Waveguides with Bragg gratings realized on a flat polymer foil are promising candidates for advanced strain sensors since such a planar approach allows precise positioning of multiple sensors in various well-defined directions, in the same foil. As such, an optical version of an electrical strain gage can be realized. Herein, several parameters are discussed which define the behaviour of such sensor foils, in particular the grating design, including the wavelength of operation and mechanical and optical properties of the used polymers. Epoxy and Ormocer®-based Bragg grating sensors operating at 850 nm and 1550 nm wavelength were realized using nano-imprint lithography and laser direct-write lithography and their strain and temperature sensitivities were compared. Finally, it is demonstrated that optical strain gage rosettes can be realized by multiplexing 3 angularly displaced sensors in the same waveguide on a single foil.
In this paper, different guided mode resonance (GMR) grating sensors are studied with the focus on investigating the possibility of high-performance configurations with simplified and potentially low-cost fabrication methods. The gratings are fabricated in polymer using nano-imprint lithography (NIL). We have chosen Ormocer materials, as they allow fabrication at room temperature, are UV patternable and have good optical and dielectric properties. The GMR gratings have a pitch around 550 nm, which corresponds to a resonance response around 850 nm with a Q factor of 2200 in simulations. In experiment, gratings imprinted on glass and on foil are characterized, showing a successful imprinting process of both devices. The measured optical response of the GMR grating for a change of the refractive index unit (RIU) of the cover (Δλ=Δ<i>n<sub>c</sub></i>) is 100 nm/RIU. The measured temperature sensitivity is -0.07 nm/K.