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3D Polymer Hybrid Photonics

Figure 1. Hybrid integrated photonics in diffusive photopolymer. This circuit couples a single-mode glass fiber coupled to a small, elliptical mode silicon waveguide using a 3D routed, tapred polymer waveguide. The fiber and chip are first encapsulated in the thermoset polymer. The lithography tool then determines the core locations and draws a customized waveguide between them. The wavegiude tapers in both shape and size to match the modes.
Diffusive photopolymers were first developed by Bruce Booth at Dupont for holography. We use a two-polymer approach introduced by Lisa Dhar at Lucent. These materials contain two monomers, in which the first monomer is thermally or optically gelled, providing a physical scaffold to support encapsulated elements or photo-patterned index structures. Linear (one photon) absorption of incident light polymerizes the second monomer, locally depleting it in the exposed region. This causes diffusional mass transport of unreacted monomer into the exposed region, resulting in an increased material density and index of refraction. A post-exposure optical flood cure bleaches remaining initiator and cross-links all remaining monomer, yielding stable, permanent index structures. The final state of the polymer can be compliant to accommodate thermal expansion or glassy for greater robustness. Deep, 3D index patterns are possible because no wet processing is required - the index structures self-develop after optical exposure.

We combine these materials with custom 3D direct-write lithography tools to implement optical integrated circuits which incorporate multiple material systems which are typically incompattible, as shown in Figure 1. A milliwatt laser focus translated through the 3D volume of a diffusive photopolymer writes an optical waveguide. This waveguide can be arbitrarily routed, tapered and shaped along its path to implement mode transformations between encapsulated subcomponents. We integrate precise detection into our lithography tool to locate the subcomponents, then write waveguides between the detected component locations to fabricate hybrid integrated circuits that require no active alignment. One technologically important application is the connection of fiber to silicon photonic waveguides.
This index measurement of a thick polymer sample shows the coupling of a polymer waveguide to a visible-light single-mode fiber. Note that the light to write the waveguide is not sent out the fiber. The core position is measured by the lithography hardware which then writes the waveguide with a translating focus.
This plot shows the index cross-section of a single-mode 1550 nm waveguide measured with a custom scanning coherent confocal microscope. Note that the analog response of the material yields a gradient-index waveguide. This reduces edge-scatter and thus loss. Index contrast of 0.01 is shown.
This phase micrograph shows the end face of a rectangular waveguide array recorded with a 4-beam interference pattern. While the DIC microscope renders this as what appears to be a bumpy sheet, the polmer surface is flat. This structure transmits and image, much like the fiber bundles used in endoscopes.

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This work has been generously funded by: