Large core, single-mode glass-based waveguides for photonic integrated circuits

We previously demonstrated light guiding in fiber-on-glass (FOG) dielectric waveguides using fluoro-tellurite glasses. These waveguides were fabricated by mechanically pressing a fiber onto a polished planar glass substrate of lower refractive index above the glass transition temperatures. However, two handling constraints have been discovered in this approach. In practice, for novel inorganic compound glasses, the minimum dimension of fiber that can be handled is preferably around 30μm. The minimum refractive index difference between the fiber and the substrate that can be reliably achieved at present with these glasses is 0.01. Our simulation results showed that, taken together, these restrictions provide a practical barrier to achieving single-mode FOG operation at telecommunications wavelengths. Here we present simulation and experimental results for a new inorganic glass FOG waveguide that simultaneously meets these handling constraints and achieves mono-mode operation around 1.55 μm. In this new design, a homogeneous glass fiber is partially embedded lengthwise in a substrate of higher refractive index glass; the non-embedded part of the fiber is air clad. Simulation results presented for fluoro-tellurite FOG waveguides confirm the success of the new design in realizing single-mode propagation at 1.55 μm for a fiber diameter of 30 μm and a fiber-substrate refractive index difference of 0.01. The design is robust, with good dimensional fabrication tolerance, but predicted losses are over 6 dBcm ^-1. A proof-of-principle demonstrator is fabricated using two commercially available multi-component silicate glasses (Schott F2 and F4). This shows multimode waveguiding at 0.633 μm, guidance around a curve, and appears mono-mode at 1.575 μm.