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激光损伤阈值测试Laser Damage Threshold TestingLaser Damage Threshold (LDT) is one of the most importantspecifications to consider when integrating an optical component intoa laser system. Using a laser in an application offers a variety ofbenefits to a standard light source, including monochromaticity,directionality, and coherence. Laser beams often contain high energies, though, and are capable of damaging sensitive optical components.When integrating a laser into an optical system, it becomes crucial tounderstand the effects of laser beams on optical surfaces and how laser damage threshold is quantified for optical components.The degree of damage induced to an optical component by a laser beam is highly dependent on the type of laser being used. Thermally-induced damage occurs under Continuous Wave (CW) laser operation. During exposure to the CW laser, the optical material may not have sufficient time to thermally relax, and failure can occur due to thermal damage to the bulk material or the optical coating. Alternatively, the damage caused by a short, intense laser pulse is due to ionization: the breakdown of the molecular bond. The electric field generated by the laser beam at the optical surface stimulates electrons at the outer energy band, causing ionization. However, it is important to keep in mind that lasers with long pulse widths (<10s) or high repetition rates(>10MHz) may also cause thermally induced damage. For these reasons, understanding laser damage threshold is crucial to designing and maintaining an optical system. -6T esting Laser Damage ThresholdLaser-induced damage threshold testing is a good method for quantifying the amount of electromagnetic radiation an optical component can withstand. There are a variety of different LDT tests. For example, Edmund Optics follows the ISO-11254 procedures and methods, which is the industry standard for determining the laser damage threshold of an optical component. Utilizing the ISO-11254 standard enables the fair comparison between optical components from different manufacturers.Edmund Optics' LDT testing is conducted by irradiating a number of test sites with a laser beam atdifferent energy densities for pulsed lasers, or different power densities for CW lasers. The energy density or power density is incrementally increased at a minimum of ten sites at each increment. The process is repeated until damage is observed in 100% of the irradiated sites. The LDT is the highest energy or power level at which no damage is observed in any of the irradiated sites. Inspection of the sites is done with a Nomarski-type Differential Interference Contrast (DIC) microscope with 100X - 150X magnification. Visible damage is observed and the results are recorded using pass/fail criteria. Figure 1 is a typical damage probability plot of exposure sites as a function of laserpulse energy.In addition to uncoated optical components, optical coatings are also subject to damage from the presence of absorption sites and plasma burn. Figure 2 is a real-world image of coating failure due to a coating defect. For additional information on the importance of LDT testing on coatings, view The Complexities of High-Power Optical Coatings.Figure 1:Exposure Histogram of Laser Damage Threshold Probability versus Exposure SiteFigure 2: Coating Failure from 73.3 J/cm3 Source due to Coating DefectDefining Laser Damage ThresholdThere are many variables that affect the Laser Damage Threshold (LDT) of an optical component. These variables can be separated into three categories: laser, substrate, and optical coating (Table 1).LDT is typically quantified with units of power or energy densities for CW and pulsed lasers, respectively. Power density is the power per cross-sectional beam area. Similarly, energy density is the energy per cross-sectional beam area of a specific pulse duration. Lasers are available with a multitude of different wavelengths and pulse durations, therefore, it is important that the optical component's LDT is suitable for the laser's parameters. As a general rule of thumb, Newton's square root scaling factor can be used to determine whether a laser can be used with an optic that is not rated at thesame LDT pulse duration specification. Equation 1 calculates a new LDT for the different pulse duration.(1)The LDT(y) is the estimated LDT for laser Y, and LDT(x) is the specified LDT for laser x. τ is the pulse duration for laser y, and τ is the pulse durat ion for laser x. Additionally, since the energy of a photon is inversely proportional to its wavelength, then theoretically the LDT scales linearly as a function of wavelength, as expressed in Equation 2. yx(2)Where PD is the Power or Energy Density at the new wavelength, PD is the Power or Energy Density at the old wavelength, λ is the new wavelength, and λ is the old wavelength. A laser with a PD of 2 W/cm at 1064nm would have a power density of 1 W/cm at 532nm, 0.667 W/cm at 355nm, etc. (y)(x)yxCW222There are some drawbacks to the scaling, as there are non-linear effects associated with the conversion. However, they are a good rule of thumb for estimating the LDT of an optic at varying wavelengths and pulse durations. Note: Optical manufacturers only guarantee the specified LDT, not scaled estimations. Laser Damage Threshold (LDT) testing is crucial when working with laser optics. Understanding how LDT is tested and defined helps choose the right optical components for the application. Laser optics thatare designed with an LDT that is suitable for a given laser ensure superior results and product lifetime, and help avoid additional expenses due to damaged components.。
一种激光损伤阈值测试新方法葛锦蔓;苏俊宏;陈磊;吕宁【摘要】随着激光器朝向大功率、高能量的方向发展，激光损伤阈值成为了衡量光学元件抗激光损伤能力的重要参数之一，因此，能否准确地测量出光学元件的激光损伤阈值成为研究的重点。
而光学元件激光损伤阈值测试的关键是能否准确地判别光学元件是否发生激光损伤。
为解决目前常见的损伤判别方法存在的精度低、识别时间长、适用材料范围窄、操作复杂等不足，提出了一种新的激光损伤的判别方法，即等离子体诊断法。
以K9玻璃为例，搭建激光损伤阈值的测试平台，利用光纤光谱仪采集强激光辐照 K9玻璃时所产生的激光等离子体闪光光谱，并对该光谱进行诊断分析，将该光谱中是否含有待测试光学元件材料中特征元素的光谱峰作为其是否收到激光损伤的标准。
同时，对K9玻璃进行了激光损伤阈值的测试，并将测试结果与等离子体闪光法和显微镜法所测的激光损伤阈值进行了对比分析。
实验表明，提出的等离子体诊断方法的判别精度高、速度快、测试装置结构简单，易实现在线测量，可以大大地提高光学元件激光损伤阈值测试工作的效率。
%With the development of the laser towards high-power and high energy,laser-induced damage threshold of optics be-comes one of the important parameters to evaluate the laser damage resistance ofoptics.Therefore,accurately measuring of the laser-induced damage threshold optics become the focal point studied.And the key to accurately measuring of the laser-induced damage threshold is whether the laser-induced damage can be accurately identified when it occurs.In order to solve low accuracy, long testing time,narrow scope of applications and complex operation of the common damage identification methods,a newtes-ting method to diagnose the laser-induced damage of optics,called plasma diagnosis,is proposed in this paper.Based on this new method,the testing platform was set up,and the spectrum obtained by fiber spectrometer was analyzed under laser radiation by different laser energies.Take whether the spectral lines of the feature element contained in the measured optics occur as stand-ard.The laser-induced damage threshold of K9 glass has been tested,and the test result was compared to that measured by the plasma flash method and the microscope method.The results show that,the plasma diagnosis method proposed in this paper has high-accurate j udgment,high-testing speed,simple testing equipment,and easy to realization,which can greatly improve the testing efficiency of the laser-induced damage threshold of optics.【期刊名称】《光谱学与光谱分析》【年(卷),期】2016(036)005【总页数】4页(P1296-1299)【关键词】激光损伤;激光损伤阈值;等离子体;光谱分析【作者】葛锦蔓;苏俊宏;陈磊;吕宁【作者单位】南京理工大学电子工程与光电技术学院，江苏南京 210094; 西安工业大学陕西省薄膜技术与光学检测重点实验室，陕西西安 710021;南京理工大学电子工程与光电技术学院，江苏南京 210094; 西安工业大学陕西省薄膜技术与光学检测重点实验室，陕西西安 710021;南京理工大学电子工程与光电技术学院，江苏南京 210094;西安工业大学陕西省薄膜技术与光学检测重点实验室，陕西西安 710021【正文语种】中文【中图分类】TH843各种高功率激光系统和强激光武器的发展，给光学元件提出了更高的抗激光损伤的要求。
本技术涉及一种光学薄膜激光损伤阈值测试方法，包括如下步骤：S1、测试得到光学薄膜单脉冲激光损伤时的激光能量密度Fth；S2、使单脉冲激光对光学薄膜进行辐照，记录下光学薄膜表面激光损伤边界不再增大时的激光损伤区域边界坐标(xi,yi)，同时记录下单脉冲激光辐照的次数n；S3、将激光能量密度的高斯分布与激光损伤区域分布对照，得到光学薄膜多脉冲激光辐照损伤时的激光损伤阈值FN；S4、不断改变入射的激光能量密度，重复执行步骤S2、S3，得到不同脉冲数目的飞秒激光辐照下光学薄膜的激光损伤阈值曲线。
有益效果是不仅仅保证多脉冲激光辐照下光学薄膜激光损伤阈值测量准确性、同时大大提高多脉冲辐照下光学薄膜损伤阈值的测试效率。
技术要求1.一种光学薄膜激光损伤阈值测试系统，其特征在于：所述测试系统包括飞秒激光器(1)、两个反射镜(2)、能量衰减系统(3)、机械快门(4)、聚焦透镜(5)、楔形片(6)、光束质量分析仪(7)、能量计(8)、供光学薄膜(9)放置的二维移动平台(10)、CCD相机(11)和电脑(12)，所述电脑(12)设有数据输出卡(13)和运动控制卡(14)；所述飞秒激光器(1)连接至数据输出卡(13)，所述二维移动平台(10)连接至运动控制卡(14)，所述光束质量分析仪(7)、能量计(8)、CCD相机(11)连接至电脑(12)，所述数据控制卡(13)用于控制飞秒激光器(1)输出飞秒激光，所述运动控制卡(14)用于控制二维移动平台(10)的水平和垂直移动，所述光学薄膜(9)安装在二维移动平台(10)上，所述CCD相机(11)摄像头对准光学薄膜(9)；所述飞秒激光器(1)、两个反射镜(2)、能量衰减系统(3)、机械快门(4)、聚焦透镜(5)、楔形片(6)在一个激光光路上，所述光束质量分析仪(7)和能量计(8)用于分别收集楔形片(6)反射方向的激光光束，所述光束质量分析仪(7)用于激光质量分析，所述能量计(8)用于测量激光的能量；所述光学薄膜(9)表面接收楔形片(6)透射方向的激光光束，所述反射镜(2)、能量衰减系统(3)用于调整飞秒激光器(1)发出的激光能量密度，所述机械快门(4)用于调整到达光学薄膜(9)表面激光的脉冲数目，所述聚焦透镜(5)用于调节激光光束焦点到光学薄膜(9)表面，所述CCD相机(11)用于记录激光光斑在光学薄膜(9)表面的位置。
光学元件的损伤阈值光学元件激光损伤阈值是衡量光学元件抗激光破坏能力的重要指标，但从高功率激光装置的应用角度上讲，损伤阈值并不是一个全面充分的指标。
公认的标准对损伤的定义是能被规定的损伤诊断装置所观察到，由激光引起的光学元件表面或内部特征永久性变化。
一般采用微分相称显微镜观察，十微米左右的损伤，而损伤阈值的界定是和测量方法和判断标准有关，所谓测量方法主要是激光参数和测试数据量的设定，判断标准就是什么样的情况算损伤，一般将损伤阈值定义为发生零损伤概率的最高激光能量密度。
光学元件损伤阈值的测试方法包括1-on-1，R-on-1，N-on-1和S-on-1，如图2所示。
a）1-on-1，即元件的每一个测试点上只辐照一个单脉冲；b）S-on-1，即用相同的激光能量脉冲以相同的时间间隔（激光脉冲重复频率）在元件上的同一点上辐照多次；c）N-on-1，即激光能量脉冲由小到大地增加，辐照在元件的同一点上。
在相邻的每个激光脉冲之间，可以没有一个固定的时间间隔；d）R-on-1，即用很小的等幅线性增加的激光能量以相同短时间间隔在元件的同一点上辐照多次。
其中，1-on-1和S-on-1测试方式通常被作为测试熔石英损伤阈值的测试方法，在国际标准11254中有明确的阐述。
N-on-1和R-on-1方式常被用作对熔石英进行激光预处理的激光辐照方式。
图2 四种损伤测试方法示意图1-on-1测试方法是目前最普遍采用的元件损伤阈值测试方法，国际标准11254中定义的测试基本步骤是：a）用相同能量的单脉冲，分别照射测试元件上的m个点（m不小于10），每个点只辐照一次，每个辐照点用相衬显微镜观测是否出现损伤，记下m个测试点中发生损伤的点数n，得出这个能量密度下损伤几率为n/m。
b）改变能量，同样测出该能量密度下的损伤频率。
要求测出多个能量点的损伤频率，其中包含损伤频率为零和损伤频率为100%的能量点。
c）以激光能量密度为横轴，以损伤频率为纵轴，得出损伤频率与激光能量点的分布散点图。
光学元件的损伤阈值光学元件激光损伤阈值是衡量光学元件抗激光破坏能力的重要指标，但从高功率激光装置的应用角度上讲，损伤阈值并不是一个全面充分的指标。
公认的标准对损伤的定义是能被规定的损伤诊断装置所观察到，由激光引起的光学元件表面或内部特征永久性变化。
一般采用微分相称显微镜观察，十微米左右的损伤，而损伤阈值的界定是和测量方法和判断标准有关，所谓测量方法主要是激光参数和测试数据量的设定，判断标准就是什么样的情况算损伤，一般将损伤阈值定义为发生零损伤概率的最高激光能量密度。
光学元件损伤阈值的测试方法包括1-on-1，R-on-1，N-on-1和S-on-1，如图2所示。
a）1-on-1，即元件的每一个测试点上只辐照一个单脉冲；b）S-on-1，即用相同的激光能量脉冲以相同的时间间隔（激光脉冲重复频率）在元件上的同一点上辐照多次；c）N-on-1，即激光能量脉冲由小到大地增加，辐照在元件的同一点上。
在相邻的每个激光脉冲之间，可以没有一个固定的时间间隔；d）R-on-1，即用很小的等幅线性增加的激光能量以相同短时间间隔在元件的同一点上辐照多次。
其中，1-on-1和S-on-1测试方式通常被作为测试熔石英损伤阈值的测试方法，在国际标准11254中有明确的阐述。
N-on-1和R-on-1方式常被用作对熔石英进行激光预处理的激光辐照方式。
图2 四种损伤测试方法示意图1-on-1测试方法是目前最普遍采用的元件损伤阈值测试方法，国际标准11254中定义的测试基本步骤是：a）用相同能量的单脉冲，分别照射测试元件上的m个点（m不小于10），每个点只辐照一次，每个辐照点用相衬显微镜观测是否出现损伤，记下m个测试点中发生损伤的点数n，得出这个能量密度下损伤几率为n/m。
b）改变能量，同样测出该能量密度下的损伤频率。
要求测出多个能量点的损伤频率，其中包含损伤频率为零和损伤频率为100%的能量点。
c）以激光能量密度为横轴，以损伤频率为纵轴，得出损伤频率与激光能量点的分布散点图。
