Femtosecond writing of high quality waveguides inside phosphate glasses and crystalline media using a bifocal approach

The fabrication of waveguides inside transparent media using ultrashort laser pulses has gained a lot of interest during the past years. When these intense pulses are tightly focused inside the material a refractive index increase in the focal volume can be achieved. Low-loss waveguides and true three-dimensional integrated optical devices have been produced by this direct writing technique in silicate glasses. However, other materials like phosphate glasses or crystalline quartz show a different behavior. In this case stress is induced around the focal volume leading to a refractive index increase in the surrounding areas (inverse profile). As a consequence high quality waveguides with a mode profile matched to conventional fibers cannot be fabricated. In this presentation we demonstrate a new approach to overcome this problem. The laser beam is split by a transmission grating and simultaneously focused at two different areas. When the distance between the two foci is appropriate the desired refractive index profile is obtained. We will demonstrate high quality waveguides in crystalline quartz and in phosphate glasses produced by this technique. The influence of the processing parameters is discussed in detail.