Arrays of fiber Bragg gratings selectively inscribed in different cores of 7-core spun optical fiber by IR femtosecond laser pulses.

In this paper, we present a new method of point-by-point femtosecond inscription of fiber Bragg gratings (FBG) arrays of different configurations in a 7-core spun optical fiber. The possibility of FBGs inscription with predefined periods in individual fiber cores allowed us to realize: 1) longitudinal FBG arrays with identical or variable resonant wavelengths in all side cores, 2) longitudinal FBG arrays inscribed only in the central or in the selected side core, and 3) an FBG array in a transverse cross section of a fiber consisting of an FBG inscribed in the central and three side cores. Based on the proposed method, by enabling the inscription through the acrylate protective coating of the fiber, a vector bend sensor has been created. Implementation of this sensor has shown that bending radii less than 4 mm can be measured with a high precision using a single-channel interrogation scheme.