Fabrication and characterization of bandpass filters based on concatenated chirped fiber gratings

Practical, low-loss (<2 dB), fiber Bragg grating single-passband transmission filters have been realized with >30 dB out-of-band rejection over ~25 mm spectral widths. The approach adopted is based on the concatenation of two or more tailored broadband chirped gratings fabricated with low short-wavelength loss to maximize the rejection and stopband width in a filter with ~1 nm passband. The effect of thermal aging for similar complex structures is considered in the context of three different fiber types: standard telecommunication, high germania (Ge)-doped and boron/germania (B/Ge)-codoped. It is shown that while hydrogenated B/Ge-codoped fiber exhibits excellent photosensitivity, the resulting decrease in guiding strength and thermal stability may make germania-doped fiber the more attractive choice