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We discuss the behavior of the transmission spectra of a long-period grating (LPG) with an external medium of refractive index higher than that of silica. We studied the evolution of the features of LPG's surrounded by several liquid media for several kinds of fiber. The study demonstrated that the behavior depends strongly on the fiber type. Efficient couplings (efficiency greater than 50%) have been obtained with an external medium of a refractive index larger than that of silica. This result indicates that a LPG can operate with surrounding materials of a refractive index higher than that of silica, promising new applications for LPG's as active optical filters.

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