Ok thanks for clarifying! The answer to your question is that you need to have a White Light bandwidth that covers the range from 1550 nm to 1558.8 nm. So you can set the center wavelength of the source to 1554.4 nm and a bandwidth of 8.8 nm. I believe I showed you how to change this from nm to Hz in another forum post. This is what determines the sample rate for your white light source. You can verify that the entire spectrum is covered by using a Spectrum Analyzer. For the LED it is the same answer.
One more very important point! The signals in OptiSystem are represented as time sampled signals, so think 2xN matrix with one column represents time and the other the complex field (there is also actually polarization info too). There is a strict relationship between the sample rate (inverse of time increment) and the bandwidth that can be represented in the frequency domain. A smaller time increment (larger sample rate) means that a larger bandwidth can be represented. The sample rate can be directly modified in the white light source, however in the LED only the bandwidth can be modified, so you need to make sure that in the global properties that the sample rate is sufficiently large.
Hope this clears it up!