Fig. 1: Conventional and topological 3-dB couplers.
From: Broadband and fabrication-tolerant 3-dB couplers with topological valley edge modes
a, b Beam splitting in (a) a conventional 3-dB directional coupler and (b) a topological 3-dB coupler composed of two different VPCs. The numbers in circles label four different ports. Light input from port 1 is divided into ports 3 and 4. The inset shows the splitting ratio as a function of wavelength, in which the dash lines represent the cases with dimensional errors. The 3 dB splitting is achieved at only one single wavelength λ1 for the conventional coupler. In contrast, the topological coupler achieves the ideal 3-dB splitting in a broadband wavelength even when dimensional errors are introduced. The white arrows with crosses indicate that no backscattering to port 1 and no straight transmission to port 2 are allowed. c Structures of Edge21 and Edge12 which are constructed with VPC1 and VPC2. Structural parameters such as s1, s2 and a are marked. The black (gray) arrows denote the propagation directions of edge modes at K (K’) valleys, respectively. d Hz field within the topological 3-dB coupler when the light is incident from port 1. No backscattering and no through transmission are observed due to the inter-valley scattering suppression. The incident light is divided equally into two output ports, resulting in the 3-dB splitting
