There is currently considerable research interest in all-optical signal processing for telecommunication applications.
In this paper it is shown that a range of all-optical signal processing functions may be carried out using passive
waveguides and phase-shifting elements as basic building blocks. Two types of multimode interference (MMI) photonic
circuits, based on silicon-on-insulator (SOI) technology, are discussed in detail. In the first type, phase-shifting elements
are added to the ports of MMI couplers. In the second type, the waveguide structure of the MMI coupler itself is
modified to achieve a range of signal processing functions. Designs are verified using 2D and 3D simulations.
Microring resonators are promising candidates for photonic signal processing applications. However, almost all
resonators that have been reported so far use directional couplers or 2×2 multimode interference (MMI) couplers as the
coupling element between the ring and the bus waveguides. In this paper, instead of using 2×2 couplers, novel structures
for microring resonators based on 3×3 MMI couplers are proposed. The characteristics of the device are derived using
the modal propagation method. The device parameters are optimized by using numerical methods. Optical switches and
filters using Silicon on Insulator (SOI) then have been designed and analyzed. This device can become a new basic
component for further applications in optical signal processing. The paper concludes with some further examples of
photonic signal processing circuits based on MMI couplers.