On-chip rotated polarization directional coupler fabricated by femtosecond laser direct writing.

We present a rotated polarization directional coupler (RPDC) on a photonic chip. We demonstrate a double-track approach to modify the distribution of the refractive index between adjacent tracks and form a single waveguide with an 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 16 dB and 20 dB for the corresponding orthogonal polarizations. We perform reconstruction of the 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° and 45° RPDCs, respectively.