Wavelength-Independent Directional Couplers for Integrated Silicon Photonics

Abstract

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 ( $\lambda \sim$ 1550 nm). The transmission characteristics of fabricated DCs are found to be indeed wavelength independent over a bandwidth of ${\text{100 nm}}$ (1525 nm $\leq \lambda \leq$ 1625 nm), consistent with the theoretical predictions. The average excess loss of such directional couplers is evaluated as $\sim\text{0.8}$ 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 $\times$ 2, 1 $\times$ 4), Mach–Zehnder interferometers (2 $\times$ 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.