A fixed phase tunable directional coupler based on coupling tuning

TL;DR

This paper presents a novel fixed phase tunable directional coupler on Lithium Niobate that allows arbitrary reflectivity tuning with stable phase, reduced fabrication errors, and improved scalability for photonic integrated circuits.

Contribution

The study introduces a coupling constant tuning-based TDC that maintains fixed phase, reduces fabrication error sensitivity, and requires fewer components compared to traditional MZI-based designs.

Findings

Enables arbitrary reflectivity tuning with fixed phase.

Exhibits fewer bending sections than MZIs.

Demonstrates resilience to fabrication errors.

Abstract

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.