Independently tailoring the birefringence axis and magnitude in femtosecond-laser-written waveguides in silica

Femtosecond-laser direct waveguide writing represents a convenient alternative to standard lithographic techniques to develop complex photonic devices, for applications ranging from classical and quantum information processing, to sensing and metrology. Laser written waveguides typically offer low modal birefringence, thus preserving coherence of polarization-encoded information. Integrated waveplates have been reported, as waveguides with tilted birefringence axis, but with limited flexibility in terms of achievable rotation angle, birefringence magnitude or control in the modal shape. Here we investigate the multi-scan approach to realize low-loss optical waveguides in fused silica substrate, working at telecom wavelengths, with finely controlled modal birefringence. We show that by tuning the horizontal and vertical shifts between subsequent scans we can independently change both the birefringence axis inclination, from vertical to over 60° tilt, and the birefringence magnitude by more than a factor of two. Efficient coupling with standard optical fibers is preserved.