Integrated optics towards third dimension

The level of integration within standard waveguide devices is usually restricted to the use of a single plane per wafer containing waveguides. Recently, concepts for waveguide structures with several planes of waveguides gain some special interest. They are mainly due to intentions to increase the number of channels per chip, to reduce the chip size and to improve device characteristics. Dense vertical stacking of waveguides turns out to be an approach to 3D integrated optics, which is within the resolution limits of up-to-date waveguide technologies. Theoretical and experimental results for 3D directional couplers are reported in detail.