Bend insensitive long-period gratings written with a CO_2 laser in fiber with parabolic-index cladding

摘要

Bending characteristics of long-period gratings (LPGs) written in fiber with parabolic-index cladding are investigated experimentally. The fiber has a uniform outer cladding and an inner cladding, where the refractive index decreases quadratically with radius. A recent theoretical analysis has shown that these LPGs can be highly insensitive to bending. A very small bend-induced wavelength shift of the attenuation band of only 5 nm was predicted for an LPG bent with a uniform curvature of 100 m"'. Experiments to prove the predicted bend insensitivity are done with LPGs written with a CO2 laser at one-side and three-side laser irradiation of the fiber. The LPG is bent in an arrangement, where the curvature is maximum in the LPG's center and decreases linearly along the bent LPG in both directions. Bend-induced wavelength shifts of an attenuation band appearing at about 1560 nm and attributed to coupling between the fiber modes LP_(01) and LP_(02), are examined for four rotational orientations of the LPG inscribed at three-side laser irradiation. The largest shift of 0.46 nm is obtained for a maximum curvature (in the middle of the LPG) of 11.6 m~(-1), (or average curvature of 5.8 nm(-1)) corresponding to the largest deflection applied on the LPG in the experiments. The depths of the bands of the above LPG at the -5.5-dB level are observed to vary by less than 0.3 dB over the whole range of applied curvatures.