Fabrication of channel waveguides in LiNb03.

Abstract: In this bachelor’s thesis, a fabrication process for channel waveguides in LiNbO 3crystals using the proton exchange technique is developed and characterized. These waveguides can be used at the laser physics department at the Royal Institution of Technology, KTH, for specifically non-linear optical applications. A waveguide is a device that guides a wave. In this case an optical waveguide is fabricated and so it guides electromagnetic waves in the optical spectrum that is light. To guide the wave diffraction has to be prevented in one or two dimensions, constraining the light to travel along a certain desired path. Waveguides can be fabricated in small sizes down to the micrometer level. The small structures can yield high-intensity guided waves with low input powers and this can lead to more efficient and compact nonlinear devices (sensitive to the field intensities). This makes it possible to produce compact and efficient devices with waveguides. To confine the light in the waveguide, the refractive index at the surface of the crystal will be increased creating a guiding layer in that region. The index increase is achieved by proton exchange. Proton exchange is a process where the lithium ions at the surface of the crystal are exchanged with hydrogen ions. This exchanged part makes the guiding layer because the hydrogen ions increase the refractive index of LiNbO 3. The fabrication process of the channel waveguides consisted of transferring a mask pattern into the LiNbO 3crystal. To do this, titanium was uniformly deposited on the crystal and on top of it a layer of photoresist was spun. The mask was transferred into the photoresist by photolithography and then etched into the titanium. The proton exchange could then take place in the mask openings. The progression of the fabrication was carefully documented after each step of the process to assess the quality of the waveguides. Several waveguides were fabricated on each sample with widths ranging from 2 to 10 microns. In the end, the full process for waveguide patterning and fabrication was developed on LiNbO 3 substrates. The fabrication recipe developed in this work allowed for reliable fabrication of uniform channel waveguides over the whole sample length, L=12 mm, with widths down to 1.02 μm. A remarkably good result if one considers that this is beyond typical resolutions (~2μm) of the lithographic system used in this work.