Prism Coupling into Polymer Integrated Optical Waveguides with Liquid Superstrates

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Abstract
We have investigated the light coupling behavior of prism-coupled waveguides adjacent to liquid superstrates. A flow cell was used for delivering the liquid superstrates to the waveguide surface. The incoupled angles and outcoupled mode patterns were collected from polystyrene waveguides subjected to superstrates of air, water, and dimethylsulfoxide. Results showed that poor coupling and significant mode scrambling were observed when liquids were infused into the flow cell volume. The poor waveguiding characteristics were found to be caused by the presence of air in the prism coupling gaps. Poor coupling was eliminated for a water superstrate by ensuring that water also occupied the prism coupling gaps. The mode coupled into the water-coupled waveguide with a water superstrate was always the most intense mode in the outcoupled pattern. In this manner it was shown that a refractive index mismatch between the coupling gap and the superstrate will lead to a waveguide with measured coupling angles that are completely insensitive to changes in superstrate refractive index.