Study and fabrication of add/drop filter based on Bragg gratings reflection coupler

Double-grating coupler based on two fiber Bragg gratings(FBGs) has the advantages of compact structure and enhanced filtering efficiency. The influences of different gratings´ position in the coupling region and grating length on the filtering spectra were investigated based on the unified coupled-mode theory. The results show that, the coupling region on the left of the graing mostly affects the drop channel characteristic; the length of the grating mostly affects the drop efficiency; the transmission characteristic is best with the whole length of the coupling region Lc integral multiple of coupling length π/(2Kf ). A coupler with uniform coupling region making of two photoconductive fibers was fabricated by improved fused taper technology, then two Bragg gratings were written in the coupling region with 248nm ultraviolet laser, and a grating reflecton coupler with maximum drop reflectivity 20dB and bandwidth 0.8nm was achieved.