Low bend loss femtosecond laser written waveguides exploiting integrated microcrack

We introduce the fabrication and use of microcracks embedded in glass as an optical element for manipulating light propagation, in particular for enhancing waveguide performance in silica integrated optics. By using a femtosecond laser to induce a strong asymmetric stress pattern in silica, uniform cracks with set dimensions can be created within the substrate and propagated along a fixed path. The smoothness of the resulting cleave interface and large index contrast can be exploited to enhance waveguide modal confinement. As a demonstration, we tackle the longstanding high bendloss issue in femtosecond laser written silica waveguides by using this technique to cleave the outer edge of laser written waveguide bends, to suppress radiative bend loss. The microcrack cross section is estimated to be 15 mu m in height and 30 nm in width, for the 10 x 10 mu m waveguides. At 1550 nm wavelength, losses down to 1 dB/cm at 10 mm bend radius were achieved, without introducing additional scattering. Both the cleave stress pattern and waveguide are fabricated with the same multiscan writing procedure, without requiring additional steps, and re-characterisation of the waveguides after 1 year confirm excellent long term performance stability.

Automated summary

This study introduces the fabrication and use of microcracks embedded in glass as an optical element for manipulating light propagation to enhance waveguide performance in silica integrated optics. Utilizing a femtosecond laser to induce an asymmetric stress pattern in silica, uniform cracks with set dimensions can be created within the substrate to improve waveguide modal confinement. The technique also addresses high bendloss issues in femtosecond laser written silica waveguides by cleaving the outer edge of laser written waveguide bends, achieving losses down to 1 dB/cm at 10 mm bend radius without introducing additional scattering.