  The properties of structures written inside dielectrics with high repetition
rate femtosecond lasers are known to depend strongly on the complex interplay
of a large number of writing parameters. Recently, ion migration within the
laser-excited volume has been identified as a powerful mechanism for changing
the local element distribution and producing efficient optical waveguides. In
this work it is shown that the transient plasma distribution induced during
laser irradiation is a reliable monitor for predicting the final refractive
index distribution of the waveguide caused by ion migration. By performing
in-situ plasma emission microscopy during the writing process inside a
La-phosphate glass it is found that the long axis of the plasma distribution
determines the axis of ion migration, being responsible for the local
refractive index increase. This observation is also valid when strong positive
or negative spherical aberration is induced, greatly deforming the focal volume
and inverting the index profile. Even subtle changes in the writing conditions,
such as an inversion of the writing direction (quill writing effect), show up
in the form of a modified plasma distribution, which manifests as a modified
index distribution. Finally, it is shown that the superior control over the
waveguide properties employing the slit shaping technique is caused by the more
confined plasma distribution produced. The underlying reasons for this
unexpected result are discussed in terms of non-linear propagation and heat
accumulation.
