Hi Sindhu!
Just want to confirm one thing first: what types/shapes of NRZ/RZ and pulses are you considering in your question for choosing filter? In general, it is known that pulses generated by optical source (e.g. laser) have Gaussian shape. It is also known, that after distortion and attenuation during propagation through the medium, the shape of this pulses remains Gaussian. Therefore, we have Gaussian pulses at the receiver in this case.