Abstract

We report for the first time that transmission of optical light centered at a wavelength of 1549.8 nm through a tapered dual-core As $_{2}$ Se $_{3}$ -PMMA fiber simultaneously inscribes an antisymmetric long-period grating and an antisymmetric apodized fiber Bragg grating. The evolutions of the transmission spectrum, and the back-reflected spectrum of the dual-core fiber are experimentally measured as the accumulated exposure time increases. The bandwidth of the antisymmetric apodized fiber Bragg grating is measured, and a narrow bandwidth of 3.5 GHz (0.028 nm) with $\sim\! \text{100}\%$ reflectivity (∼32 dB contrast) is observed. A theoretical model of an antisymmetric apodized fiber Bragg grating in a dual-core fiber computationally reproduces the experimentally observed evolution of the transmission spectrum and the back-reflected spectrum. Both theory, and experiment reveal for the first time that such structure requires dual phase-matching conditions. It is also the first time that one single reflection peak of a fiber Bragg grating is observed on a dual-core multimode fiber that supports two modes due to the satisfying of dual-phase matching conditions. The performance of this device as an optical filter is investigated. The antisymmetric apodized structure provides a novel method for achieving apodized fiber Bragg gratings with narrow bandwidth, high reflectivity, and suppression of side-lobes. The simultaneously inscribed antisymmetric long-period grating and antisymmetric apodized fiber Bragg grating in the dual-core As $_{2}$ Se $_{3}$ -PMMA taper opens the path towards the realization of novel sensors and devices.

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