1. Introduction
To increase the transmission capacity of the optical communication network, mode division multiplexing (MDM) technology has received attention in recent years [1]. Waveguide-type mode (de)multiplexers (MUXs) are key devices for connecting single-mode and multi-mode systems, enabling coupling or separation between different modes. Combining wavelength division multiplexing (WDM) and MDM technologies is effective in expanding transmission capacity from long-distance fiber communication to very short-reach chip-based communication. For chip communications, the size of the device is an important indicator to achieve high integration density. Nanophotonic technologies based on Si wire waveguides and photonic crystals can confine light to smaller devices due to the high refractive index difference of materials. Especially, photonic crystals (PCs) are widely studied because of their unique performance and high integration density. In [2], 2-mode MUX has been demonstrated by PC waveguides. In [3], WDM/MDM hybrid multiplexing technology was proposed, which is composed of PC waveguides for both wavelength MUXs and mode MUX. To achieve much higher transmission density, PC mode MUX for the higher number of modes is desired. In this study, we propose a 5-mode multiplexer by cascading four asymmetrical directional couplers (ADCs) based on 2D pillar square-lattice PC waveguides. The device can selectively excite 5 target modes (TE0, TE1, TE2, TE3, and TE4 modes) by choosing the input ports.