The Low Pass Bessel Filters have a cut-off frequency of 0.75*Symbol rate. You should change that for the 0.75*Bit rate, which will cut out a lot more noise. I’m not exactly sure what your application is, but you might want to make sure you are using the gratings correctly in your design. If you are interested in some sort of WDM system, I would think you should use the reflected spectrum from your gratings and not the transmitted spectrum.
Look at the spectrum response of fiber bragg gratings at this webpage: