FPI-POFBG Angular Movement Sensor Inscribed in CYTOP Fibers With Dynamic Angle Compensator

We present the development and application of an angular movement sensor based on Fabry-Perot interferometer (FPI) in series with a fiber Bragg grating (FBG) inscribed in cyclic transparent optical polymer (CYTOP) using a femtosecond laser. The FPI was fabricated with UV-curable resin between the CYTOP and a silica pigtail, which provides flexibility and the possibility in situ fabrication. In this case, the FPI is an edge-filter for the FBG response, which presents both wavelength shift and optical power attenuation as the bending angle increases. A dynamic compensator for the material's viscoelastic response resulted in a six-fold reduction in the sensor's transient response error. The FPI-POFBG angular movement sensor was validated in knee angle monitoring during gait cycles, where the sensor presented errors as low as 2.5°. This is a low-cost sensor with high flexibility on fabrication and with the capacity of following subtle angle variations, as the ones commonly occurred in gait cycles.