Spectral Nonuniformity of DMD Reflectance in the near-infrared and its Mitigation with Germanium Interference Filters

Digital micromirror devices (DMDs) are key components of spatially modulated optical sensing systems, including single-pixel (SPI) cameras. Although optimized for visible wavelengths, DMDs can also operate in the near-infrared (NIR), where pronounced diffraction effects arise. Diffraction is shown to induce a strong directional dependence in the DMD reflection spectrum, leading to significant deviations from a flat spectral response when diffracted orders overlap the specular direction. To mitigate these effects, compact, high-efficiency germanium-based interference filters are designed to control DMD reflectance in the NIR. The filters are well suited for integration into SPI infrared cameras and other DMD-based sensors. An ellipsometric dispersion model for amorphous germanium is developed, showing excellent agreement between modeled and experimentally measured filter transmittance. The analysis further indicates that germanium layers deposited by electron-beam physical vapor deposition are best described as composite materials comprising amorphous germanium, crystalline germanium, and germanium oxide. SPI imaging in the visible and NIR bands using the germanium filter is demonstrated.