Low Bend Loss, High Index, Composite Morphology Ultra-fast Laser Written Waveguides for Photonic Integrated Circuits

We demonstrate a novel, composite laser written 3D waveguide, fabricated in boro-aluminosilicate glass, with a refractive index contrast of 1.12 x 10^-2. The waveguide is fabricated using a multi-pass approach which leverages the respective refractive index modification mechanisms of both the thermal and athermal inscription regimes. We present the study and optimisation of inscription parameters for maximising positive refractive index change and ultimately demonstrate a dramatic advancement on the state of the art of bend losses in laser-written waveguides. The 1.0 dB cm^-1 bend loss cut-off radius is reduced from 10 mm to 4 mm, at a propagation wavelength of 1550 nm.