3D laser-written silica glass step-index high-contrast waveguides for the 3.5 mu m mid-infrared range

We report on the direct laser fabrication of step-index waveguides in fused silica substrates for operation in the 3.5 mu m mid-infrared wavelength range. We demonstrate core-cladding index contrasts of 0.7% at 3.39 mu m and propagation losses of 1.3 (6.5) dB/cm at 3.39 (3.68) mu m, close to the intrinsic losses of the glass. We also report on the existence of three different laser modified SiO2 glass volumes, their different micro-Raman spectra, and their different temperature-dependent populations of color centers, tentatively clarifying the SiO2 lattice changes that are related to the large index changes.