Optical characteristics of a hybrid fiber grating based on a surface long-period grating incorporating a fiber Bragg grating

A hybrid grating based on a surface long-period grating (SLPG) incorporating a fiber Bragg grating (FBG) was proposed, and its optical characteristics were experimentally investigated by changing the external temperature and the ambient refractive index. For the fabrication of the hybrid grating, a FBG was firstly fabricated by using a beam scanning technique with a frequency-doubled Ar+ laser; then, some portion of the cladding region without the FBG was removed by using a side polishing technique. The SLPG was formed on a flat surface of the side-polished fiber (D-shaped fiber) by using a standard contact lithography technique. The transmission spectrum of the hybrid grating was superimposed on those for the SLPG and the FBG. Variations in the external refractive index and the temperature modified the transmission characteristics of the proposed hybrid grating. The ambient index sensitivity of the resonant peak induced by the SLPG was measured to be 810.7 nm/RIU while the resonant peak generated by the FBG was kept constant as the ambient index changed in the index range from 1.30 to 1.45. The temperature sensitivities of two resonant peaks for the SLPG and the FBG were measured to be −0.52 nm/°C and 0.01 nm/°C, respectively. Accordingly, we expect discrimination between two concurrent sensitivities, such as the external refractive index and the temperature sensitivities, to be possible when the hybrid fiber grating is implemented.