Design method for nonsymmetric imaging optics consisting of freeform-surface-substrate phase elements

Phase elements can be used in optical systems to achieve similar design goals to traditional geometric optical elements. If we replace traditional geometrical optical elements in optical systems by phase elements (such as diffractive optical elements and metasurfaces) which have phase functions loaded on the geometric surface substrates, it is possible to generate imaging optical systems that offer better performance, increased compactness, lighter weight, and easier alignment and manufacturing than conventional imaging systems. Here we propose a design method for imaging systems consisting of freeform-surface-substrate phase elements. The design process begins from an initial system that uses simple geometric planes or other predefined geometric surfaces without phase functions. After point-by-point construction and iteration steps, the geometric substrate surfaces and closed-form phase functions can then be calculated quickly and efficiently. The resulting design can be used as a good starting point for further optimization. To illustrate the generality and feasibility of the proposed design method, we present two high-performance compact systems as design examples. Both systems meet the design requirements, with small distortions after optimization. Their modulation transfer function (MTF) curves are close to the diffraction limit. This design framework can be used to design next-generation imaging systems using phase elements for applications including near-eye-displays, high-performance cameras and remote sensing and detection. The proposed method also offers insight into design of imaging systems that are constrained to conformal substrate shapes or integrated substrates.