Low bend loss femtosecond written waveguides exploiting microcrack enhanced modal confinement

While recent advances in femtosecond writing have allowed rapid and flexible realization of lab-on-chip and integrated optics systems in various media, miniaturisation of such fs-written waveguide architectures remains hampered by high bend loss α b . Despite efforts to reduce bend loss, e.g. by annealing the waveguides [1] or writing stress structures [2], the minimum bend radius at telecommunications wavelengths remains large (> 16mm) in silica [1]. To overcome this, we demonstrate a novel bend loss reduction method by inducing stress to fabricate a microcrack on the outer bend edge of the waveguide. The large index difference at the core-microcrack interface (1.45 vs 1.0) enhances modal confinement and inhibits radiation loss. Laser induced stress cracking is often used for dicing glass sheets with excellent smoothness (roughness < 30nm) [3]; hence such cracks will not exacerbate scattering losses when integrated alongside a waveguide.