High-efficiency edge couplers enabled by vertically tapering on lithium-niobate photonic chips

TL;DR

This paper introduces a novel, efficient edge coupler design for lithium niobate photonic chips that significantly reduces coupling loss and is compatible with standard fabrication processes, advancing integrated photonics applications.

Contribution

The paper presents a new wedge-shaped TFLN waveguide edge coupler with silicon oxynitride cladding, achieving low coupling loss and robust fabrication tolerance, suitable for scalable photonic integration.

Findings

Coupling loss of 1.52 dB/facet with 3-μm MFD fiber

Potential reduction to 0.43 dB/facet theoretically

Achieved minimum waveguide linewidth of 600 nm

Abstract

In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have advanced in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler connecting PICs to light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride (SiON) cladding. Experimental results show that the coupling loss between the TFLN PIC and a 3-{\mu}m mode field diameter (MFD) lensed fiber is low at 1.52 dB/facet, with the potential for improvement to 0.43 dB/facet theoretically. The coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 {\mu}m is reduced to 0.92 dB/facet. This design maintains robust fabrication and alignment tolerance. Importantly, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be easily achieved using foundry-available i-line stepper lithography. This work benefits the development of TFLN integrated platforms, such as on-chip electro-optic modulators, frequency comb generation, and quantum sensors.