# On-chip Rotated Polarization Directional Coupler Fabricated by Femtosecond Laser Direct Writing

## TL;DR

This paper introduces a novel on-chip rotated polarization directional coupler fabricated via femtosecond laser direct writing, demonstrating high extinction ratios and fidelity in polarization projection, advancing integrated photonic polarization control.

## Contribution

The work presents a new method to fabricate a rotated polarization directional coupler with arbitrary birefringent axes on a photonic chip, using a double-track approach for index modification.

## Key findings

- Achieved average extinction ratios of 16dB and 20dB for orthogonal polarizations.
- Reconstructed Stokes vectors with fidelities up to 98.1% and 96.0%.
- Demonstrated effective polarization projection on a photonic chip.

## Abstract

We present a rotated polarization directional coupler (RPDC) on a photonic chip. We demonstrate a double-track approach to modify the distribution of refractive index between adjacent tracks and form a single waveguide with arbitrary birefringent optical axis. We construct a RPDC with the two axis-rotated waveguides coupled in a strong regime. The obtained extinction ratios on average are about 16dB and 20dB for the corresponding orthogonal polarizations. We perform the reconstruction of Stokes vector to test the projection performance of our RPDC, and observe the average fidelities up to 98.1% and 96.0% for the perfectly initialized states in 0$^\circ$ and 45$^\circ$ RPDCs respectively.