Inscription of Waveguides and Beam Splitters in Borosilicate Glass Using a Femtosecond Laser with a Long Focal Length

Femtosecond-laser enable the internal fabrication of photonic components in various materials. The extremely high peak intensity due to the ultrashort pulse width leads to a nonlinear interaction in the focal point, which makes the processing of transparent materials such as glass possible. Nonlinear ionization mechanisms can lead to a local, permanent refractive index change of the material, which opens up a promising potential for the fabrication of integrated optics. If the femtosecond laser, focused by a long focal length, is moved precisely relative to the glass sample, it is possible to inscribe three-dimensional geometries directly into glass. Thus, a variety of photonic components such as waveguides, beam splitter and other micro-optical components can be fabricated directly in the volume. In general, the change of the refractive index is due to various processes in the glass network structure, such as photo-induced defects, the reorganization of the glass network structure, or thermal processes and the resulting densification of the material. [1] Depending on the inscription parameters and the used glass type itself the refractive index change of the laser modified region can be either positive or negative. [2] Therefore, different types of waveguides have to be distinguished. Type-I waveguides form the simplest realization. There the laser track itself results in a positive index change surrounded by material with a negative refractive index. Light can be guided directly in the laser track. [3] For borosilicate glass, where the laser track itself results in a negative refractive index while the surrounding volume experiences a positive refractive index, Type-II waveguides can be used. Type-II waveguides are realized out of two parallel inscribed waveguides. The light-guiding region is found in the overlapping volume of the two laser tracks. [4]