Fibre Bragg gratings are written across all 120 single-mode cores of a multi-core optical fibre. The fibre is interfaced to multimode ports by tapering it within a depressed-index glass jacket. The result is a compact multimode "photonic lantern" filter with astrophotonic applications. The tapered structure is also an effective mode scrambler.
Birks, T. A., Mangan, B. J., Díez, A., Cruz, J. L., & Murphy, D. F. (2012). "Photonic lantern" spectral filters in multi-core fibre. Optics Express, 20(13), 13996-14008. https://doi.org/10.1364/OE.20.013996