Effect of fabrication errors on binary optical element imaging quality

Abstract

Compared to conventional optical elements, the binary optical element (BOE) can not only revise aberration, but also reduce optical system’s weight, miniaturize a system and increase the freedom of system design. Nowadays, we need to improve the manufacturing process before the industrialization of the BOE. However, the fabrication accuracy of BOE affects the imaging quality of the optical system. This paper analyzes the influence of fabrication errors on BOE’s diffraction efficiency and transmission wavefront, to guide the processing and tolerance analysis of the optical systems design stage. Generally, in fabrication processes of the multiple-phase step BOE, there are alignment errors, linewidth errors and depth errors for various reasons. Due to the fabrication errors, the diffraction efficiency of the BOE would be reduced, thus stray light is introduced into the system. Besides, BOE graphical structure is also changed, then introduces wavefront aberrations to the optical imaging system, and thereby reduces the imaging quality of the system. Based on scalar diffraction theory, we have come to some conclusions like that the formula of wavefront PTV which can derive the fabrication requirements of alignment accuracy and linewidth accuracy; if 4-step BOE’s linewidth errors are controlled within a certain range, diffraction efficiency declines little, however, when the linewidth errors exceed this range, the diffraction efficiency will decline rapidly; the influence of deep and shallow etching depth errors on the diffraction efficiency is equivalent, the influence of multiple etching depth errors on the diffraction efficiency is independent and symmetric, and so on. This article focuses on analyzing the fabrication requirements of BOE based on how the fabrication errors affect the diffraction efficiency and wavefront imaging quality of BOE, then guide the design and fabrication processes of BOE imaging optical systems.