Tailoring the birefringence of femtosecond-laser-written multi-scan waveguides in glass

Abstract

Femtosecond-laser direct waveguide writing is progressively emerging as an alternative to conventional 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 with controlled modal birefringence. We show that by tuning the horizontal and vertical shifts between subsequent scans we can independently change both the magnitude and the axis inclination of the birefringence, while keeping efficient mode coupling with standard fibers.

Figures (4)

• Figure 1: a) Schematic representation of the effect of the scan writing order on the orientation of the fast axis of the waveguide (depicted as the blue arrow in the drawings). b) Optical microscope images of the corresponding waveguides: the darker dot on the cross section is related to optical damage of the substrate with resulting stress accumulation. Scalebar corresponds to 10 $\mu$m. c) Cut-back data of the evaluated phase delay between two orthogonal polarization states propagating inside the waveguide for different sample lengths. The plot reports each data point together with its 2$\pi$-periodicity; the dashed line is of the form of Eq. \ref{['eq:phase']}, with $\lambda = 1550$ nm and $m=0$.
• Figure 2: Evaluated birefringence modulus in units of $b_0$ (birefringence of the reference waveguide of Fig. \ref{['fig:stdWg']}), and inclination angle $\theta_b$ of the fast axis, as a function of the horizontal scan spacing, for waveguides with a rectangular cross-section.
• Figure 3: a) Schematic representation and optical microscope picture of a waveguide with tilted cross-section ($\alpha = 35^\circ$). Scalebar corresponds to 10 $\mu$m; $\alpha$ and $\theta_b$ are shown in the drawing with their positive orientation. b) Evaluated birefringence modulus in units of $b_0$, and fast-axis axis $\theta_b$, as a function of the cross-section inclination angle $\alpha$.
• Figure 4: Evaluated birefringence modulus in units of $b_0$, and variation of the inclination of the fast axis, as a function of the horizontal scan spacing, for a waveguide with fixed cross-sectional tilt $\alpha = 35^{\circ}$ (corresponding to $\theta_{b,0} \simeq - 45^{\circ}$).