Temperature-dependence of polymer grating couplers for single-mode waveguides

Polymer optical sensors have attracted significant scientific interest due to the advantages of flexibility and low- cost mass-production possibilities. A main challenge in integration of all-polymer systems is interfacing sensor parts to light sources and detectors. Coupling strategies such as end-facet coupling, 45° mirrors coupler and gratings coupler have been investigated. However, facet polishing is complicated for large flexible foils, while 45° mirrors often require a metal reflection layer. Thus grating couplers have the biggest potential and are compatible with roll-to-roll processes. Polymers have higher thermo-optic-coefficients and thermal-expansion-coefficients compared to inorganic materials, making them very sensitive to temperature changes. Consequently, polymer grating couplers show a shift of the coupling angle with temperature and thus a decay of efficiency. We therefore present a temperature characterization of all-polymer based grating couplers on waveguides based sensors. Cost-effective manufacturing methods, including hot-embossing and spin-coating, were used for the fabrication of waveguides and gratings on PMMA foils. Single mode waveguides were realized by modifying the dimensions of their cross-section. Gratings with a period of 560 nm were subsequently bonded on the waveguides for input coupling. Thermal response was characterized by monitoring the coupling angle at different temperatures. Temperature dependences of the incident angle at two main peaks of 0.0027 °/K and 0.0054 °/K were determined respectively at the wavelength of 852 nm and a linear response over the evaluated range between 298 K and 323 K was observed, which opens up possible applications for on-chip temperature monitoring and thermal compensation when integrated with polymer sensors.