Refractive index modifications are fabricated in the volume of rare-earth-doped glass materials namely Er- and Pr-doped
ZBLAN (a fluoride glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF), an Er-doped nano-crystalline glass-ceramic and
Yb- and Er-doped phosphate glass IOG. Femtosecond laser radiation (τ=500fs, λ=1045nm, f=0.1-5MHz) from an Ybfiber
laser is focused with a microscope objective in the volume of the glass materials and scanned below the surface
with different scan velocities and pulse energies. Non-linear absorption processes like multiphoton- and avalanche
absorption lead to localized density changes and the formation of color centers. The refractive index change is localized
to the focal volume of the laser radiation and therefore, a precise control of the modified volume is possible. The width
of the written structures is analyzed by transmission light microscopy and additionally with the quantitative phase
microscopy (QPm) software to determine the refractive index distribution perpendicular to a waveguide. Structures
larger than 50μm in width are generated at high repetition rates due to heat accumulation effects. In addition, the
fabricated waveguides are investigated by far-field measurements of the guided light to determine their numerical
apertures. Using interference microscopy the refractive index distribution of waveguide cross-sections in phosphate glass
IOG is determined. Several regions with an alternating refractive index change are observed whose size depend on the
applied pulse energies and scan velocities.
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