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强光防护面板是光斑分布探测器的非常关键的部件,直接关系到强光参数测量中探测器的安全性和可靠性。为避免防护面板在激光辐照升温后强度下降造光学取样孔变形,进而影响激光参数测量精度,选用耐热铝合金材料(在300 ℃以下使用强度无变化)。防护面板表面工艺需进行特殊处理,在尽量降低光热耦合效率的前提下,其设计还需考虑反射光对实验光路上其他设备和激光光源的影响。防护面板表面采用喷砂镀高反射膜层(短时间使用温度需小于320 ℃)的处理工艺,实测其反射系数大于95%。防护面板为三维轴对称圆形结构设计,在无其他热源的情况下,强光辐照时温度场分布T可按照经典Fourier热传导方程进行分析[11]:
$$ \rho {c_p}\frac{{\partial T}}{{\partial t}} - k\left[ {\frac{{{\partial ^2}T}}{{\partial {r^2}}} + \frac{1}{r}\frac{{\partial T}}{{\partial r}} + \frac{{{\partial ^2}T}}{{\partial {z^2}}}} \right] = (1 - R){I_0}\beta {\rm{e}^{ -\beta \textit{z}}} $$ (1) 式中:
$\; \rho $ 、$ {c_p} $ 、$ k $ 分别为防护面板材料的密度、比热容和热导率;$ R $ 为防护面板材料表面对特定波长激光反射系数;$\; \beta $ 为防护面板材料激光能量耦合系数;I0为入射激光强度。在不考虑传导散热和对流换热的情况下,边界条件为:
$$ \overline q = \sigma \varepsilon ({T^4} - T_b^4) $$ (2) 公式(2)为热辐射边界条件,
$ \varepsilon $ 为材料表面发射效率,Tb为环境温度,$ \sigma $ 为Boltzmann常数。$$ T = T(r,z,t) $$ (3) 公式(3)为第一类边界条件,是强制边界条件,给定边界上的材料初始温度。
防护面板的强光防护能力以膜层和材料使用强度温度上限为依据。数值模拟了在直径10 cm、功率密度500 W/cm2均匀光斑辐照下,防护面板整体温度场分布和温升极大值。防护面板温度分布为圆周中心对称分布,图2给出了其在不同激光辐照时长下温度场数值模拟四分之一截图。
通过对激光辐照下防护面板温度场分布的数值模拟的结果可以看出,在给定的激光光斑尺寸和功率密度下,在60 s激光辐照时刻,防护面板最高温度为292 ℃,满足膜层和防护面板材料正常使用要求。
Detector array for measuring spot distribution of high power continuous and pulsed laser
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摘要: 通过测量发射到远场的激光功率密度时空分布给出所需要的到靶总功率、光束质量、桶中功率、功率时间曲线等关键指标参数,是目前准确评价激光系统性能的重要技术手段。介绍了一种基于光电探测器阵列实现近红外脉冲激光功率密度时空分布的测量方法,可以实现900~1700 nm波长、动态范围大于2000倍的激光光斑参数测量。该阵列探测器具有测量面积大、单元一致性好、测量精度高等特点,并可同时实现脉冲和连续激光参数测试要求。给出了阵列探测器的总功率测量结果,测量值与激光器输出功率偏差在5%以内,且激光光斑分布测量结果准确可靠。该阵列探测器已在多套激光系统的参数测试中得到成功应用,可以作为响应波段内的脉冲/连续激光光斑参数测试一种有效技术方案。Abstract: It is an important technical means to accurately evaluate the performance of laser system by measuring the spatio-temporal distribution of laser power density emitted to the far field and obtaining the key index parameters such as total power on the target, beam quality, power in the bucket and power-time curve. A method for measuring the spatio-temporal distribution of near infrared pulse laser power density based on photodetector array is introduced. The detector array can be used to measure laser spot parameters in the wavelength range of 900 nm to 1700 nm with dynamic range more than 2000 times. The detector array has the characteristics of good unit consistency and high measurement accuracy, large measurement area, and can meets the needs of pulse and continuous laser parameter measurement. The experimental results for performance of the detector array indicate that the deviation between the measured value and the laser output power is less than 5%, and the measurement results of laser spot distribution are accurate and reliable. The detector array has been successfully applied in the parameter test of several laser system, can be used as an effective technical scheme for the measurement of pulse and continuous laser spot parameters in response band.
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