We discuss the development of multi-core fiber Bragg gratings (FBGs) to be applied to astrophotonics, more specifically to near-infrared spectroscopy for ground-based instruments. The multi-core FBGs require over 100 notches to reject the OH lines in a broad wavelength range (160 nm). The number of cores of the fiber should correspond to the mode number in the multi-mode fibers and should be large enough to be able to capture a sufficient amount of light from the telescope. A phase-mask based technique is used to fabricate the multi-core FBGs.