High-order mode rotator for Si integrated circuits

Abstract

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 E ij modes with i =1 and j > 1 rather than E ij 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 E 21 mode is rotated into the E 12 mode by setting length L to half the beat-length, π/(β MR1 − β MR2 ).