For uniform FBG, there are a series of peaks aside the main reflection peak, which are called side modes or side lobes. For certain applications, such as DWDM, the side lobes will be a disadvantage and affect the channel isolation of an optical device. By using function to modulate the refractive index modulation amplitude,  the side modes of a grating could be effectively suppressed, which is called apodization. Apodized gratings offer significant improvement in side-mode suppression while maintaining reflectivity and a narrow bandwidth. The side mode suppression ratio (SMSR) is an indicator of how much the side modes is suppressed.