**Acknowledgements**

where Δ*nmax* is the maximum reduction on the refractive index in the track, and *σ<sup>x</sup>*

Nd:YVO4

**6. Conclusions**

and fused-silica.

116 Advanced Surface Engineering Research

waveguide in the core of the waveguide and in the bulk [56, 60].

the width of the damaged region along the horizontal and vertical directions respectively, resulting in a *Δn*~5 × 10−3, which was found to be in the same range of previously reported for other waveguides inscribed by femtosecond laser writing, such as Nd:YAG ceramics,

**Figure 5.** Spatial variation of the Nd3+ fluorescence intensity for a double-filament waveguide at room temperature (a) and at 550°C (b) and emission of Nd3+ at 1060 nm under excitation at 808 nm for a 20 μm diameter depressed cladding

**Figure 4.** (a) Output power measured in double-filament and depressed cladding waveguides after isochronal heat treatment between room temperature (RT) and 750°C. (b) Evolution of the FWHM TE mode as a function of the annealing temperature between room temperature (RT) and 750°C for the 20 μm diameter cladding waveguide [56, 60].

Fabrication of integrated photonic devices has been a hot topic in research since the first report on femtosecond laser written waveguides in glass by using the Ultrafast Laser Inscription Technique (ULI), in 1996. In this chapter, we have reviewed the fundamentals of this laser

and *σ<sup>y</sup>*

are

Dr. Daniel Sola thanks the Ministry of Economy and Competitiveness of the State General Administration under the project MINECO FIS2016-76163-R and the PIT2 program of the University of Murcia's own research plan for the financial support of his contract.
