# Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits

## TL;DR

This paper presents a low-loss fiber-to-chip coupling method for lithium niobate photonic circuits, significantly reducing insertion losses and enabling practical fiber integration.

## Contribution

The authors develop and experimentally demonstrate a mode size converter with inverse tapers that achieves fiber-to-chip coupling loss below 1.7 dB, improving over previous >10 dB losses.

## Key findings

- Fiber-to-chip coupling loss is reduced to below 1.7 dB.
- The mode converter shows high fabrication tolerance and repeatability.
- Enables practical interfacing of lithium niobate photonic circuits with optical fibers.

## Abstract

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly > 10 dB) dominates the total insertion losses of typical LN photonic integrated circuits, where on-chip propagation losses can be as low as 0.03 - 0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetric guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.