High-Extinction-Ratio and Compact 1310/1550 nm Wavelength Diplexer on SOI Platform Based on an SWG-Structured Two-Mode Interference Coupler

We propose a 1310/1550 nm wavelength diplexer on a 220-nm silicon-on-insulator (SOI) platform. The device is based on a compact two-mode interference (TMI) coupler enabled by a subwavelength grating (SWG) slot. The ideal beat length ratio of 2:1 is achieved with the transverse magnetic (TM) mode by fine-tuning the SWG slot parameters, resulting in a TMI length of only 37 µm. We reveal that the key to high extinction ratio (ER) is the careful design of the tapers, and the device achieves high ERs of 28.05/42.54 dB at 1310/1550 nm with simulation. The measured bandwidths for ER $>$ 15 dB are 82 nm and 56 nm at O- and C-band. Moreover, the design guarantees large calculated 1-dB-insertion-loss (IL) bandwidths of 192/123 nm at 1310/1550 nm. To the best of our knowledge, this is the first experimental demonstration of a high-performance compact silicon 1310/1550 nm diplexer based on a TMI coupler.