Simultaneous measurement of bend direction and degree of curvature using micromachined grating fibers

We report the design and fabrication of a novel fiber optic bend sensor capable of measuring the bend direction and degree of curvature simultaneously. The sensor has a serial arrangement of two fiber Bragg gratings (FBGs) with two different reflection center wavelengths and similar asymmetrical cross sections with different etching faces on an independent single-mode fiber. These cross sections were fabricated by anisotropic reactive ion etching (RIE) using CFplasma. The reflection center wavelengths of the two FBGs shifted sinusoidally with a phase difference of π/2 as a function of the bend direction under constant bend feeder displacement. Because it was shown that the reflection center wavelength shifted linearly as a function of the degree of curvature of an FBG with an asymmetrical cross section in our previous experiment, this sensor could be used to detect both the bend direction and the curvature from a pair of central reflection wavelength shifts for two FBGs. These experimental results were compared with theoretical values calculated from a strain of the core in an etched optical fiber.