In crystalline materials, the fabrication of optical waveguides by femtosecond laser irradiation is not as easy as in glasses [7] because, in many cases, it is not possible to produce a refractive index increase, able to directly confine and guide light along a certain trajectory. On the contrary, the most typical situation is that the refractive index of the crystal is decreased by the effect of the high intensity of the laser, but even in those cases it can be used anyway to design efficient waveguides [22].
In our study, we designed and fabricated waveguides with different configurations and geometries in the search for the best performance, helping us to understand the confinement mechanisms in Pr: LLF. The following waveguide types were tested: