Higher order coupling mode for orientation-dependent bend measurement using an off-axis FBG inscription over few-mode fiber

An off-axis fiber Bragg grating (FBG) was proposed and demonstrated for orientation-dependent bend measurement. The FBG was formed in a few-mode fiber over core and cladding via high peak power femtosecond laser and phase mask technique. Three clear resonances were observed in the reflection spectrum, which were corresponding to the fundamental mode (LP₀₁), the first-order mode (LP₁₁), and the coupling mode (LP_01-11). The LP_01-11 mode presents highly intensity response to the fiber bend without any wavelength change due to its unique mode field, and the FBG eccentric inscription further enhances the polarization of LP_01-11 so that the sensor was directionally sensitive to the fiber bend by the intensity interrogation. Moreover, the wavelength-referenced temperature information could be demonstrated together with the fiber bend. The sensor was a good candidate for bend and temperature measurement simultaneously.