Hello Numan,
I agree with both members, and would like to add/elaborate from myself that it is important what type signal (doamin) do you have in the beginning. If you are working with optical digital system, then you are in time domain. On the other hand, if you work with analog system that is built by frequency(wavelength)-multiplexed channels, you are in frequency domain. The conversion algorithm is provided in a link provided by Alistu.