针尖增强拉曼系统高数值孔径物镜对焦方法研究

Research on Focusing Method of High Numerical Aperture Objective Lens in Tip-enhanced Raman System

为提高针尖增强拉曼系统对焦准确性和效率,提出通过分析CCD相机拍摄的光斑图像的像素值实现自动对焦的方法。首先快速粗调焦缩小调焦范围,然后再慢速细调焦提高调焦精度。实验表明:与Tenengrad评价函数相比,方差评价函数更适用于针尖增强拉曼光谱测量系统自动对焦,聚焦到样品表面时对焦结果的无偏估计标准差<0.25μm,具有较高的重复性和鲁棒性;快速粗调焦可提高调焦效率,避免陷入局部最优解;慢速细调焦评价函数曲线在焦点附近呈现“M”形,可通过二次拟合提高调焦精度。

In order to improve the focusing accuracy and efficiency of tip-enhanced Raman system(TERS), an automatic focusing method is proposed by analyzing the pixel value of the spot image captured by CCD camera.Fast coarse focusing to narrow the focusing range, and then slow fine focusing to improve the focusing accuracy.The experimental results show that, compared with Tenengrad evaluation function, the variance evaluation function is more suitable for automatic focusing of tip-enhanced Raman system.When focusing on the sample surface, the unbiased estimation variance of focusing result is less than 0.25 μm, which has high repeatability and robustness.Fast coarse focusing can improve the focusing efficiency and avoid falling into the local optimal solution.The curve of slow fine focusing evaluation function presents an “M” shape near the focus, which can be adjusted by quadratic fitting to improve the focusing accuracy.