光盘光学系统标量衍射理论的有效性分析

Validity analysis of scalar diffraction theory in optical system of optical disk

衍射计算是设计光盘光学系统的基础。由于光盘信息符（坑点）尺寸可以和波长相比较或更小，标量衍射理论的有效性必需进行检验。首先分析了标量衍射理论的能量计算误差。对于典型的坑点尺寸，标量理论的能量计算误差高达１９％。然后用耦合波理论分析了入射波偏振态的影响。计算表明ＴＭ波和ＴＥ波的衍射特性与标量理论的结论相差较大，ＴＭ波和ＴＥ波的衍射行为也有差别。这种差别是由电磁场边界条件引起的，且会随着坑宽的缩小而增大。对于ＤＶＤ，标量理论的误差已经很大，要得到准确的结果就必须采用矢量衍射理论。

Diffraction calculation is the basis of designing optical system of optical disk. Validity of scalar diffraction theory must be checked, because the size of information mark (pit) of optical disk is comparable with or smaller than the laser wavelength. The energy calculation error of scalar theory was investigated firstly. The error was as large as 19% for typical pit size. Then the polarization effect of incident wave was analyzed with coupled wave theory. It is found that the diffraction characteristic of TE and TM polarization is far from the result of scalar theory, and difference also exist between TE and TM polarization. The difference can be ascribed to the boundary condition of the electromagnetic field, and it will increase with the decrease of the pit width. The error of scalar diffraction theory is very large for DVD, and vector diffraction theory must be employed to get correct result.