I haven’t worked with SAC OCDMA (simulation work) but I think in case of MD codes, you just implement MD codes (instead of Hadamard codes) with FBGs and that should be it. There is a paper named “Development of a new code family based on SAC-OCDMA system with large
cardinality for OCDMA network” that I refer you to, which has not used FBG I think.