Fabrication of tapered waveguides by i-line UV lithography for flexible coupling control

A tapered bus-waveguide is demonstrated to enhance the waveguide-to-cavity coupling by mass-productive, cost-effective i-line UV lithography. Through enlarging the overlap between the evanescent wave and waveguide resonator, we experimentally show that the coupling strength of silicon nitride waveguides can be 7 times stronger than the conventional coupling of a uniform, straight bus-waveguide. For the first time, strong over-coupling is identified at a 400 nm gap and quality factor approximate to 105 without elongating the coupling length. This design relieves the fabrication limits and provides the flexibility for coupling control, especially in the strongly over-coupled regime with i-line UV lithography.(c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement