Narrow-band directional couplers made of dissimilar single-mode fibers with different cladding refractive indexes

A wavelength-selective directional coupler, consisting of two polished single-mode fibers with different cladding refractive indexed, has been fabricated. In contrast to symmetrical couplers, where the power transfer characteristic is a quasiperiodic function of the wavelength, couplers made of dissimilar fibers show a true bandpass-filter characteristic. They consist of two fibers with different core diameters and refractive-index profiles, having the same cladding refractive index. The parameters of the fibers must be chosen in such a way that if their propagation constants beta /sub 1/, beta /sub 2/ are plotted over the wavelength, the curves intersect at a cross-over wavelength lambda /sub 0/ equal to the center wavelength of the filter. The 3-dB bandwidth of the coupler's power transfer characteristic is 13.6 nm, the best value achieved up to now.<
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