Abstract

Small-period long-period gratings (SP-LPGs) have been fabricated by a UV laser of 244 nm in hydrogen-loaded single mode fibers (SMFs). The period of the SP-LPG is determined to be 25 μm by a custom-designed amplitude mask, which is placed close to the SMF. The fabricated SP-LPGs were annealed at 80°C for 48 h to stabilize the property. The transmission spectrum measured with non-polarized light clearly shows a series resonances of dual-peak pairs (DPPs). Fig. 1(a) bottom trace depicts the typical transmission spectrum showing two pairs of dual-peaks at1474.7 nm and 1481.0 nm as one pair and 1575.6 nm and 1582.1 nm as the other pair, which agrees well with the simulation in ref [1], in which only cladding modes with the lowest azimuthal order are taken into account. Coupling to high azimuthal order cladding modes should be zero in a circularly symmetrical refractive index (RI) perturbation for the SP-LPGs [2], thus we see from Fig. 1(a) very weak DPP appears around the 1525nm.

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