Mode-division-multiplexing (MDM) can increase the total on-chip transmission capacity. Silicon-on-insulator (SOI) based MDM multiplexer (MUX) and demultiplexer (DEMUX) using asymmetrical directional couplers (ADCs) are promising; however, they usually require long coupling lengths for mode conversion. Here, for the first time up to the authors' knowledge, we propose, simulate, fabricate and demonstrate a size reduced SOI based $4 \times 4$ MDM MUX and DEMUX using enhanced evanescent-wave coupling (EEC). Here, we illustrate by experiments and numerically analyses that by size reducing the ADC access coupling region, evanescent-wave coupling is enhanced. Significant coupling length reduction of ~80 % can be achieved, while similar MDM MUX and DEMUX performance can be observed. To experimentally evaluate the broadband and high-speed transmission operations of the proposed device, 48 wavelength channels each modulated by >60 Gbit/s orthogonal frequency division multiplexing (OFDM) signals are successfully mode multiplexed and demultiplexed, achieving a transmission capacity of 11.73 Tbit/s.
