Ultra-Broadband and Low-Modal-Crosstalk Mode Multiplexer Based on Cascaded Vertical Directional Couplers Formed by Adiabatic-Tapered Waveguides Without Mode Conversion

We present an ultra-broadband and low-modal-crosstalk mode multiplexer based on cascaded vertical directional couplers formed by adiabatic-tapered waveguides without mode conversion. The input and output ports of the mode multiplexer are designed in the same planar layer to meet the standard package requirement. We design and fabricate a three-mode mode multiplexer, which serves to spatially combine and separate the E-11, E-21 and E-12 modes in the few-mode-core outputs without mode conversion with polymer material, where adiabatic tapers are utilized in the design of directional coupler to extend the operation wavelength and two pairs of directional couplers serve to suppress the modal crosstalks. We fabricate a typical device to demonstrate the idea. The fabricated device has a length of 29 mm, which provides coupling ratios higher than 97% and modal crosstalks lower than -19.6 dB over the C+L band and beyond with weak polarization dependence. The proposed mode multiplexer is fiber capable and allows to connect with the few-mode fibers directly by the use of fan-in and fan-out technology. The proposed mode multiplexer is simple, scalable, and effective to extend the operation wavelength as well as suppressing the modal crosstalks, which provides a powerful device to manipulate the modes in a mode division multiplexing system and an effective method to realize an all-optical chip for simplifying the Multi-Input Multi-Output signal processing with low power consumption.