Compact lithium niobate photonic integrated circuits

TL;DR

This paper introduces a fully-etched lithium niobate photonic integrated circuit platform that achieves ultra-low propagation loss and high integration density, enabling compact, high-performance photonic devices such as microring resonators and soliton microcombs.

Contribution

The authors develop the first fully-etched LN PIC platform with ultra-low loss and high integration density, surpassing previous partial-etch approaches.

Findings

Achieved bending radius down to 20 μm with smooth sidewalls.

Realized high-Q microring resonators with Q/V of 7.1×10^4 μm^-3.

Demonstrated soliton microcombs at 500 GHz repetition rate.

Abstract

Lithium niobate (LN) is a promising material for future complex photonic-electronic circuits, with wide applications in fields like communications, sensing, quantum optics, and computation. LN took a great stride toward compact photonic integrated circuits (PICs) with the development of partially-etched LN on insulator (LNOI) waveguides. However, integration density is still limited for future high-compact PICs due to the partial edge nature of their waveguides. Here, we demonstrate a fully-etched LN PIC platform which, for the first time, simultaneously achieves ultra-low propagation loss and compact circuit size. The tightly-confined fully-etched LN waveguides with smooth sidewalls allow us to bring the bending radius down to 20 $\mu$m (corresponds to 1 THz FSR). We have achieved compact high-$Q$ microring resonators with $Q/V$ of 7.1 $\times$ 10$^{4}$ $\mu$m$^{-3}$, almost one order of magnitude larger than previous demonstrations. The statistical mean propagation losses of our LN waveguides is 8.5 dB/m (corresponds to mean $Q$-factor of 4.9 $\times$ 10$^{6}$) even with a small bending radius of 40 $\mu$m. Our compact and ultra-low-loss LN platform shows great potential in future miniaturized multifunctional integration systems. As complementary evidence to show the utility of our platform, we demonstrate soliton microcombs with an ultra-high repetition rate of 500 GHz in LN.