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实验中通过调节1064 nm脉冲激光器电压与衰减片对损伤激光能量进行控制,使损伤激光能量密度为41.355 mJ/cm2,通过电脑端输出的CCD接收的图像观察CCD成像情况,以CCD成像情况判断CCD损伤状态,CCD点损伤、线损伤及全靶面损伤时成像图片如图1所示。
做了多组实验,对CCD不同损伤状态下光学成像系统猫眼回波功率与偏振度的数据进行记录,绘制变化曲线,如图2所示。
图 2 CCD不同损伤状态下猫眼回波功率与偏振度变化曲线
Figure 2. Change curve of cat eye echo power and degree of polarization under different CCD damage states
由图2(a)、(b)可以看出四组实验对应CCD损伤状态变化规律相同,但相对应的猫眼回波功率与偏振度变化趋势有所差异,呈现不同的变化趋势,因此,猫眼回波功率、偏振度的变化趋势与CCD未损伤至全靶面损伤状态变化规律没有良好的相关性;由图2分析可得CCD受脉冲激光辐照未损伤至全靶面损伤进程中不能以猫眼回波功率与偏振度变化规律作为损伤状态的判断依据。
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光线照射漫反射目标时反射光较弱,然而当光线照射目标发生“猫眼”效应时,反射光会按照原路返回[7]。设激光器发射功率为P0
, 经过距离为z的大气传输后照射目标,只考虑大气衰减对激光传输的影响,“猫眼”回波由一个探测器接收,探测器所接收的光功率可表示为[8]: $$ P = \frac{16{A}_{m}{{{{{A}_{d}\tau }_{m}}^{2}{{\tau }_{a}}^{2}P}_{0}\tau }_{d}\rho }{{\pi }^{2}{{\theta }_{t}}^{2}{{\theta }_{r}}^{2}{{\textit{z}}}^{4}} $$ (1) 式中:
${A}_{m} $ 为目标的接收面积;$ {A}_{d} $ 为接收系统的孔径面积;$ {\tau }_{m} $ 为目标系统的透过率;$ {\tau }_{a} $ 为空气透过率;$ {\tau }_{d} $ 为探测系统的透过率;$\; \rho $ 为光敏面反射率;$ {\theta }_{t} $ 为激光发散角;$ {\theta }_{r} $ 为反射光发散角;z为激光器与猫眼目标间的距离。结合文中光学成像系统猫眼回波探测实验系统,实验过程中猫眼目标受激光辐照后损伤处材料与材料表面形貌发生变化,材料不同,其表面反射率不同,材料表面形貌发生变化,其粗糙度改变,由参考文献[9]可知材料表面粗糙度(目标表面粗糙度参数为面均方根高度,均方根(root mean square, RMS)高度与粗糙度成正比关系)越大,材料对入射光的吸收率越高,其反射率越低[10],而其他参数不变,所以由公式(1)可知文中实验中探测目标的猫眼回波功率P仅与光敏面的反射率$\; \rho $ 有关,且二者之间成正比关系。为进一步研究CCD未损伤至全靶面损伤损伤进程中猫眼回波功率变化趋势不同的原因,使用Olympus公司生产的型号为DSX110光学数码显微镜,以600倍放大率采集各损伤状态下损伤处显微图像,CCD像元大小为10 μm×6 μm,如图3(a)~(c)所示,CCD在产生点损伤、线损伤与全靶面损伤时损伤处微透镜层颜色发生改变,表面形貌无明显变化,使用ZYGO公司型号为ZGPH3X02-1Z-03V,垂直精度为0.15 nm的白光干涉仪采集两组CCD产生点损伤至全靶面损伤进程中CCD损伤处的表面轮廓图,其中一组图像如图3(d)~(f)所示,其损伤处材料表面均方根高度如表1所示。由图3(a)~(c)可知:CCD产生点损伤至全靶面损伤进程中损伤至微透镜层,所以损伤处材料不变,由表1可得两组实验CCD损伤处粗糙度变化趋势不同,由此可得CCD在未损伤至全靶面损伤的损伤进程中,猫眼回波功率变化趋势是由微透镜粗糙度变化无固定规律造成的。
图 3 不同损伤状态损伤处显微图像(a)~(c)与轮廓图(d)~(e)
Figure 3. Microscopic images (a)-(c) and contour images (d)-(e) of damage sites under different damage states
表 1 材料表面均方根高度测量结果
Table 1. RMS height measurement results of material surface
RMS height Sq/μm Point damage Line damage Full target damage The first group 4.132 3.919 4.006 The second group 3.895 4.036 3.940 -
材料目标表面散射偏振特性与入射角、探测角、折射率、表面粗糙度等参数有关[9]。实验系统一样,影响猫眼目标猫眼回波偏振度的因素为材料折射率与表面粗糙度。基于微面元理论,结合Jones矩阵与Mueller矩阵相互转换,可得偏振双向反射分布函数的表达式为:
$$ \begin{split} {f}_{j,k}\left({\theta }_{i},{\phi }_{i},{\theta }_{r},{\phi }_{r},\lambda \right)=&\frac{1}{2\pi }\frac{1}{4{\sigma }^{2}}\frac{1}{{{\rm{cos}}}^{4}\theta }\frac{{\rm{exp}}(-({{\rm{tan}}}^{2}\theta /2{\sigma }^{2}\left)\right)}{{\rm{cos}}\left({\theta }_{i}\right){\rm{cos}}\left({\theta }_{r}\right)}\times\\ &{M}_{j,k}({\theta }_{i},{{\phi }_{i},\theta }_{r},{\phi }_{r}) \end{split}$$ (2) 式中:
$ \sigma $ 为目标表面的粗糙度参数;由半球定向反射理论可得,光波入射到猫眼目标表面后按照原路返回的光波积分运算为:$$ {\rho }_{HDR}^{\tau }\left({\theta }_{i}\right)={\int }_{0}^{2\pi }{\int }_{-\tau /2}^{\tau /2}f\left({\theta }_{i},{\theta }_{r},\Delta \varphi \right){{\rm{cos}}}\left({\theta }_{r}\right){{\rm{sin}}}\left({\theta }_{r}\right){\rm{d}}\left({\theta }_{r}\right){\rm{d}}\left(\Delta \varphi \right) $$ (3) 以Mueller矩阵建立入射光Stokes向量与目标表面散射光Stokes向量之间的数学关系,当线偏振光入射目标表面时,线偏振光在目标表面散射回波的Stokes向量为:
$$ {S}^{r}={\rho }_{HDR}^{\tau }\cdot {S}^{i} $$ (4) 式中:
$ {S}^{i} $ 为线偏振光的Stokes向量;因此得到线偏振光入射目标表面其回波散射的偏振度表达式为:$$ DOP=\frac{\sqrt{\left({\left({S}_{1}^{r}\right)}^{2}+{\left({S}_{2}^{r}\right)}^{2}+{\left({S}_{3}^{r}\right)}^{2}\right)}}{{S}_{0}^{r}} $$ (5) 使用matlab仿真可得不同材料猫眼目标回波偏振度与目标表面粗糙度的关系曲线如图4所示,猫眼目标材料表面粗糙度越大,猫眼回波偏振度越小,且相同粗糙度时,猫眼回波偏振度由高到底为Al、树脂(Resin)、Si。
图 4 不同材料“猫眼”目标回波散射偏振度与目标表面粗糙度的关系曲线
Figure 4. Relation curves between the polarization degree of echo scattering and the surface roughness of target for cat’s eye of different materials
由上文可知CCD损伤进程中损伤处材料不变,材料表面粗糙度变化无固定趋势,由图4可得相同材料其表面粗糙度与猫眼回波偏振度成反比,所以CCD在未损伤、点损伤、线损伤与全靶面损伤损伤进程中猫眼回波偏振度变化趋势不同是CCD损伤进程中损伤处微透镜粗糙度变化无固定规律造成的。
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实际光电对抗中多采用高于目标损伤阈值的高能量重频脉冲激光对目标进行多脉冲损伤,结合实际应用,实验中调节损伤激光能量使CCD受到首个激光脉冲辐照后达到完全损伤的效果,对CCD不同位置依次进行0~8个脉冲辐照损伤,不断加深CCD器件的损伤状态,分别记录光学成像系统CCD探测器件受0~8个脉冲辐照过程中光学成像系统猫眼回波功率与偏振度数据,进行多组实验,分析实验数据,如图5所示:CCD芯片受激光辐照0~8个脉冲损伤进程中,光学成像系统猫眼回波功率与偏振度呈现先增大后减小,再增大最后不断减小的变化规律。
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采集CCD受激光辐射0~8个脉冲损伤进程中部分损伤处表面形貌图像,如图6所示,采集CCD损伤处表面轮廓图像,整理测量所得材料表面均方根高度数据,如表2所示。
表 2 材料表面均方根高度测量结果
Table 2. Root mean square height measurement results of material surface
Item Pulse number 0 1 2 3 4 5 6 7 8 RMS height Sq/μm 4.135 0.012 0.007 0.491 0.440 0.370 0.352 0.516 0.650 光学成像系统猫眼回波功率与偏振度受CCD损伤处材料与表面粗糙度影响。在激光辐照0~8个脉冲损伤进程中,如图6(b)所示,CCD首先被损伤至遮光铝膜层,其损伤处显露出被熔融的微透镜所覆盖的金属网状结构,随激光辐照脉冲数增加,熔融的微透镜逐渐消失,裸露出遮光铝膜,且如表2所示,材料表面粗糙度减小,因此,光学成像系统猫眼回波功率、偏振度增加;当激光辐照三个脉冲时,如图6(c)所示,损伤处少部分单一网格处遮光铝膜呈熔融态,材料表面粗糙度显著增加,如表2所示,所以猫眼回波功率、偏振度有所减小;激光辐照脉冲个数继续增加,如图6(d)所示,损伤处熔融态金属铝范围不断扩大,且其表面均方根高度变化不断减小,如表2所示,其粗糙度不断减小,猫眼回波功率、偏振度不断增大;当激光辐照六个脉冲时,如图6(e)所示,CCD损伤处中心位置出现黑洞,此时CCD损伤至硅基底,材料硅对671 nm激光反射率仅为0.3,远小于金属铝,且材料表面均方根高度不断增大,粗糙度不断增大,所以猫眼回波功率、偏振度不断减小。
Cat eye echo characteristics of optical imaging system in CCD damage process
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摘要: 建立了CCD损伤进程中光学成像系统猫眼回波探测系统,记录了CCD损伤进程中光学成像系统猫眼回波功率与猫眼回波偏振度数据并绘制了变化曲线,分析了猫眼回波特性的变化机理、CCD损伤状态与猫眼回波功率、偏振度变化之间的联系,研究得出:光学成像系统CCD探测器件受脉冲激光辐照产生点损伤、线损伤至全靶面损伤进程中,猫眼回波功率、偏振度变化与CCD损伤状态变化没有良好的相关性,提高损伤激光能量使CCD受到首个脉冲辐照时完全损伤,在0~8个脉冲损伤进程中,猫眼回波功率与偏振度先增大后减小,再增大最后不断减小,可以此规律对CCD是否完全损伤进行判断。Abstract: The cat eye echo detection system of optical imaging system in the process of CCD damage was established. The data of cat eye echo power and degree of polarization of optical imaging system during CCD damage process were recorded and the curve of change was drawn. The change mechanism of cat eye echo characteristics, the relationship between CCD damage state and the change of cat eye echo power and degree of polarization were analyzed. The results show that there is no good correlation between the change of cat eye echo power and degree of polarization and the change of CCD damage state during the process of point damage, line damage and full target damage of CCD detector in optical imaging system by pulsed laser irradiation. The CCD is completely damaged when the first pulse is irradiated by increasing the damage laser energy. In the 0-8 pulse damage process, the cat eye echo power and degree of polarization increase first, then decrease, then increase and finally decrease, which can be used to judge whether CCD is completely damaged.
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Key words:
- cat eye echo /
- power /
- degree of polarization /
- roughness
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表 1 材料表面均方根高度测量结果
Table 1. RMS height measurement results of material surface
RMS height Sq/μm Point damage Line damage Full target damage The first group 4.132 3.919 4.006 The second group 3.895 4.036 3.940 表 2 材料表面均方根高度测量结果
Table 2. Root mean square height measurement results of material surface
Item Pulse number 0 1 2 3 4 5 6 7 8 RMS height Sq/μm 4.135 0.012 0.007 0.491 0.440 0.370 0.352 0.516 0.650 -
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