Ultra-Low Crosstalk and Fabrication-Tolerant Silicon-Nitride O-Band (de)Multiplexer Using Bragg Grating-Assisted Contra-Directional Coupler

A wavelength division (de)multiplexing (WDM) filter with ultra-low channel crosstalk (XT) and high tolerance was proposed for a 1 × 4 O-band coarse-WDM (CWDM) system on a silicon-on-insulator (SOI). The filter consists of four waveguide Bragg grating (WBG) structures, each one having a multi-mode waveguide and corrugations at both waveguide side walls. To relax the critical dimension, low-material-index silicon nitride (SiN$_{x}$) was utilized instead of silicon. For efficient design of each SiN$_{x}$ multi-mode WBG (MMWBG), the core and corrugation widths were engineered over the perturbed-permittivity coupled-mode theory, leading to an ultra-high side-lobe suppression ratio $>$25.3 dB. The overall CWDM filter offers flat-top responses with ultra-low excess losses (ELs) $< $0.6 dB, a high channel uniformity $>-$0.45 dB, broad 1-dB bandwidths (BWs) $\sim$13.45 nm, ultra-low XTs $<$$-$28 dB, and ultra-broad available bandwidths of 14.35 nm for XTs $< $$-$28 dB (ABW$_{28-}$_dB $\sim$14.35 nm). Analysis of the tolerance showed that XTs of the overall filter remained <−25 dB even for extreme cases with $\pm$18-nm over-etching errors without compromising the ELs or BWs. Given its ultra-low XT, ultra-broad ABW, and high tolerance, the proposed MMWBG shows the great potential and high attractiveness for the O-band CWDM telecommunication systems.