基于旁瓣光束衍射反演的强激光远场焦斑测量方法

针对大型激光装置使用纹影法无法实现旁瓣光束弱信号区域光强分布精确测量的问题,提出了基于旁瓣光束衍射反演的纹影法强激光远场焦斑测量方法。采用逆向推演间接测量的研究方法,沿光路传播逆方向推导,以旁瓣光束衍射光强图像和相位图像作为输入,通过计算获得未遮挡前旁瓣光束远场焦斑分布。相比传统基于纹影的远场焦斑测量方法,本文的主要改进和优化措施为:首先,基于旁瓣光束衍射反演的原理和间接测量的思想,改进纹影法强激光远场焦斑测量数学模型,从理论角度揭示该模型的合理性;其次,实验仿真强激光远场焦斑测量的整个过程,主要步骤为旁瓣光束衍射、加噪去噪、旁瓣光束衍射反演、焦斑重构等,验证了该方法的可行性;最后,将改进的DnCNN算法用于去除主瓣和旁瓣光束12位科学CCD图像不同级别(0~75 dB)的噪声,提升了远场焦斑的重构精度。实验结果表明:该方法不仅消除了纹影小球对旁瓣光束衍射的影响,而且获得了真实的旁瓣光束弱信号区域的光强分布,包括旁瓣光束各个波峰的幅值和位置、动态范围比值等远场焦斑测量重要参数,其中重构焦斑动态范围比值与理论焦斑动态范围比值之间的误差为3.20%,提高了基于纹影的强激光远场焦斑测量的可信度和实验精度。

Measurement for far-field focal spot of high power laser based on the diffraction inversion of sidelobe beam

In order to solve the problem that the schlieren method cannot accurately measure the weak sig?nal region of the sidelobe beam in large laser devices of the national large scientific facility,a new schlieren method based on the diffraction inversion of the sidelobe beam is proposed to measure the far-field focal spot for high-power laser.The key point of this method is that an indirect measurement approach is used based on reverse deduction,while deducting along the reverse direction of the optical path propagation.The diffraction intensity image and phase image of the sidelobe beam are the inputs to calculate the far-field focal spot distribution of the front sidelobe beam,which is not shielded.Compared with the traditional far-field focal spot measurement based on the schlieren method,the improvements and optimizations proposed in this paper are as follows.First,the mathematical model of far-field focal spot measurement using the schlieren method is improved to reveal the rationality of the model,theoretically based on the principle of diffraction inversion of the sidelobe beam and the indirect measurement approach.Then,the feasibility of this method is verified by simulating the whole experimental process of high power laser far-field focal spot measurement,which consists of sidelobe beam diffraction,denoising,sidelobe beam diffraction inversion,and focal spot reconstruction.Finally,the improved DnCNN algorithm is used to remove the noise of dif?ferent levels(0-75 dB)of 12 bit scientific CCD images in the mainlobe and sidelobe beams,and the recon?struction accuracy of far-field focal spot is improved.The experimental results show that this method not only eliminates the influence of the schlieren sphere on the diffraction of the sidelobe beam but also obtains the real intensity distribution of the sidelobe beam in the weak signal region,including the important param?eters of far-field focal spot measurement,such as the amplitude and position of each peak of the side lobe beam,the dynamic range ratio,the amplitude and position of each peak of the sidelobe beam,and the ratio of dynamic range.The error between the reconstructed and theoretical focal spots of the dynamic range ra?tio is 3.20%.It is significant to improve the reliability and experimental accuracy of the far-field focal spot of high-power laser measurement using the schlieren method.