Slit shaping technique for femtosecond laser direct write fabrication of two-dimensional symmetric waveguide arrays in silica glass

To obtain low loss, depth-insensitive, 2D waveguide arrays with symmetric circular cross sections in silica glass, we applied slit shaping technique to fabricate waveguides with a low NA water immersion objective lens (10×, 0.3NA). We systematically investigate the influences of slit width, pulse energy, writing speed, the number of scans, processing depth, and spherical aberration on the fabrication resolution. We have applied this technique to write low loss (0.34 dB/cm) single-mode waveguides, 1D (1 × 5) and 2D (5 × 5) waveguide arrays in silica glass substrates. This technique will be useful for entangled quantum photonic integrated circuits and topological photonics.