CO2 laser direct writing of optical waveguides in silica thin films on silicon

The development of low-loss, low-cost, and stable passive waveguide devices has been a major focus of research in the past few decades. Several techniques have been proposed in which direct laser writing is one of the more appealing methods since complex photolithography may no longer be required. These techniques are based on the creation of a core region by laser-induced refractive index change. Direct UV writing is one of these techniques but its application is normally restricted to highly photosensitive materials such as germania doped silica [1]. Femto-second laser writing has been used to circumvent this problem: the technique allows the creation of waveguides in a variety of bulk materials [2]. However, these methods modify the properties of the material in the exposed core region, the waveguide formation process is slow due to the writing speed of the order of 100 μm/sec [2], and are expected to cause damage in nonlinear materials. In this paper we demonstrate a simple and novel technique that allows the creation of optical waveguides in silica thin films on silicon by removing material to leave behind a core region, being defined as a region between two trenches. The proposed process is distinct from other laser writing techniques as the structure of the core region is preserved since only the cladding area is modified. Even though two trenches are required to create a waveguide the overall fabrication time is orders of magnitude shorter than with other direct laser writing techniques as the writing speed is of the order of 50 mm/sec. We believe this technique is also useful for the fabrication of waveguide devices in nonlinear materials e.g. PPLN, KTP, BBO etc. and particularly in poled glasses.