Direct femtosecond laser written waveguides in bulk Ti³⁺:Sapphire

We report on direct inscription of type-II waveguides in bulk titanium-doped sapphire with an ultrafast chirpedpulse oscillator. Ti ³⁺:Sapphire is of particular interest due to its large emission bandwidth which enables a broadband tunability and generation of ultra-short pulses. However, its lasing threshold is high and powerful high brightness pump sources are required. The fabrication of a waveguide in Ti³⁺:Sapphire could thus enable the fabrication of low-threshold tunable lasers and broadband fluorescence sources. The latter are of interest for optical coherence tomography where the obtainable resolution scales with the bandwidth of the light source. The fabricated waveguides are formed in-between two laser induced damage regions. This technique has been applied to other crystalline materials (e.g. LiNbO₃) but not in Ti³⁺:Sapphire, yet. The size of the structural changed regions is strongly dependent on the writing laser polarization. These damage regions of changed structure cause a stress-field inside the crystalline lattice which consequently increases the refractive index to form a waveguide. The written structures exhibit a strong birefringence and two waveguides that support orthogonal polarized modes are formed between each pair of damage lines. Linearly polarized light parallel to the crystal's surface is guided between the two damage regions while a waveguide for the orthogonal polarization is formed underneath. The propagation properties of the waveguides are characterized by their near-field profiles and insertion losses with respect to the writing parameters. Further the fluorescence output power is measured and the emission spectra of the waveguides are compared to the bulk material.