High-Order-Mode-Pass Mode (De)Multiplexer With a Hybrid-Core Vertical Directional Coupler

We propose a high-order-mode-pass mode (de)multiplexer based on the structure of a vertical waveguide directional coupler formed with a few-mode core and a single-mode core that has a higher refractive index. Unlike a conventional directional coupler, where the two cores have the same refractive index, the hybrid-core structure allows the design of a directional coupler to couple only the fundamental mode of a few-mode core to a single-mode core without affecting the high-order modes of the few-mode core. To demonstrate the idea, we design and fabricate a vertical coupler formed with a three-mode core and a single-mode core with polymer material. Our experimental device has a length of 15 mm and a coupling ratio for the fundamental mode varying from 92.5% to 98.7% in the C-band (from 1530 to 1565 nm), which corresponds to a relative residual power of the fundamental mode in the three-mode core varying from -10.9 to -18.9 dB. The crosstalks to the single-mode core from the two high-order modes of the three-mode core are smaller than -13.8 dB in the C-band. The performance of the device is polarization insensitive. The proposed hybrid-core mode (de)multiplexer allows an effective control of the fundamental mode of a few-mode waveguide and thus opens up new possibilities for the design of mode-controlling devices, such as mode-group (de)multiplexers, mode-dependent-loss compensators, and mode add-drop multiplexers, for mode-division-multiplexing applications based on circular or elliptical few-mode fibers.