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 β₁, β₂ are plotted over the wavelength, the curves intersect at a cross-over wavelength λ₀ 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