  Atoms ionization by the simultaneous absorption of multiple photons has found
applications in fiber optics, where it leads to unique nonlinear phenomena. To
date, studies of the ionization regime have been limited to gas-filled
hollow-core fibers. Here, we investigate multiphoton ionization of standard
optical fibers, where intense laser pulses ionize the atoms constituting the
fiber structure itself, instead of that of the filling gas. We characterize
material modifications produced by optical breakdown. Their formation affects
laser beam dynamics over hours long temporal scales. The damage features are
studied by means of optical microscopy and X-ray microtomography. In the
framework of glass photonics, our results pave the way for a novel glass
waveguide micromachining technique.
