Broad-band optical coupler based on evanescent-field coupling between three parallel long-period fiber gratings

We propose a broad-band 3×3 optical coupler based on the principle of evanescent-field coupling between three parallel identical long-period fiber gratings. In particular, we demonstrate experimentally the situation where light is launched into one fiber and coupled equally to the other two fibers. The effects of changing the surrounding refractive index and introducing an offset distance between the gratings on the transmission characteristics of the coupler are investigated. Using three identical 35-mm-long gratings, we find that the peak coupling efficiency increases by 9 and 14 dB, as the surrounding refractive index and the offset distance increase from 1.0 to 1.440 and from 0 to 50 mm, respectively. The side-lobe suppression ratio varies between 16.0 and 23.0 dB. We achieve a total power conversion efficiency of 85% at the resonance wavelength with a surrounding refractive index of 1.420 and an offset distance of 50 mm.