A polymer optical waveguide with out-of-plane branching mirrors for surface-normal optical interconnections

We have developed a simultaneous fabrication method using temperature control reactive ion etching (RIE) for channel optical waveguides incorporating plural out-of-plane branching mirrors made from polymer film. By using this method, the etching rate can be adjusted locally by controlling the temperature. This technology also enables the formation of trenches of various depths on the same polymer optical waveguide. We noted from scanning electron microscope (SEM) observations that simultaneous control of the mirror tilt angle and a smooth core surface could be achieved. To be specific, a heat treatment temperature of 130-135°C appears to be the optimum to maintain a rectangular cross section and to achieve a sufficiently smooth core surface for a polymethyl methacrylate (PMMA) waveguide. The measured propagation loss is small, in spite of the presence of a high-Δ waveguide (Δ=5.4%). For example, losses of 0.1, 0.3, and 0.7 dB/cm are measured at wavelengths of 650 nm, 850 nm, and 1.3 μm, respectively. From far-field pattern (FFP) measurements, we found that the mirror plane was almost rectilinear, and that the reflected light can be captured efficiently by a photodiode. In operational temperature tests, we showed that intensity fluctuations of the coupling light can be reduced to less than 1.5 dB for the temperature range between -25°C and +85°C by adopting a sandwich structure with glass plates