Compact vector bend sensor using Dual-Off-Axis innermost Cladding-Type FBGs

A compact fiber-optics sensor is proposed and demonstrated experimentally using vector bend measurements. The device consists of a section of multiple-cladding fiber containing a dual-off-axis fiber Bragg grating (FBG) inscription over both the fiber core and cladding. The FBGs are distributed on both sides of the fiber core axis using the high-peak power of a femtosecond-laser and improved FBG inscription technology. Four resonances were observed in the reflection spectrum corresponding to two 'cladding' and two 'core' grating reflections. Because of the weak confinement of light propagation in the innermost fiber cladding, the cladding resonances were sensitive to fiber bending for only power interrogation. Moreover, the asymmetric distributions over the fiber cross section of the gratings produced vector responses to the bend application on the fiber in a certain coordinate plane.