Study on Polarization Compatibility of Ultra-low-loss Waveguides for the Full Visible Spectrum

In recent years， visible-light integrated photonics has undergone rapid development， demonstrating significant applications across various fields. In biosensing， integrated optical waveguide sensors based on the evanescent wave effect enable high-sensitivity real-time monitoring of analytes by detecting optical parameter variations induced by changes in analyte properties. For consumer electronics， miniaturized on-chip spectrometers provide real-time spectral analysis capabilities for smart devices， supporting applications such as food safety testing and health monitoring. In optical computing， metasurfaces and diffractive neural network technologies have facilitated the development of low-power intelligent sensing chips with ultra-fast parallel processing capabilities， capable of performing tasks such as image recognition and object tracking.Extending the operational wavelength range of these devices to cover the entire visible spectrum holds considerable importance， as it can enhance the detection accuracy and applicability of miniaturized spectrometers， broaden the bandwidth of visible-light communication systems， and enable biosensors to simultaneously detect multiple biomolecules in complex samples. As fundamental components of integrated photonic devices， waveguides serve as the core elements for directional photon transport and device interconnections in most on-chip systems. However， current research on full-visible-spectrum waveguides remains incomplete， with most studies focusing on waveguide dimension design for specific narrow bands. The investigation of low-loss transmission across the entire visible spectrum in single waveguide structures remains insufficient， which may limit their applications in on-chip full-visible-spectrum devices. Therefore， this study proposes a waveguide design that supports the entire visible spectrum （435~760 nm）.Generally， single-mode waveguides exhibit superior performance compared to multimode waveguides. However， research has revealed conflicting requirements for single-mode operation across different wavelengths： shorter wavelengths require smaller dimensions to suppress higher-order modes， while longer wavelengths necessitate larger dimensions to prevent energy leakage. Consequently， none of the three commonly used waveguide structures （rectangular waveguides， rib waveguides， and slab waveguides） can achieve single-mode operation across the entire visible spectrum. As an alternative approach， this study systematically investigates key parameters for minimizing propagation loss under multimode transmission conditions.Considering the respective advantages and research value of polarization-sensitive and polarization-insensitive waveguide platforms， dimension designs were completed for both TE-polarized thin waveguides and polarization-insensitive thick waveguides based on silicon nitride platforms with thicknesses of 500 nm and 1 μm， respectively： For thin waveguides， the thickness was optimized at 0.5 μm after comprehensive consideration of loss and fabrication challenges， while thick waveguides were designed with 1 μm thickness to achieve polarization-insensitive low-loss transmission. The width of thin strip waveguides was selected as 1.5 μm for lower loss， whereas thick waveguides were designed with 2 μm width to mitigate scattering loss induced by sidewall roughness. Regarding spatial configuration， thin waveguides require minimum waveguide spacing of 0.6 μm and minimum bending radius of 15 μm， while thick waveguides permit tighter configurations with 0.4 μm spacing and 10 μm bending radius. For both waveguide types， fabrication dimensional tolerances exhibit negligible impact on optical transmission. Additionally， thin waveguides demonstrate high sensitivity to sidewall roughness， whereas thick waveguides maintain relatively low transmission loss when the root-mean-square roughness is ≤10 nm.Comparative analysis of the two waveguide configurations reveals distinct advantages　thin waveguides offer easier fabrication and higher integration density， though they are limited to TE-mode operation； thick waveguides feature comparable effective refractive indices for TE and TM modes， making them suitable for polarization-insensitive applications in systems employing broadband LED sources （e.g.， biosensors and color imaging）， despite requiring specialized stress-control processes during fabrication. Both configurations present unique advantages that can be selected based on specific device requirements.In summary， this study analyzes propagation loss and mode distribution in waveguides to determine optimal parameters for low-loss transmission， including waveguide dimensions， bending radii， and spacing. Furthermore， it evaluates the impact of fabrication tolerances on waveguide performance， providing valuable references for the design and manufacturing of on-chip photonic devices operating across the full visible spectrum.