Hi Umer Syed,
I agree with Aasif dar that distortion is an unwanted phenomena in optical communication, in general sense distortion is unwanted change in amplitude and phase while transmission, signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delays.
In theoretical basis we assume that the signal transmitted (with a definite bandwidth) has same transmission characteristics but In practical cases suppose for a bandwidth of 10 GHz i.e. 0.1 nm wavelength difference (delta f) is there so for each wavelength the characteristics of the fiber is different (negligible in this case) but when transmitted long distance along the fiber this little difference becomes noticeable, i.e. each single wavelength follow different different path and arrive at the receiver with a time delay for the same signal, in this way the pulse gets broaden in time domain (and respective in frequency domain as these two are interrelated).
So dispersion may be defined as the pulse broadening due to multi-path propagation and its unit is ps/nm-km, where ps-picosecond is the pulse boradening, nm is the wavelength of the signal and km is the fiber distance.
attenuation is due to the fiber loss in terms of power.
Regards,
Dhiman