Ultra-compact hybrid multimode division multiplexer based on a photonic crystal nanobeam.

In this Letter, we propose and fabricate an ultra-compact hybrid four-mode multiplexer based on a photonic crystal nanobeam to enhance the integration of on-chip multidimensional multiplexing systems. By introducing the nanobeam into the system, mode conversion can be achieved synchronously during reflection, eliminating the need for a dedicated mode converter and enabling higher chip integration. Besides, subwavelength gratings and tapered waveguides are gradually introduced to further reduce the length of the coupling region. As a result, the proposed tapered-waveguide-assisted ultra-compact hybrid four-mode multiplexer has a coupling region length of only 1.625 μm, and the minimum feature size is 130 nm. Experimental results demonstrate that this device exhibits an insertion loss less than 3.66 dB and cross talk better than -13 dB within a bandwidth of 60 nm. Attributed to its compact structure and low lithography precision requirements, the proposed device will undoubtedly contribute to achieving high integration of on-chip multidimensional multiplexing systems.