Refractive index profiling of direct laser written waveguides: Tomographic phase imaging

We present a technique to measure the refractive index profile of direct laser written waveguides. This method has the potential for straightforward implementation in an existing laser fabrication system. Quantitative phase microscopy, based on the Transfer of Intensity equation, is used to analyse waveguides fabricated with an ultrashort pulsed laser embedded several hundred micron below the surface of fused silica. It is shown that the cumulative phase change induced by the waveguide perpendicular to its axis may be monitored in real-time during the fabrication process. Results are verified through comparison with interferometry. Tomographic measurements using illumination from a high numerical aperture condenser lens are used to infer the waveguide cross-section. Results are compared with measurements of the waveguide cross-section from a third harmonic generation microscope. © 2013 Optical Society of America.