Achromatization of efficiency of multifocal diffractive optical elements using tailored double-layer structures

Multifocal diffractive optical elements (MFDOEs) form images at different scene distances using interleaved sawtooth patterns. However, single-layer MFDOEs exhibit wavelength-dependent efficiency distributions, causing chromatic variations. This work introduces multilayer MFDOEs with two materials of different dispersion characteristics being separated by a sawtooth structure, enabling efficiency achromatization across multiple diffraction orders. The focus is on selecting two materials with a wavelength-dependent refractive index difference Δ<i>n</i> (<i>λ</i>), for which the refractive index contrast Δ<i>n</i>(<i>λ</i>)/<i>λ</i> remains nearly constant. Exemplarily, the inorganic glass K-SSK3 and the polymer PS are investigated. The achromatization degree is assessed using mean absolute error (<i>MAE</i>) and root mean square error (<i>RMSE</i>). Optimal achromatic efficiency is demonstrated for orders <i>m</i> = 0, 1, 2. For specific geometries, simultaneous achromatization is achieved, distributing efficiency as 50% in <i>m</i> = 1 and 25% in <i>m</i> = 0, 2 over the wavelength range from 0.5 to 0.8 µm. These results advance the development of highly efficient achromatic MFDOEs.