Symmetric polarization-insensitive directional couplers fabricated by femtosecond laser writing

Opt Express. 2018 Jun 11;26(12):15101-15109. doi: 10.1364/OE.26.015101.

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 yield a polarization-independent power splitting and, at the same time, preserve the polarization state of the propagating light. More in detail, we devise two different approaches to realize such devices: the first one is based on local birefringence engineering by additional refractive index modification tracks, while the second one exploits ultra-low birefringence waveguides (b = 1.2 × 10-6), obtained by thermal annealing.