First Demonstration of Single-Mode Polymer Optical Waveguides with Circular Cores for Fiber-to-Waveguide Coupling in 3D Glass Photonic Interposers

A simple and low-cost fabrication method for single-mode polymer optical waveguides with circular cores was demonstrated for fiber-to-waveguide coupling. The waveguide structure consists of trenches with semicircular cross sections, fabricated with dry film benzocyclobutene (BCB) as the bottom cladding layer, circular cores embedded inside the trenches, and another layer of dry film BCB as the top cladding layer. Simple photolithography is used to pattern both trenches in the bottom cladding layer and cores, and only one mask is used for both lithography steps. The advantages of single-mode circular waveguides on ultra-thin 3D glass interposers are discussed by comparing optical properties of those with conventional polymer waveguides with trapezoidal cross sections. To the best of the author's knowledge, this is the first demonstration of single-mode polymer waveguides with circular cross sections. Fabrication of circular-core waveguides are discussed and geometry characterization and analysis are performed.