Efficient and Broadband Trident Spot-Size Convertor for Thin-Film Lithium Niobate Integrated Device

Thin-film lithium niobate on insulator (LNOI) has recently emerged as a promising platform for high-speed optical communication devices. For practical applications, an efficient, polarization-insensitive, misalignment-tolerant and broadband fiber-to-chip optical coupler is necessary. In this letter, we present a fiber-to-chip edge coupler based on trident spot-size convertor (SSC). Experiment shows 1.18/1.10 dB per facet low loss at 1550 nm for TE/TM polarization respectively. A relatively large alignment tolerance (AT) has also been demonstrated. Over a broadband wavelength range from 1490 nm ~ 1640 nm, the edge coupler exhibits a maximum loss of 1.30 dB, a wavelength dependent loss (WDL) of 0.23 dB for TE mode and a polarization dependent loss (PDL) of 0.33 dB when coupled to a single mode fiber (SMF) with a mode field diameter (MFD) of ~ $6~\mu \text{m}$ .