Compact and Broadband Edge Coupler Based on Multi-Stage Silicon Nitride Tapers

We have proposed and experimentally realized an ultra-compact and broadband silicon nitride edge-coupler that enables high coupling efficiency. The proposed coupler was realized by concatenating short tapers in four stages, whose angles were designed to minimize the footprint while preserving the coupling efficiency. The constituting taper segments were designed by carefully sectioning a long adiabatic taper while adapting to an appropriate taper angle for each segment. The designed coupler exhibited an extremely short footprint of 76 μm. A coupling efficiency of 92% was experimentally attained at 1550 nm wavelength when coupled to a single-mode fiber having a mode field diameter of ∼4 μm. Further, an efficiency of over 90% throughout the C and L bands was observed. A 3-dB bandwidth of 965 nm, spanning λ = 1015-1980 nm, was achieved in the simulation. Additionally, the fabricated device exhibited an enhanced cleaving tolerance by virtue of its elongated tip, along with relaxed alignment tolerances ranging up to 3.5 μm. The proposed design was also found to comply with the waveguides having widths between 1 μm and 4 μm without affecting the overall footprint and efficiency. This work is anticipated to provide a promising foundation for the development of compact photonic devices.