Multifunctional Waveguide Grating Couplers for Integrated Optics
This thesis describes a novel waveguide component in integrated optics called a multifunctional grating coupler. It couples light in or out of optical waveguides and provides additional optical functions such as beam splitting, arbitrary focusing, polarization sensing, and wavelength separation (wavelength division demultiplexing). The couplers are quasi-periodic grating structures etched into the surface of an optical waveguide and thus they can be conveniently integrated monolithically with other components such as lasers, detectors, or switches.
Multifunctional grating couplers might be used in optical fiber communication to couple light into waveguide detectors or to couple light from an in-plane waveguide laser out into air and focus it into optical fibers. The multifunctionality reduces the need for additional optical components, both integrated and external. Therefore, optical systems using the grating couplers are potentially robust, small-size, and simple to align and package.
This thesis presents a new design algorithm for multifunctional waveguide grating couplers. A grating for outcoupling is demonstrated for focusing into a three-dimensional image above the waveguide. The emphasis, however, is on incoupling. Different optical functions are demonstrated, as incoupling and focusing into any arbitrary positions within the waveguide. Furthermore, it is shown that different polarization states (TE- and TM-polarization) can be separated in the waveguide (polarization sensor) or combined to obtain polarization insensitive grating couplers. It is also possible to separate different wavelengths incident on the coupler, thus obtaining wavelength division demultiplexing or wavelength encoding.
The performance of the grating couplers was evaluated by numerical simulations and tested experimentally in InP-based waveguides for approximately 1550nm wavelength. The results were in good agreement with the simulations.