Integrated Microwave Photonics

Silicon and silicon-based materials (i.e. silicon nitride) are attractive for integrated microwave optics since they exhibit low loss at wavelengths near 1.5 microns, which makes it simple to leverage existing telecommunications components. In addition, the fabrication and manufacture of silicon photonics can leverage the expertise and infrastructure of CMOS-based silicon electronics processing. Finally, the large-scale integration of photonic components on a silicon chip will enable a wide range of increasingly complex devices and microwave photonic systems on a chip.

(a): Top view of a cascaded Fabry-Perot microcavity filter with microheaters for thermo-optic tuning, (b) a cleaved waveguide facet, and (c) cross-section of a silicon/air grating mirror.
(a): Top view of a cascaded Fabry-Perot microcavity filter with microheaters for thermo-optic tuning, (b) a cleaved waveguide facet, and (c) cross-section of a silicon/air grating mirror.

Our research is focused on the use of micro- and nanomachining techniques to fabricate state-of-the-art integrated waveguide structures. These techniques can be used to develop high-index-contrast waveguides as well as grating mirrors which can be over 99% reflective over bandwidths spanning >100 nanometers. Using these straightforward fabrication techniques enables the development of a large variety of devices spanning a broad range of application areas. These devices include waveguide splitters such as directional couplers and multi-mode interference (MMI) splitters as well as Mach-Zehnder interferometers (MZI’s). Other microwave photonics devices include Fabry-Perot microcavities and add-drop filters, in a silicon-on-insulator (SOI) or silicon nitride system. We are also developing active components that rely on thermo-optic effects or micro-electro-mechanical systems (MEMS). Our aim is to incorporate these structures directly into microwave photonic systems.

(a): A silicon nitride unbalanced Mach-Zehnder Interferometer (MZI) and (b): measured MZI wavelength spectrum.
(a): A silicon nitride unbalanced Mach-Zehnder Interferometer (MZI) and (b): measured MZI wavelength spectrum.

Low Power, High-Speed Electro-Optics

Our research in high-speed electro-optics is focused on the use of compound semiconductor heterostructures in high-index-contrast waveguides to achieve ultra-low modulation voltages. State-of-the-art heterostructures such as coupled quantum wells should enable large optical phase shifts at low bias voltages without introducing significant loss. The integration of these materials into suspended high-index-contrast waveguides (in which the core is surrounded by air) reduces the necessary bias voltage further by dramatically reducing the size of the optical mode while maintaining excellent overlap with the semiconductor heterostructure. Our most recent devices have achieved Vπ L coefficients as low as 110 mV-cm in suspended waveguides with MQW cores only 215 nm thick.

A schematic (a) and SEM (b) of a suspended rib semiconductor quantum well waveguide. By suspending the waveguide (using a selective etch to remove the sacrificial layer) the electro-optic phase shift is enhanced by as much as 40%.
A schematic (a) and SEM (b) of a suspended rib semiconductor quantum well waveguide. By suspending the waveguide (using a selective etch to remove the sacrificial layer) the electro-optic phase shift is enhanced by as much as 40%.