We prove the formation of birefringent nanoporous structures in the volume of various doped silica (TiO2, F, GeO2), GeO2 and borosilicate glasses by femtosecond laser. Using quantitative birefringence measurements and scanning electron microscopy we determined that this birefringence is mainly induced by porous nanogratings. So far, the form birefringence induced by these modifications was known mainly in fused silica. We determined the repetition rate of the irradiating laser pulses as crucial processing parameter. The retardation in the laser modified doped silica or GeO2 is comparable to fused silica, while the birefringence in Borosilicate is two times weaker.
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