A Fabry Perot optical filter is an optical band-pass-filter.
For example, such filters are used as laser-resonators, generating a very narrow banded spectrum. This device consists of two highly reflecting mirrors facing each other with distance d and often filled with air. In principle this filter, based on interference, can be adjusted by distance d of both mirrors, influencing the resonator conditions what makes this filter tunable and which also enables an exact adjustment of the filter edge regarded to the FBG spectrum.
We can observe the birefringent FBG signals and the adjusted filter curves on the graph below.