Silicon based integrated optical circuit can confine the light field in sub-micron scale, and the fabrication process is compatible with mature microelectronic CMOS process. It can realize large-scale, high-density and low-cost on-chip photonic integration, which has become a hot direction of academic research and industrial applications.
Advanced multiplexing technologies such as wavelength division multiplexing, polarization multiplexing and mode multiplexing can rapidly expand the information transmission capacity on the chip. Among them, mode multiplexing uses multiple intrinsic modes in multimode waveguide to expand information transmission capacity without increasing the number of light sources, which shows advantages of high efficiency and low cost. However, when the multimode waveguide is bent, it is easy to cause high-order mode radiation loss, mode wavefront disorder and inter-mode crosstalk. Generally, a large bending radius of 500 mm is needed to ensure the efficient transmission of four TE modes, and the bandwidth is limited. Therefore, it is still a great challenge to design ultra compact and wide bandwidth multimode curved waveguide.
The research group led by Prof. Xinliang Zhang and Prof. Dingshan Gao from Huazhong University of Science and Technology, in cooperation with the group from South-Central University for Nationalities, demonstrated a multimode curved waveguide with small bending radius and large bandwidth based on the principle of transformation optics. The research results are published in Photonics Research, Vol. 8, No. 12, 2020 (Shuyi Li, Lifeng Cai, Dingshan Gao, Jianji Dong, Jin Hou, Chunyong Yang, Shaoping Chen, Xinliang Zhang. Compact and broadband multimode waveguide bend by shape-optimizing with transformation optics [J]. Photonics Research, 2020, 8(12): 12001843).
Firstly, the arc-shaped multimode curved waveguide in physical space is transformed into virtual space by coordinate conformal mapping, and the straight waveguide structure with non-uniform spatial refractive index distribution is obtained. It is found that the non-uniformity of refractive index at the upper and lower boundaries of the virtual space waveguide is the source of high-order mode field scattering and inter mode crosstalk. The research team proposed a new idea to optimize the shape of the upper and lower boundaries of the waveguide directly in virtual space to eliminate mode field scattering and crosstalk.
Boundary optimized multimode curved waveguide and refractive index distribution (a) virtual space; (b) physical space
Then, the conformal transformation is used to map back to the physical space, and a 90-degree multimode curved waveguide with optimized shape can be obtained quickly, which is characterized by continuous and smooth transition of boundary curvature radius and waveguide width.
The theoretical simulation results show that the multimode curved waveguide supports four TE modes, and the transmission loss of each mode is less than 0.1 dB and the inter-mode crosstalk is less than - 20 dB in the very large bandwidth of 1160 ~ 1660 nm. The research team has fabricated devices on commercial silicon on insulator (SOI) wafers by one-step lithography. The test results show that the transmission loss at 1550 nm is less than 0.6 dB, and the crosstalk is less than - 17 dB.
Shuyi Li, the first author of this paper and Ph.D. graduate of Huazhong University of Science and Technology, believes that this research shows that the transformation optics design method can deeply and intuitively understand the influence of any waveguide shape distortion on the mode field transmission, thus providing a unique solution for the design of some complex shape photonic devices.
