Reply To: With the increase in Power the Ber first decreases then increases.

[Abdallah IsmailParticipant]

If you are using a fiber,This is right, because The non-linearity start to appear as the power increased,but in your curve the non-linearity appeared late may be the effective area of the fiber is large or nonlinear refractive index is too small.
Cheers,
Abdallah