Thanks Prakash for your reply.
Yes, it is possible to see the difference visually . But i need to get a spectrum by taking the difference of the two spectrums. Can we do it in optisystem? Is it possible to subtract two spectrums?

i have attached the image of the circuit which i am currently working on.Can you please say how it can be modified as a back to back circuit. Does it means removing the 2km HNL-DSF shown in the figure ?

Why in some cases BER is plotted against received power while log of BER is plotted against received power in other cases? Is there any relation between the two plots ?

Hi MHD NA,
I have downloaded it again.But it shows the same error. Is there any other way? I am badly in need of the software. Pls help me out with this.

Also how to measure the Four wave mixing crosstalk level from an optical spectrum. Is there any equation ? i have attached the image of the spectrum from which i have to measure it. In that paper it is said that the crosstalk level is -19.45. Can some one help me out with this, to how to calculate it.