Single-Channel Bandpass Filters Formed by a Metal-Doped Fiber and Long-Period Gratings

We demonstrated all-fiber single-channel bandpass filters formed by a high-attenuation metal-doped fiber as a core mode blocker spliced between two long-period fiber gratings (LPFGs). The LPFGs were written in dissimilar fibers, and their grating periods were adjusted so as to match the resonance wavelengths of only one pair of the same order cladding modes and also avoid matching those of other cladding modes. Three types of single-mode fibers with a relative index difference of about 2%~2.5$ %, 0.8%, and 0.46% were used for fabricating LPFGs. The insertion loss in passband and the full-width at half-maximum bandwidth of fabricated single-channel bandpass filters were 6.6 dB and 4.7 nm, 6.9 dB and 6.4 nm, and 7.7 dB and 7.1 nm.