Selective fiber Bragg grating inscription in multicore fiber by femtosecond laser and phase mask

An effective and stable method for selective fiber Bragg gratings (FBGs) inscription in multicore fibers (MCFs) using a femtosecond laser and phase mask is presented. The nonlinear photoluminescence imaging technique is adopted for visually displaying the position of the femtosecond laser focus-line inside the MCF, which helps to guarantee that the laser focal-line is precisely positioned in a preset fiber core during the inscription process. FBGs with different wavelengths are successfully inscribed into each core of a seven-core fiber (SCF). The FBGs show good consistency with a side-mode suppression ratio more than 27 dB. In addition, the largest difference of the peak intensity in the reflection spectrum between different FBGs is less than 1.4 dB and the 3-dB bandwidth for the seven FBGs is less than 0.3 nm. Confirmatory experiments show that the FBGs in the SCF have perfect directional bending response. This method provides an effective scheme for selective FBGs inscription in a multicore fiber.