High-performance and fabrication-tolerant edge coupler on thin film lithium niobate based on a three-dimensional inverse taper

Abstract

Thin film lithium niobate has been widely considered as a promising photonic integration platform. However, due to the non-vertical sidewall of lithium niobate etching, it is challenging to design an efficient fiber coupler for the submicron-sized lithium niobate waveguide. Here, a high-performance edge coupler to a cleaved fiber is introduced on the thin film lithium niobate platform using a sharp three-dimensional taper. The proposed taper structure, which ensures a nearly adiabatical transition of both transverse-electric (TE) and transverse-magnetic (TM) modes, is formed by two standard patterning steps for lithium niobate. A silica ridge waveguide is used as the intermediate transition structure between the taper and a high-numerical-aperture fiber. The simulated coupling losses are as low as −0.1 dB per facet, while the experimental mean values reach −0.29 and −0.24 dB for TE and TM modes, respectively. The coupling spectra exhibit a flat wavelength response, which almost coincides for both polarizations. The 0.5-dB coupling bandwidth is beyond 180 nm. Within this bandwidth, the worst coupling losses are −0.61 and −0.56 dB for TE and TM modes, respectively, and the polarization-dependent loss is below 0.17 dB. The present edge coupler also exhibits a good fabrication tolerance, and its fabrication can also facilitate wafer-scale processing without wafer dicing and end-facet polishing.