Flexible add-drop multiplexers for mode manipulation using mode exchangers

Flexible mode manipulation is essential for the large-scale construction of mode-division multiplexing (MDM) systems, enabling arbitrary signal exchanging, routing, and multiplexing to significantly enhance optical interconnection capacity. A compact and flexible mode manipulation scheme is demonstrated, centered on a mode exchanger (ME) optimized by intelligent algorithms. Taking four-mode exchangers based on a Bézier curve profile waveguide and corresponding mode add-drop multiplexers (MADMs) as examples, theoretical analysis and experimental verification of their flexible mode manipulation capabilities are presented. Experimental measurements on the fabricated ME_01/23, ME_02/13, and ME_03/12 mode exchangers reveal insertion loss (IL) below 1.8 dB and crosstalk (CT) better than -10 dB, spanning the wavelength ranges of 1515-1570 nm, 1508-1580 nm, and 1519-1580 nm, respectively. All three devices feature a compact footprint of less than 6 × 3 μm². Meanwhile, characterization of the three-mode add-drop multiplexers (MADMs) operating over the C-band (1530-1565 nm) verifies IL of less than 2.4 dB and CT exceeding -10.0 dB for all transmission modes. Notably, the proposed mode manipulation strategy offers theoretical scalability to higher-order modes, overcoming a key limitation of conventional MDM systems: the inability to directly access specific low-order modes in multimode bus waveguides. These findings thus provide critical technical foundations for integration into next-generation high-density optical communication systems.