HELLO AASIF BASHIR..
Well these spikes can be either due to high launching power or the distortion in the fiber channel.. and to get best results try to increase global layout iterations from 1 (which is a default value) to 2 or 3 if you are using any bidirectional component in your design implementation.. That can help
Also i would like to suggest u to use power meters also for keeping an eye on the power level variations.. i hope it will be helpful..
Thanks & regards