hi aasif
if the parameter is channel length then the graph is wrong one..
The expected power penalty due to dispersion is given by a parabolic function of the ratio of symbol rate to dispersion limited
bandwidth times a coefficient, “c,” which relates to the roll-off of a raised cosine receiver response. If we wish to examine
this in terms of dispersion power penalty versus total dispersion. First we need to know the spectral width of the laser.
For multilongitudinal mode (MLM) laser, usually Fabry-Perot (FP) type, the spectral width is the root-mean-square spectral
width, and for single-longitudinal mode (SLM) lasers, usually distributed feedback (DFB) lasers, the spectral width is the
width at the 20 dB down points divided by 6.07. This is the Gaussian spectral width at the 20 dB down point.
with regards