Enhanced Modal Coupling Loss in a Two-Mode SOI Waveguide of Broken Parity-Time Symmetry

Effect of optical loss, presented in a superstructure grating with a complex refractive index profile, on the mode coupling is studied in a two-mode silicon-on-insulator waveguide. In this scheme, both our theory and experimental results indicate that the optical loss not only provides optical absorption of a targeted output mode and thus the corresponding modal coupling loss, but also induces a broken parity-time symmetry phase that strongly prohibits power coupling from the incident mode to the output mode. As a result, the latter can give rise to a much-enhanced modal coupling loss of the targeted output mode. Under a reasonable device length about 37 
μ
m, the enhancement factor can be as high as ∼20 dB in simulation and ∼17 dB in experiment. This paper may offer a different way to exploit the role of optical loss in boosting modal coupling loss that can directly translate to power extinction ratio. It thus may find potential applications in novel intensity modulator designs.