Low Thermal Sensitivity Grating Devices Based on Ex-45$^{\circ}$ Tilting Structure Capable of Forward-Propagating Cladding Modes Coupling

The authors describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45deg. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small tilting structures, the ex-45deg TFBGs facilitate the light coupling to the forward-propagating cladding modes. The authors have also theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium, and large tilt angles. In particular, experiments are conducted to investigate the spectra and far-field distribution, as well as temperature, strain, and refractive-index sensitivities of ex-45deg devices. It has been revealed that these ex-45deg gratings exhibit ultralow thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long-period gratings when the low thermal cross sensitivity is required