Influences on the bandwidth at the drop port of the add/drop filter using a fiber grating coupler by uniform and linearly chirped fiber Bragg grating

Writing a fiber Bragg grating (FBG) in an optical fiber coupler is a very effective way to implement an optical add/drop filter (OADF). In order to analyze the influences on the bandwidth at the drop port of the OADF using fiber grating-assisted mismatched coupler, a uniform FBG and a linearly chirped FBG were written on the coupler, and two different OADFs were fabricated. The fiber grating is written in a 2×2 mismatched coupler, which is fused and tapered with a standard single mode fiber (SMF) and a high germanium doped photosensitive fiber, by using a 248 nm ultraviolet laser. For the OADF using the uniform FBG, the 3 dB and 20 dB bandwidths are 0.1 nm and 0.3 nm, respectively; for the OADF using the linear chirped FBG, the 3 dB and 20 dB bandwidths are 0.8 nm and 2 nm, respectively. The experimental results meet the theoretical analysis well. The bandwidth at the drop port of the device can be adjusted by adjusting the chirped coefficient of the linearly chirped FBG.