Schematic diagram of the optical structure. (a) The optical waveguide network with three unit cells, one entrance, and one exit, where E_I, E_R, and E_O are, respectively, the input, reflected, and output EM waves. Each black thick dashed line represents two segments of PT -symmetric waveguides with the lengths of d₁ and d₂, respectively, and the black thick solid curves at the entrance and exit represent PT -symmetric waveguides with the length of d₁. (b) PT -symmetric waveguide composed of three subsegments, where the lengths of the three subsegments are the same.