On-Chip Reconfigurable Wavelength-Selective Optical Mode Exchanger for WDM-Compatible Mode-Division Multiplexing

Wavelength-division multiplexing (WDM)-compatible mode-division multiplexing (MDM) approach is a promising optical communication technique that leverages multiple spatial modes on different wavelengths within an optical waveguide to boost the data capacity. In a WDM-compatible MDM transmission system, a wavelength-selective mode exchanger is a key component which can selectively fulfill the mode conversion on a specific wavelength of the light. Here we propose a compact and reconfigurable wavelength-selective mode exchanger on a silicon photonic chip. The device employs a symmetric Mach-Zehnder interferometer (MZI) structure, with optical mode couplers at both input and output for mode conversion. Each MZI arm features a single-mode rib waveguide, integrated with multiple ultra-compact microdisk resonators (MDRs) for wavelength selectivity, and a metallic micro-heater on each MDR for tuning. By adjusting the micro-heaters, a controlled phase-shift difference between the two arms enables wavelength-selective mode conversion. The mode conversion bandwidth is determined by the MDR resonance bandwidth, and can be enhanced by aligning the resonances of multiple MDRs. Experimental results demonstrate successful transverse-electric (TE0 and TE1) mode conversions across three wavelength channels. The device exhibits strong reconfigurability, allowing tunability of the central wavelength, channel spacing, and bandwidth. Using the chip in an optical transmission system, WDM-compatible MDM transmission is demonstrated. The proposed on-chip wavelength selective mode exchanger holds key advantage of scalability since the WDM channel can be hugely increased by adding more ultra-compact MDRs, which paves the way for future large-scale WDM-compatible MDM optical communication systems.