Hello esakkimuthu
The main reason of the more non linearities impairments in DCF is the small effective area of DCF as compared to SMF.Generaly the Aeff area of DCF is around 22um^2 and around 80um^2 in case of SMF.The value non linear phase shift given as
φ=2π/Λ n2 P/Aeff L
So less the value of Aeff,more the phase shift and more the non linearities.However this also have some benefits and drawbacks in signal amplification.
Please visit the link i have given below to read very good discussion on this particular topic