Compact and broadband TFLN two-mode multiplexer using fast quasi-adiabatic dynamics

Thin-film lithium niobate (TFLN) has emerged as a strong candidate for next-generation on-chip communication platforms. Mode multiplexers with compact size and large bandwidths are crucial for high-capacity mode-division multiplexing systems. Adiabatic mode multiplexers ensure that the target mode is transferred smoothly without abrupt changes, leading to better bandwidth, power efficiency, and robustness performance. However, the traditional adiabatic mode multiplexers based on linear taper should be long enough to satisfy the broadband adiabatic condition. Here, we overcome this length-bandwidth limitation using fast quasi-adiabatic (FAQUAD) dynamics in TFLN mode multiplexer to achieve adiabaticity quickly during the mode evolution process. Our demonstration represents improvements in length-bandwidth (95 μm-132 nm) performance compared to linear adiabatic mode multiplexer (465 μm-104 nm) with mode conversion efficiency > 90%. Meanwhile, the proposed FAQUAD TFLN two-mode multiplexer presents robust fabrication tolerance in 62 nm bandwidth when the width and rib height errors are ±30 nm and ±10 nm, respectively.