Effect of refractive index-mismatch on laser microfabrication in silica glass

We studied the peculiarities of femtosecond laser microfabrication in silica glass with a refractive index that did not exactly match the value for which the focusing optics is designed. Spherical aberrations resulting from a small refractive index mismatch were found to increase the size and distort the shape of photodamaged regions, thus reducing the spatial resolution of the microfabrication. However, these undesirable effects can be minimized, providing that the focusing depth inside the glass is not too large, and the laser intensity is kept close to the light-induced damage threshold.