Broadband C-band silica four-mode (de)multiplexer based on adiabatically tapered asymmetric directional couplers

We design and experimentally demonstrate a C-band four-mode multiplexer/demultiplexer based on a silica planar lightwave circuit (PLC) platform. The device is fabricated on a 2.0%-index-contrast SiO 2 waveguide platform and employs three cascaded asymmetric directional couplers to multiplex the E 00 , E 10 , E 20 , and E 30 modes. To reduce the sensitivity of conventional asymmetric directional couplers to phase-matching conditions and fabrication errors, an adiabatically tapered waveguide is introduced into the mode-conversion coupling region of each stage. This design enables smooth modal power transfer within the coupling regions, thereby broadening the operating bandwidth and improving fabrication tolerance. At a wavelength of 1550 nm, the insertion losses of the four mode channels for a single multiplexing process are 0.94, 1.08, 1.13, and 1.10 dB, respectively. Over the measured wavelength range of 1500–1600 nm, the insertion loss remains below 1.7 dB for all mode channels, the polarization-dependent loss (PDL) is below 0.7 dB, and the worst-case modal crosstalk is lower than − 20.1 dB. These results indicate that the adiabatically tapered asymmetric directional coupler (ADC) design can simultaneously improve bandwidth and fabrication tolerance in a 2.0%-index-contrast silica PLC platform, providing a scalable route toward low-loss on-chip mode-division multiplexing.