Determining the Orientation of Tilted Fiber Bragg Gratings Using a Planar Substrate

The tilted fiber Bragg grating (TFBG) is known as orientation-dependent (rotating along the axial direction) because of the directionality of the grating plane. Upon the orientation of the tilt plane the mode field distributions cross the fiber is no longer acting as an isotropic circularly symmetric pattern (i.e., the high order cladding modes), which implies that cross the fiber section the sensitivities of TFBG to mechanical, physical, and bio- chemical sensing is not a constant. Therefore, it is very important to know the orientation of TFBGs prior to the real measurements for vector parameters such as vibration, rotation, bending, and pressure. Here we proposed and demonstrated a simple method to determine the orientation of TFBG via a planar substrate (glass slide for instance). When attaching TFBG on its surface and rotating the fiber, we found out that the cladding resonances of TFBG present an asymmetrical amplitude modulation and the strongest modulated mode to rotation identifies the grating plane direction. Meanwhile, by calculating the spectral area of TFBG transmission and selecting a substrate with a suitable refractive index, we can obtain well-improved results in terms of quantification of the directionality of the grating plane.