The substrate material for a 0.23 inch optical waveguide module is typically high-refractive-index glass, specifically a borosilicate-based glass compound or a lithium-niobate (LiNbO₃) crystal, depending on the specific design and application. For the 0.23 inch optical waveguide module used in AR smart glasses, the substrate is often a fused silica glass with a refractive index of 1.458 at 587.6 nm, or a Schott N-BK7 glass with a refractive index of 1.5168. These materials are chosen because they offer low optical loss, high thermal stability, and compatibility with micro-OLED displays. The waveguide core is usually made of doped silica or tantalum pentoxide (Ta₂O₅), with a thickness of 0.5 to 1.5 micrometers, to ensure efficient light propagation. The cladding layer is often silicon dioxide (SiO₂) with a thickness of 10 to 20 micrometers, providing a refractive index contrast of 0.5% to 1.5%. This structure supports single-mode or multimode operation at visible wavelengths, typically 450 nm to 650 nm, with a numerical aperture of 0.12 to 0.25. The substrate material must have a coefficient of thermal expansion (CTE) below 5 ppm/°C to match the silicon-based micro-OLED driver, preventing misalignment during thermal cycling. In commercial prototypes, Corning Eagle XG glass is used, with a CTE of 3.17 ppm/°C and a Young’s modulus of 73.6 GPa, ensuring mechanical durability. The waveguide module’s total thickness is 0.5 mm to 1.0 mm, with a substrate thickness of 0.3 mm to 0.5 mm, to fit into compact AR frames. The optical efficiency is 15% to 25%, with a field of view of 30° to 40°, depending on the grating design. The substrate material also affects the polarization-dependent loss, which is kept below 0.5 dB for uniform brightness. In some designs, polycarbonate plastic is used as a substrate for cost reduction, but it has a refractive index of 1.586 and a CTE of 70 ppm/°C, leading to thermal drift issues. Therefore, glass substrates remain the standard for high-performance modules.
The material choice directly impacts the waveguide’s dispersion characteristics. For a 0.23 inch module, the substrate must support diffractive gratings with a period of 300 nm to 500 nm, etched into the surface. These gratings are typically made of titanium dioxide (TiO₂) or silicon nitride (Si₃N₄), with a duty cycle of 50% and a depth of 100 nm to 200 nm. The substrate’s surface roughness must be less than 1 nm RMS to minimize scattering losses, which are typically 0.1 dB/cm for glass substrates