FBGs with narrow spectral bandwidth are good elements for filtering optical signals. Such FBGs are widely used as optical add/drop multiplexer in WDM systems. High level of SLSR is allowed to avoid adjacent channel crosstalk in systems. A flat – top reflection spectrum and steep spectral roll-down are a feature these FBGs. Athermal packaging FBG with wavelength stability < 0.16 nm for temperature range 0 ÷ 70 ⁰C is required for stable operation.