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.
Highlights
Linear integrated optical devices [1] are used for splitting, combining or elaborating signals in classical optical communications, and stand at the basis of complex experiments in quantum photonics [2, 3]
The waveguide birefringence could be further reduced by applying beam-shaping techniques during the femtosecond laser irradiation process [25]. With such a low birefringence value, polarization encoded photonic qubits with coherence lengths of a few tens of microns could propagate without losing coherence for several meters. We have studied both theoretically and experimentally the behavior of polarization insensitive directional coupler (PIDC) composed of birefringent waveguides
We have shown that a birefringence unbalance in the PIDC interaction region leads to a polarization transformation that is dependent on the specific path taken by the light within the device
Summary
Linear integrated optical devices [1] are used for splitting, combining or elaborating signals in classical optical communications, and stand at the basis of complex experiments in quantum photonics [2, 3] As they rely, for their operation, on field superposition and interference, they often require control on the polarization of the propagating light [4]. As we will study in more detail in the following, despite the equal power splitting, all couplers made of birefringent waveguides may produce polarization rotations that depend on the specific input-output path taken by the light into the coupler This effect may represent a strong limitation to achieve a true polarization independent operation and in developing integrated devices for polarization-state engineering. Propose and validate experimentally two different strategies for realizing PIDCs free of any asymmetry using the FLM technology
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