We report on the fabrication and characterization of integrated Mach-Zehnder interferometers in polymer foil without an interaction window. The interferometers are based on inverted rib waveguides, which allow single mode behavior even for waveguide widths larger than a few micrometers. The phase change between the two interferometer arms upon a refractive index change of the analyte that serves as the upper cladding is generated by the asymmetricity of the two interferometer arms. A difference of the waveguide width in the straight part of the interferometer leads to different effective refractive indices and thus to a change in the interference signal. We show in small scale the process chain, which is compatible with a cost-effective roll-to-roll fabrication process. For a proof of principle we apply deionized water and a glucose solution as analytes to the sensor foils and detect the transmitted intensity as a measure of the induced phase change. A detection limit of 3·10⁻³ refractive index units is reached for homogeneous sensing at a total system length of 9.3 mm and a total waveguide core thickness of 3 μm.
New design concepts for all-polymer integrated optical Mach-Zehnder interferometers (MZIs) optimized for environmental sensing are presented. Fabricated using large-area printing techniques, these polymer-based components are designed for low cost fabrication while maintaining suitable sensitivities to external refractive index changes. One key aspect in their design is obviating the need for an "interaction window" over one arm of the interferometer, as is usually defined in semiconductor or glass components, but requiring additional lithography, deposition and etching steps not suitable for polymer printing technology. We thus employ an asymmetric MZI, and derive the form of structures with optimized sensitivity and operating characteristics. Using the derived design criteria, experimental verification using a chemical test system finally demonstrates the utility of the approach.
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