Glass substrate with integrated adiabatic waveguide bends for high-density fiber-to-chip coupling

Glass substrates have remarkable optical and radio frequency characteristics, making them the appealing platform for copackaged optics and on-board optical interconnects. Ion-exchange (IOX) diffusion in glass produces optical waveguides with minimal fiber-to-chip coupling loss but suffer from limited bend radius due to low refractive index contrast. This paper introduces adiabatic IOX waveguide designs with >40% increase in the refractive index contrast of the bend region, reducing bend losses from 27 dB/cm to 0.9 dB/cm at a 6 mm radius, and shrinking the radius for a bend loss of 0.1 dB/cm from ~20 mm to ~9 mm. This represents a more than 4-fold decrease in footprint area as compared to standard IOX waveguides. Importantly, these low-loss waveguides can be implemented without altering the existing wafer-scale IOX process. The approach leverages the graded index nature of the IOX, enabling self-induced tapered transitions and demonstrating efficient mode conversion using a fully adiabatic waveguide bend.