Low short-wavelength loss fiber Bragg gratings inscribed in a small-core fiber by femtosecond laser point-by-point technology.

A femtosecond-laser-induced fiber Bragg grating (FBG) usually has a higher insertion loss at the shorter wavelength than at the reflection wavelength, i.e., so-called short-wavelength loss. High-quality FBGs are inscribed in different types of small-core single-mode fibers (SMFs) by the use of femtosecond laser point-by-point technology in order to investigate the effect of the fiber core diameter on the grating inscription efficiency and on the short-wavelength loss. A lower laser pulse energy is required to achieve the same grating reflectivity in a smaller-core fiber than in a large-core fiber. The short-wavelength loss of the small-core FBG is lower than that of the large-core FBG with the same reflectivity. Furthermore, a series of FBGs with a low short-wavelength loss are inscribed in a small-core SMF along the fiber axis to achieve so-called series-integrated FBGs (SI-FBGs). Finally, the effect of the input light direction on the reflection peak of the SI-FBGs is investigated to reduce the influence of the grating short-wavelength loss in the sensing and communication applications.