Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

It has been shown that the wavelength-dependent performance of a directional coupler (DC) in silicon-on-insulator (SOI) platform can be greatly engineered by suitable design optimizations. Semianalytical coupled mode theory is used to optimize a nearly wavelength-independent design of a DC in an SOI substrate with a device layer thickness of 220 nm, operating in TE-polarization (λ ~1550 nm). The transmission characteristics of fabricated DCs are found to be indeed wavelength independent over a bandwidth of 100 nm (1525 nm ≤ λ ≤ 1625 nm), consistent with the theoretical predictions. The average excess loss of such directional couplers is evaluated as '0.8 dB and there are scopes for its further reduction. These DCs are then used further to demonstrate integrated optical building blocks like power splitters (2 × 2, 1 × 4), Mach-Zehnder interferometers (2 × 2), and all-pass microring resonators. Their performances are also found to be uniform within the wavelength range mentioned and, thus, making them suitable for integrated silicon photonics for broadband applications.