What are the types of DCM in DWDM link?

Two types of DCM are used in the DWDM link and are called post-compensation and pre-compensation. Since DCMs are considered part of the transmission line, the prefixes “post-” (after) and “pre-” (before) refers to the section of the transmission line that requires the compensation. 

 For the post-compensation DCM deployment, DCMs are placed after the fiber span that needs compensation. For G.652 fiber compensation, dispersion remains positive throughout the link. 

For the pre-compensation DCM deployment, DCMs are placed before the fiber span that needs compensation. For G.652 fiber compensation dispersion remains negative throughout the link.

Both methods are acceptable since optical amplifiers do not add dispersion into the link provided that the DCM maximum power specifications are not exceeded. Placing DCMs after the optical amplifier can reduce link OSNR, but may increase nonlinear distortions due to high power levels if DCMs use DCF fiber. DCF fiber is more susceptible to nonlinear effects due to its smaller core area. Optical amplifiers are available with intermediate stage access designed to accept DCM connections. This allows for dispersion compensation with less impact on the link loss, OSNR, and nonlinear distortions.