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 ≈ 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.