Adiabatic couplers in SOI waveguides Liang Cao, Ali Elshaari, Abdelsalam Aboketaf, Stefan Preble Rochester Institute of Technology, 77 Lomb Memorial Drive, Rochester, NY, 14623, USA lxc3973@rit.edu Abstract: Adiabatic 3-dB couplers based on highly confined silicon waveguides are studied and experimentally demonstrated. The couplers are inherently broadband and insensitive to fabrication imperfections, which enables reliable photonic circuits. ©2010 Optical Society of America OCIS codes: (130.3120) Integrated Optics Devices; (230.7370) Waveguides 1. Introduction Symmetric directional couplers are essential components in integrated optical circuits for coupling/splitting light to different waveguides. However, the coupling length is extremely sensitive to wavelength and fabrication imperfections which make it difficult to achieve the desired coupling ratio from the coupler. Asymmetric adiabatic couplers have been developed to overcome these drawbacks. Previous reports of adiabatic couplers in LiNbO 3 [1], and polymeric [2] waveguide materials show that adiabatic couplers do not require precisely defined power-transfer length. In addition, they have small wavelength and fabrication variation dependence but with a trade-off of longer coupling lengths than directional couplers. In recent work adiabatic couplers were used in a high index-contrast SOI platform to efficiently couple light between two waveguides in a Mach-Zehnder Interferometer (MZI) based electro-optic modulator [3]. However, there were little details on the coupler design and overall there is no information in the literature on how the high confinement of silicon wire waveguides introduces any tradeoffs in the coupler performance, such as polarization dependence, fabrication robustness, or overall length. Here we present a detailed analysis and design of an adiabatic 3-dB coupler that is broadband and robust to fabrication variations. We also integrated the adiabatic coupler into a MZI in order to experimentally verify robust 3dB coupling. 2. Device design and analysis Adiabatic couplers work by adiabatically converting the mode of a single waveguide into either the even or odd mode of two identical waveguides separated by a small gap. Although adiabatic couplers look similar to directional couplers they work very differently in that adiabatic couplers ensure only one fundamental mode is excited at all times within the coupler. In a conventional directional coupler both the fundamental even and odd mode of the structure are excited initially which then interfere with each other and results in the characteristic power oscillation between the two waveguides. Therefore, the key to designing an adiabatic coupler is ensuring that only one mode of the system is excited at all time and the transition of the mode from the that of a single waveguide to the even/odd mode of the two waveguides is done slowly (adiabatically) enough. Once this adiabatic transition occurs the light naturally stays in the even/odd mode, which automatically has 50% of the power in each waveguide. Consequently the adiabatic coupler is inherently broadband, insensitive to fabrication imperfections and  length (as long as it is long enough). Fig. 1. Adiabatic coupler geometry and simulated modes . The even or odd mode can be realized based on which input waveguide is excited. a1651_1.pdf OSA / CLEO/QELS 2010 CThAA2.pdf 978-1-55752-890-2/10/$26.00 ©2010 IEEE