Subperiod broadband diffraction efficiency dispersion mechanism for bi- or multi-focal diffractive optical elements

Bi- or multi-focal diffractive optical elements (DOEs) typically experience a rapid decrease in diffraction efficiency when deviating from the central design wavelength. This is a severe issue for single-layer DOEs and limits their applications. Currently, the usual solution is to use multilayer DOEs (MLDOEs), which are unfavorable for lightweight design, and their assembly introduces additional problems. This is usually due to an unclear understanding of the impact of dispersion on diffraction efficiency and the underlying optimization mechanism. Here, we report an interesting broadband dispersion mechanism. First, through a special method-derivation-we present the influence mechanism between broadband diffraction efficiency and the dispersion factor from a new, to the best of our knowledge, perspective, and explain that the decline in broadband diffraction efficiency of traditional DOEs is due to the inability to eliminate the component of the dispersion factor on diffraction efficiency. By designing bi- and multi-focal DOEs using a subperiod approach, we found that a multi-subperiod design can minimize the component of the dispersion factor on diffraction efficiency, thereby minimizing the deviation in broadband diffraction efficiency. Through relatively simple phase transformations, the dispersion characteristics on diffraction efficiency of single-layer DOEs can be effectively improved without the need for complex phase transformations or multilayer DOEs. Furthermore, we believe that the elucidation of this mechanism can further deepen the understanding of DOEs.