TOPAS based humidity insensitive polymer planar Bragg gratings for temperature and multi-axial strain sensing

摘要

We demonstrate the application of humidity insensitive polymer planar Bragg gratings written into planar TOPAS substrates for measuring temperature as well as multi-axial tensile and compressive strain. The polymer planar Bragg gratings are realized by a rapid fabrication process using a KrF excimer laser which illuminates a stacked mask configuration consisting of an amplitude mask and a phase mask with a grating period of 1036.79 nm. We determine the UV-induced refractive index modification of the integrated waveguide by phase shifting Mach Zehnder interferometry and investigate its light guiding properties. The integrated polymer planar Bragg grating (PPBG) reflects a wavelength of 1576.5 nm with a reflectivity of about 93 % and shows almost no sensitivity against relative humidity. Therefore, the TOPAS-PPBG is well-suited for measuring temperature as well as tensile and compressive strain. Temperature measurements are feasible up to 116°C. An observed temperature hysteresis disappears after three consecutive temperature cycles, after which the TOPAS-PPBG is characterized by a sensitivity of -6.5 pm/°C. Furthermore, tensile and compressive strain is applied at different angles showing an angle-dependent sensitivity of the PPBG making it a suitable candidate for multi-axial strain sensing.