| Popis: |
The diffraction grating, a common diffractive optical element (DOE) used in many low-cost optical systems, is often fabricated in single-layer devices with stamping and rolling processes. Recently, there has been interest in micro-scale, customizable, and multi-layer DOEs that are increasingly being fabricated using expensive nanofabrication techniques, such as lithography, which requires numerous costly processes and clean rooms. Low-cost 3D printing of DOEs has, so far, not been feasible as the step change in illumination from a pixelated light source produces well-known diffraction effects of their own, which distorts the desired effect by the uneven surfaces and non-homogeneous refractive index. This paper reports a simple yet effective technique, coined ‘liquid immersion’, to characterize the uneven surface profile and the refractive index changes that result from pixelated light. Given this characterization, we can compensate for the undesired distortions by ‘pre-warping’ the desired grating, allowing the use of low-cost 3D printing with pixelated light intensity to achieve the required phase modulation for a DOE made from a photopolymer resin. This recovers the use of low-cost, single-layer DOE and paves the way for future multi-layer applications. Experiments show that the non-uniform light exposure generated both uneven surfaces (the diffraction gratings, ~100nm grating height for a 300-µm thick DOE) and non-homogeneous refractive index inside the printed sample (dN = 0.00013±0.00001) sufficient for high-quality reproduction of the desired optical effect. |
