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衍射计算是设计光盘光学系统的基础。只读光盘信息符（ 坑点） 尺寸可以和波长相比拟或在更小时标量衍射理论不再适用。本文将光盘信息符抽象成二维光栅，应用耦合波理论导出了当用平面波检测光盘时衍射波场的计算公式，得到了衍射强度坑深曲线，并提出了临界坑深的概念。分析表明，临界坑深随着信息符横向尺寸的变小而变小，这将导致衍射波光强变化的减小，从而限制了光盘的存储密度。本文还分析了检测平面波的偏振态对衍射波强度的影响
Diffraction calculations are the basis for designing a disc optical system. CD-ROM information (pit) size can be compared with the wavelength or smaller scalar diffraction theory is no longer applicable. In this paper, the disc information symbol is abstracted into two-dimensional grating, and the formula of diffracting wavefield when the disc is detected by the plane wave is deduced using the coupled wave theory. The diffraction intensity-depth curve is obtained and the concept of critical depth is proposed. The analysis shows that the critical pit depth decreases with the decrease of the horizontal size of the information symbol, which leads to the decrease of the light intensity of the diffracted wave, which limits the storage density of the optical disk. The paper also analyzes the influence of detecting the polarization state of the plane wave on the diffraction intensity