Symmetric polarization insensitive directional couplers fabricated by femtosecond laser waveguide writing

TL;DR

This paper presents the design and experimental realization of polarization-insensitive directional couplers using femtosecond laser waveguide writing, overcoming birefringence-induced polarization dependence for improved optical device performance.

Contribution

The authors introduce two novel approaches to fabricate polarization-insensitive directional couplers, enhancing their robustness and applicability in integrated photonics.

Findings

Demonstrated polarization-independent power-splitting ratios

Developed two fabrication methods: birefringence engineering and thermal annealing

Validated the approaches through experimental measurements

Abstract

We study analytically the polarization behaviour of directional couplers composed of birefringent waveguides, showing that they can induce polarization transformations that depend on the specific input-output path considered. On the basis of this study, we propose and demonstrate experimentally, by femtosecond laser writing, directional couplers that are free from this problem and also yield a polarization independent power-splitting ratio. More in detail, we devise two different approaches to realize such devices: the first one is based on local birefringence engineering, while the second one exploits ultra-low birefringence waveguides obtained by thermal annealing.