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High-order mode rotator for Si integrated circuits

As the need for high-capacity on-chip optical interconnects is becoming increasingly critical, mode-division multiplexing (MDM) has been exploited to solve this problem for silicon-on-insulator (SOI)-based planar integrated circuits (PICs). For MDM, a mode multi/demultiplexer (MUX/DEMUX) is a key component. Seeking vertical expansion of high-order mode field distributions, namely Eij modes with i =1 and j > 1 rather than Eij modes with i > 1 and j = 1 which have already been utilized in MDM system would not only enable to further increase the capacity of on-chip MDM links while preserving their footprint, but may also find applications in 3D intra-chip optical interconnections for multi-core processor architectures to realize mode coupling between waveguides in mutually perpendicular planes. Here i and j represent the number of lobes of mode intensity distribution in horizontal and vertical direction of the transverse plane, respectively. We propose a mode rotator (MR) that enables the spatial distributions rotation of the high-order modes (shown in Fig. 1(a)) by using an etched multimode waveguide. Fig. 1(b) shows the structure of the high-order MR, which is composed of two straight waveguides with width W and height H, and a middle waveguide with an etched corner over a certain length L, which supports two orthogonal eigen modes (MR1 and MR2), as is shown in Fig. 1(c). As the light propagates along the z axis, the input E21 mode is rotated into the E12 mode by setting length L to half the beat-length, π/(βMR1 - βMR2). We analyzed the structure using a film mode matching (FMM) mode solver and a 3D vectorial eigen mode expansion simulator. The calculated length and the mode conversion efficiency (MCE, defined as MCE= P12/P21, where P12 and P21 are the power levels of the output E12 mode and input E21 mode, respectively) of the mode rotator device are presented as a function of design parameters for both TM and TE polarizations. As can be seen in the above figures, high mode conversion efficiencies (above 98%) can be achieved and the etching parameters can be chosen depending on the target mode conversion efficiency. However, there is a trade-off among MCE, device length and fabrication tolerance. A high target value of MCE will result in longer devices for both TM and TE polarizations, along with a relatively tight fabrication tolerance in the dimension of etching width for TM operation, or etching depth for TE operation. Depending on the required device performance (MCE) and the processing conditions, the etching parameters can be chosen accordingly. As a concrete example of application, a two-mode multiplexing system based on E21 and E12 is also numerically simulated, showing a mode extinction ratio exceeding 15 dB over a wide wavelength range of 100 nm.