Enhancement of free spectral range of the resonance peaks in a long-period fiber grating by controlling material dispersion of cladding modes

The transmission spectrum of a long-period fiber grating was analyzed theoretically for an optical fiber whose material dispersion in the cladding region was modified by doping a transition metal, Cr3+. By doping Cr3+ ions into the cladding, we were able to change the effective indices of cladding modes and subsequently the phase matching conditions for coupling with the core mode in a long-period fiber grating. Enhancement of the free spectral range in the long-period fiber grating over 400 nm were rigorously predicted in the proposed fiber structures, which could be applicable in novel filters for a wide band optical communication systems. (C) 2001 Published by Elsevier Science B.V.