispersion and therefore helps reverse pulse spreading due to fiber dispersion. The length of the chirped fiber grating is typically between 10 and 100 cm.