I. Introduction
Wavelength division (de)multiplexers are key components to compose highly integrated and large capacity photonic systems for telecom, datacom, and biomedical sensing applications. Wavelength division multiplexing is commonly used in today's silicon photonic products such as optical transceivers and active optical cables (AOC). For silicon photonic applications, SOI-based wavelength splitting devices have been developed, including arrayed waveguide gratings, cascaded Mach-Zehnders, ring resonators, and echelle gratings [1]–[4]. However, these devices exhibit high losses and high sensitivity to both fabrication and temperature variations. To solve this problem, angled multimode interferometer (AMMI) (de)multiplexers [5]–[10] are proposed and demonstrated with capability of providing low insertion loss (IL) with strong fabrication tolerance. The AMMI structure is easy to fabricate with a single-step lithography/etching process and the device footprint is acceptable (millimeter scale) for a 4-channel WDM with relatively large channel spacing (10−20 nm). However, the device length increases greatly with the narrowing of channel spacing or adding more channels. Its spectra response also deteriorates significantly, leading to a high IL or large cross-talk (XT) [5]. It is a big challenge to realize denser channel spacing while maintaining the excellent performances.