Measurement of the refractive index profile of waveguides using defocusing microscopy

Defocusing microscopy (DM) is a bright-field optical microscopy technique often used to obtain structural parameters of objects with low difference in refractive index in relation to the surrounding medium (phase objects). We show a use of this technique to measure the refractive index (n) profile of waveguides produced by femtosecond laser micromachining inside the bulk of a sodalime glass. The results are used to analyze the influence of production parameters on n. The methodology requires only a bright-field optical microscope and has proved to be easily applied. Results provide important insights on the waveguide microfabrication process, since translation speed, rather than intensity, has shown to be more important for achieving greater variations in refractive indices. Index of refraction differences between the waveguide and the substrate of the order of 10^-4 were measured for a series of straight waveguides fabricated with different parameters. Low sample scan speeds and pulse energies near 1.20 μJ used for fabrication showed the highest values of refractive index change for waveguides in sodalime glasses.