Direct femtosecond laser writing of waveguide structures and Bragg gratings for integrated NIR optics using multi scan technique

Abstract

Femtosecond laser direct writing techniques are very useful tools for rapid prototyping of integrated optics, since for example 3D- waveguide structures can be easy implemented without much effort in fused silica. Typical waveguide diameters processed by femtosecond lasers in glass limit the application for integrated optics to wavelengths of less than 1000 nm. Here, multi scan techniques were used to produce integrated optics for NIR light sources. The design and properties of a multi scan waveguides, which consist of a bundle of parallel, slightly overlapping single waveguides, were presented. Based on this an interferometer in a fused silica substrate, working for wavelength around 1500 nm was produced. This interferometer is used to detect the vibrations of a photo acoustic tuning fork, which was integrated in the fused silica substrate, too. It was also shown that the multi scan technique is very well suited for implementation of Bragg gratings. In general it is difficult to integrate grating structures with high refractive index contrast in such waveguides due to the fact, that a waveguide itself, generated with femtosecond pulses, has already experienced a small refractive index contrast to the bulky unmodified glass. Different approaches have reported for production of waveguide Bragg gratings in glass. A weak second order waveguide grating has been demonstrated for the first time in bulk fused silica with a two-step laser process by Marshal et al.. Here the region of the Bragg structure was larger than the waveguide itself. Zhang et al. got much stronger Bragg reflections with a different grating design, which consisted of a periodically segmented waveguide written with ultra short pulse lasers. Brown et al used a multi scan technique to produce a first order Bragg grating, which consists of parallel lines of reftractive index modifications. For this they had to precisely control and spatially synchronize the refractive index modulations imprinted in the material with femtosecond laser pulses by each scan with the help of a position sensitive trigger signal of the translation stages. In comparison to this procedure, a more simple but very efficient technique to produce the Bragg grating was used. The Bragg structure is generated by segmentation of a single waveguide of the waveguide bundle. The advantages of this design are the high index contrast and the reduced propagation loss, because of the continuous intact waveguide structure.