Broadband high coupling efficiency edge coupler with low polarization-dependence on the silicon-nitride platform

In this paper, we present an ultra-broadband double-tip edge coupler with high coupling efficiencies for both TE and TM polarizations on a silicon nitride platform. The coupler is designed for the high NA fiber which provides a smaller mode size. In simulations, both TE and TM coupling efficiencies remain above 90% within the wavelength range 1000-2000 nm. In comparison, the TE and TM coupling efficiencies of the conventional single-tip coupler design can drop below 73% over this range. An ultra-high coupling efficiency above 95% is predicted over a bandwidth of 760 nm for the TE mode with the double-tip design, compared to 450 nm for the single-tip design. Experimentally, the highest measured coupling efficiencies for the double-tip and single-tip couplers are 97.1% and 95.7%, respectively. For the double-tip design, the coupling efficiency remains above 90% within the measurement range (1450-1640 nm), as the polarization state changes with the wavelength. These measurements confirm the polarization insensitivity of the double-tip design. In addition, we measure the performance of the double-tip design with a laser source operating at shorter wavelengths. The measured 94% coupling efficiency at 1280 nm indicates the broad bandwidth of the coupler. This work is anticipated to provide a promising foundation for the development of compact efficient photonic devices.