Design of efficient and compact adiabatic couplers based on adiabatic mode evolutions

In this study, two different adiabatic couplers are proposed based on the adiabatic mode evolution mechanism in the two-waveguide system. The mode conversion/transfer in vertically asymmetric adiabatic couplers is investigated. The proposed adiabatic couplers couple the beam modes from one waveguide to another using adiabatic mode evolution and enable the mode conversion/transfer between the designed modes. The k-vector frequency-dispersion diagram for the first adiabatic coupler is calculated, and the variation of the guided mode in the adiabatic coupler with the waveguide width is shown to guide the design of adiabatic couplers. The design results demonstrate that the proposed adiabatic mode couplers are substantially shorter than the linear-shape connection, but can achieve the same power mode conversion efficiency. The main function of the first adiabatic mode coupler designed is to convert the TE0 mode in one port to the TM0 mode in the other port, the structure only needs a length of 108 μm to achieve 90 % power conversion efficiency, and the traditional linear-shape connection requires 1540 μm to achieve the same conversion efficiency. As a result, the proposed adiabatic coupler size is 14 times shorter than the conventional design. Moreover, in the design of the second 3-dB adiabatic coupler, the adiabatic mode evolution transfer of the two lowest-order even and odd eigenmodes is implemented in a two tapered ridge waveguide system for 3-dB energy splitting. The proposed coupler will achieve a transfer efficiency greater than 90 % in the range of 1400 nm to 3760 nm when the device length is 47.5 µm, which is an ultra-wide operating bandwidth range.