Planar waveguide Bragg grating sensors for composite monitoring

Composite materials are extensively used in a wide array of application markets by virtue of their strength, stiffness and lightness. Many composite structures are replaced today not only after failure but also before, for precautionary reasons. Adding optical sensing intelligence to these structures not only prolongs their lifetime but also significantly reduces the use of raw materials and energy. The use of optical based sensors offer numerous advantages i.e. integrability, high sensitivity, compactness and electromagnetic immunity. Most sensors integrated in composites are based on silica fibers with Bragg gratings. However, polymers are an interesting alternative because they present several advantages. They have high values in the opticalconstants involved in sensing, are cost-effective and allow larger elongations than silica. Moreover, planar optical waveguides represent an interesting approach to be further integrated e.g. in circuits. We present a comparison between Ormocer (R)-based and epoxy-based polymer waveguide Bragg grating sensors. Both polymers were screened for their compatibility with composite production processes and for their sensitivity to measure temperature and stress. Ormocer (R)-based sensors were found to exhibit a very high sensitivity (-250 pm/degrees C) for temperature sensing, while the epoxy-based sensors, although less sensitive (-90 pm/degrees C) were more compatible with the epoxy-based composite production process. In terms of sensitivity to measure stress, both materials were found to be analogous with measured values of (2.98 pm/mu epsilon) for the epoxy-based and (3.00 pm/mu epsilon) for Ormocer-based sensors.