Multiple mode band engineering in 1D grating-based photonic crystal waveguides for multidimensional silicon on-chip filters

Researchers at Hebei University, working with collaborators at Fuzhou University and Southeast University, have developed a multimode band-engineering framework in one-dimensional grating-based photonic crystal waveguides designed to meet the growing demand for scalable and flexible filtering in mode-division and hybrid multiplexing systems. The work, published in PhotoniX Synergy, combines multimode photonic band engineering, scalable higher-order-mode filtering, and reusable filter building blocks to enable broadband manipulation of spatial modes, polarizations, and wavelengths, providing a unified route toward compact and multifunctional silicon photonic devices.

As optical communication and information-processing systems continue to scale, multidimensional multiplexing is becoming increasingly important for expanding on-chip transmission capacity. Mode-division multiplexing offers a promising solution by using orthogonal spatial modes as independent information channels. However, increasing the number of supported modes introduces increasingly complex modal dispersion and bandgap relationships, making it difficult to transmit a selected higher-order mode over a broad bandwidth while simultaneously suppressing multiple lower-order modes. Existing approaches often rely on mode conversion, cascaded filters, or auxiliary structures, which can increase device complexity, optical loss, and design difficulty.

To address these challenges, the researchers developed a multimode band-engineering strategy based on one-dimensional grating photonic crystal waveguides. By jointly engineering the transverse waveguide dimension and grating parameters, the approach controls the supported Bloch modes, modal dispersion, Bragg coupling, and the relative positions and widths of their photonic bandgaps. This coordinated design creates three distinct transmission regimes—all-pass, selective-pass, and all-block—within a unified band diagram. In the selective-pass regime, lower-order modes fall within their respective bandgaps and are Bragg-reflected, whereas the target higher-order mode remains in an allowed band and is directly transmitted.

“Our goal was to move multimode filtering beyond device-by-device optimization and establish a systematic way to engineer multiple modal bands within a shared photonic structure,” said Shengbao Wu, corresponding author at Hebei University. “By placing lower-order modes inside photonic bandgaps while maintaining the target higher-order mode in a transmission band, we can realize direct and scalable mode selection without relying on cascaded filtering stages or intermediate mode conversion.”

To translate the band-engineering principle into practical devices, the team developed a unified architecture integrating a chirped–uniform–chirped grating section with strip-to-subwavelength-grating waveguide mode converters. Devices fabricated on a standard 220-nm silicon-on-insulator platform achieved TE₁-, TE₂-, and TE₃-pass filtering with measured bandwidths exceeding 200 nm, 123 nm, and 70 nm, respectively, while maintaining extinction ratios above 20 dB and insertion losses below 2 dB. Using these higher-order-mode filters as reusable building blocks, the researchers further demonstrated a broadband polarizer, a polarization beam splitter, and a wavelength demultiplexer.

“Broadband multimode control requires more than opening a photonic bandgap—it requires precise engineering of the relative dispersion and bandgap positions of multiple Bloch modes,” said Lei Zhang, co-first author of the paper. “The chirped transitions and mode-conversion structures allow the engineered band response to be translated into low-loss and broadband device operation, while the extension to polarization and wavelength filtering demonstrates the broader versatility of the multimode band-engineering framework.”

See the article:

Multiple mode band engineering in 1D grating-based photonic crystal waveguides for multidimensional silicon on-chip filters

https://doi.org/10.1007/s44519-026-00010-w