Multimode interference couplers as compact and robust static optical phase shifters

The ability to realise compact and wavelength insensitive static optical phase shifters is of great value for many photonic integrated circuits. The work in this paper presents a design for a compact static optical phase shifter based on 1 × 1 MMI couplers which has the potential to replace conventional tapered waveguide phase shifters. The operation of these phase shifters is described in detail and designs are presented for typical InP and Si based waveguide structures. For Si waveguides, a factor of 3 reduction in length is possible using the proposed 1 × 1 MMI phase shifter compared to using standard tapers. Such devices offer excellent performance over a wide operational wavelength range and are tolerant to fabrication errors. Although reflection and insertion losses in MMIs are higher than those found in adiabatic tapers, the overall reduction in length makes MMI based phase shifters an interesting choice for use in photonic integrated circuits where robust performance and compact size is required.