In recent years femtosecond lasers have proved to be of great utility for micromachining within the bulk of transparent materials, nevertheless little is still known about the fundamental physics driving the process. Depending on the laser intensity delivered to the sample 3 different types of features can be written into the glass. We observe that in the intermediate regime there is a correlation between the negative sign of the effective index change, the presence of anisotropic reflection and birefringence. We propose a consistent model which can explain all these three principal characteristics.

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