Optical Waveguides and Light Guides in Vat-Printed Photopolymers: Refractive-Index Contrast, Propagation Loss, Scattering, Coupling and Qualification

Vat photopolymerisation can generate three-dimensional polymer optical paths whose geometry is difficult to obtain by conventional planar photonics. Recent work demonstrates DLP-printed elevated waveguides, DLP-printed photoluminescent waveguides and splitters, stereolithography-printed methacrylate waveguides whose optical output changes after solvent uptake, complex SLA waveguides for three-dimensional light redistribution, and two-photon-polymerised waveguides with sub-micrometre features and engineered core-cladding structures. These demonstrations also expose the qualification problem. Bulk refractive index and total transmittance do not determine propagation loss; surface stair-stepping, sidewall roughness, internal inhomogeneity, conversion state, bends, branches, interfaces and coupling can dominate. Propagation loss must be separated from insertion or coupling loss, and a waveguide tested at one wavelength or environmental state cannot be assumed equivalent at another. This review defines a state-based evidence chain from cured optical material to finished waveguide and integrated device, proposes minimum reporting requirements, and positions bulk-plastic standards such as ISO 13468-1:2019 and ASTM D1003-21 as supporting material-characterisation tools rather than substitutes for waveguide loss measurements. The 3Dresyns refractive-index material platform is included only as manufacturer implementation context; no waveguide loss, modal behaviour or device performance is inferred from nominal resin refractive index.