A freeform optical element, defined as an optical part that has no axis of rotational symmetry, had gained lots of attention in recent years thanks to its high flexibility in design and excellent optical performance. It has shown many advantages over conventional spherical and aspherical optics, such as high degree of integration and enhanced imaging ability. With the rapid development of manufacturing technologies, freeform optics have found wide applications in various areas, including illumination (Feng, et al., 2010), astronomy observation (Hugot, et al., 2014), green energy (Garcia-Botella, et al., 2006), imaging (Chrisp, et al., 2016) and virtual reality (Wei, et al., 2019). Nowadays, freeform optics are playing an increasingly important role in modern optics and are considered the next-generation optical components.
However, due to the complexity of the surface geometry, the fabrication of freeform optics has faced major challenges such as low production efficiency and imperfect surface quality. Additionally, the non-symmetry nature and high degree of freedom of freeform optics have also raised many problems in metrology. Conventional metrology methods for axisymmetric optics are not adequate to characterize freeform surfaces, while existing freeform metrology techniques such as coordinate measuring machine and stitching method based on interferometry also have many drawbacks, e.g., high cost, low efficiency, limited accuracy and small dynamic range. In order to analyze and cope with these major problems in fabricating and measuring freeform optics, this dissertation will focus on investigating the fundamental knowledge of optical design, fabrication and measurement of freeform optics, exploring the applications of freeform optics in imaging and non-imaging fields, and putting forward a novel method for freeform optics metrology.
First, to address the problem of low-cost high-accuracy metrology of freeform optics, a special type of microlens array was designed and fabricated, based on which a low-cost Shack-Hartmann wavefront sensor (SHS) was built. With a self-developed algorithm, this SHS can achieve a measurement accuracy of 0.3λ. This SHS is then adopted to measure an Alvarez lens, which is a typical freeform lens. In the meantime, an infrared SHS which works in longwave infrared range (8 ~ 12μm wavelength) was developed for metrology of infrared optics. These SHSs have offered a novel and affordable option for high accuracy metrology of freeform optics.
Second, a freeform compound eye infrared imaging system was designed and developed. This system consists of two freeform lens arrays and a 3D-printed microstructure array sandwiched between them to prevent crosstalk between adjacent imaging channels. This 19-channel compound eye was able to achieve a total field of view of 80°. Surface geometry accuracy of the system was maintained by ultraprecision diamond milling and precision glass molding processes. Experiments showed that this system has met the designed field of view and resolution requirements.
Finally, a novel fabrication process by combining single point diamond turning and electroforming was developed to generate a precision nickel mold for glass molding. First, a harmonic diffractive lens (HDL), which is a special type of freeform lens, was designed. Then diamond turning was adopted to fabricate the designed geometry on a plastic disk. After coating a thin conductive layer onto the disk, electroforming was conducted to generate a nickel disk that is thick enough to be a mold in precision glass molding process. Precision glass molding was applied to fabricate the designed glass HDL in the last step.
Overall, this dissertation focuses on applying different processes to fabricate and developing novel method to measure freeform optical elements.