  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.
