Long reach 8 (LR-8) silicon-nanowire optical demultiplexer for 400GbE and beyond in datacenter applications

This report describes the experimental demonstration of a silicon-nanowire-based flat-topped demultiplexer, specifically designed for Long Reach 8 (LR-8) wavelength-division multiplexing targeting 400 GbE interconnects. The motivation stems from the limitations of conventional bulk and silica-based demultiplexers. The proposed device is based on a multi-stage, multiple delayed interferometric (MDI) structure. Its core consists of four MDI stages, featuring precisely engineered optical path differences and multimode interference (MMI) couplers with varied splitting ratios. The third stage incorporates an intentionally shortened path difference to disrupt spectral periodicity. To further enhance band suppression ratio and ensure flat-topped responses, additional band rejection filters are introduced in the fourth stage. The device achieves an ultra-compact footprint of 600 × 300 μm2 and employs passive silicon waveguides (220 nm thick, 410 nm wide). The measured discrete, non-uniformly spaced channel grid in the O-band closely aligns with theoretical simulations based on coupled mode theory. A measured band suppression of 11 dB and box-like spectral shapes validate the design, with minor distortions attributed to fabrication-induced phase errors. This study also experimentally confirms the impact of statistical excess phase errors by evaluating an alternative waveguide design with a width of 350 nm. Importantly, the non-periodic, flat-topped LR-8 spectral characteristics were successfully demonstrated under TE-polarized input. The report also discusses performance limitations such as spectral crosstalk and suboptimal suppression ratio, identifying phase imbalance in certain asymmetric MMI couplers as the main cause. These degradations can be mitigated by integrating discrete phase shifters in the third MDI stage, without adding design or fabrication complexity. This work sets a new benchmark for integrated LR-8 demultiplexer designs, combining spectral flatness, non-periodicity, compactness, and thermal stability - key features for next-generation high-capacity interconnects.