全息光栅曝光系统中空间滤波器孔径与激光束腰关系的选择方法

在全息光栅制作过程中, 由空气悬浮颗粒及光学器件缺陷引起的散射光会被记录在光刻胶上, 既影响光栅衍射效率, 又会带来光栅杂散光. 为消除系统散射光, 需要对激光光束的高频成分进行滤波处理. 但是, 激光光束通过空间滤波器后发生衍射, 进而影响干涉场质量, 因此选择合适的空间滤波器孔径是搭建全息光栅曝光系统的前提条件. 根据标量衍射积分理论, 通过卷积计算滤波光束的振幅分布分析光束的相位变化, 以光束相位突变位置远离光束中心为依据确定滤波器针孔半径与光束束腰半径的比值下限; 根据滤波光束的能量守恒关系, 通过滤波光束的高斯参数变化描述光束的衍射强弱规律, 以光束发生衍射的临界条件确定滤波器针孔半径与光束束腰半径的比值上限. 结果表明, 当空间滤波器针孔半径与激光束腰半径之比满足1.52< a/ω₀<2.2时, 滤波光束曝光区域相位恒定, 光束能量通过率大于99%, 在此区间空间滤波器滤波效果最好.

The ratio choice method of the pinhole aperture to the waist of the Gaussian laser in the fabrication of holographic gratings

In the fabrication of the holographic diffraction gratings, the scatter light which is produced by the reflection of the granule in air or the diffraction defect of the optical apparatus will be recorded simultaneously in the resist. Those noises not only reduce the diffraction efficiency of gratings, but also give rise to the scattered light in diffraction. So an appropriate spatial filter should be used to stop the higher spatial frequencies corresponding to the Gaussian beam. Because the diffraction effect after the pinhole aperture can demolish the wavefronts of laser, it is the most important to choose the optimal pinhole for adjusting the waist of the laser. On the basis of standard scalar diffraction theory, the field amplitude is given by the convolution calculation, the phase distortion of diffraction wavefronts is also analyzed, and the lower limit ratio of the aperture size with respect to the beam radius is given when the phase transitions are further away from the center. Using the energy conservation of Gaussian beam and comparing the characteristics change of diffraction beam, the effect of diffraction is presented and the upper limit ratio of the spatial filter is given by the critical condition of the diffraction. The conclusion shows that if the ratio of the aperture radius to beam waist is between 1.52< a/ω₀<2.2, the phase is invariable in the area of the exposal, and the power transmitting the spatial-filter pinhole aperture is slightly more than 99%, so spatial filter performs its function only in this range of the radio.