Low-loss and low-crosstalk multimode waveguide bend on silicon

A low-loss and low-crosstalk multimode waveguide bend is proposed and demonstrated for mode-division-multiplexed optical interconnects. The proposed 90°-bend is composed of two identical 45°-bends, which are defined as modified Euler curves. For the designed 90° Euler-bend with a core width of 2.36 μm for supporting four TM-polarization modes, it is allowed to achieve an effective radius as small as 45 μm, which is about 1/4 of the radius (~175 μm) for a regular 90° arc-bend. In theory, this proposed 90° Euler-bend has very low excess losses (<0.1 dB) and very low inter-mode crosstalks (<-25 dB) over a broad wavelength-band. A silicon photonic integrated circuit is designed, fabricated and characterized by integrating a pair of mode (de)multiplexers and a multimode bus waveguide with a Euler S-bend consisting of two cascaded 90° Euler-bends. The measurement results show that the fabricated Euler S-bend has low excess losses of <0.5 dB and low inter-mode crosstalks of <-20 dB over a broad band from 1520 nm to 1610 nm for all the 4 mode-channels of TM polarization.