Development of a new fabrication method for stacked optical waveguides using inorganic-organic copolymers

The number of channels in integrated optical modules is limited by the horizontal dimension of the chips. The transition from planar technologies to vertical structuring schemes yields higher integration densities, but makes suitable optical materials as well as an adapted fabrication method necessary. We here present a recently developed technology for the stacking of optical waveguides using hybrid inorganic-organic polymers (ORMOCER(R)s). Among advantages with respect to the use of low-cost conventional processing methods (e.g., photopatterning, spin coating), this material system offers a high thermal stability and a low optical attenuation at the telecom wavelengths. The stacking process faces several fundamental problems such as broadening of the structures during UV patterning or the appearance of index inhomogeneities generated by diffusion effects. In addition, the nonlinear index change of the polymer materials during polymerization has to be considered carefully to ensure defined index distributions in all layers. A solution meeting these requirements substantially reverting to standard processes will be presented. The key points, an LTV absorber method and a combined UV and thermal curing, are investigated in detail. Exemplary, the realization and test of an optical fan-out element composed of four waveguide layers, each with 8 channels of equal path lengths, is shown.