In the case of fibre Bragg gratings (FBGs), the main parameter behind choosing the filter for a specific wavelength is mainly the "Periodicity" of the particular grating. Which can be controlled minutely at the time of fabrication of the grating. Other parameter involved such as effective RI also has an influence as Central Bragg wavelength is dependent on the effective RI too. The former one is the most effective parameter in this regards.