High-performance 90°hybrids based on MMI couplers in Si₃N₄ technology

Low-loss, broadband, and low phase deviation optical 90°hybrids based on 2 × 4 multimode interference couplers are firstly demonstrated using Si₃N₄ on SiO₂ technology. Only a simple traditional design method of MMI based on self-imaging theory and a single-stripe Si₃N₄ waveguide geometry with only one step etching process are utilized. The characterization of the fabricated devices is carried out over a wide spectral window of 1520–1610 nm, showing that the measured phase error is less than 8°, and common-mode rejection ratios (CMRRs) are better than −20 dB. The measured total loss of the fabricated device including a 90°hybrid and a 3-dB coupler is <1 dB in a bandwidth over 80 nm. Our analysis also shows large fabrication tolerance with width of ±0.1μm or length of ±3μm.