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根据研制的目标要求,在设计中采用了以下的方法:在两块光学玻璃之间加一层金属网丝(铜丝),然后使用胶粘的工艺,将两片光学玻璃紧紧粘合在一起,制备成金属网丝夹层的屏蔽光窗。这个原理还是利用了截止波导管的原理,即金属丝网的每个网眼均可看成一个个小的波导管,电磁波频率高于波导管的截止频率时,可自由通过;电磁波频率低于波导管截止频率时,将会被截止;设金属丝网为矩形,网眼宽度为W,则截止频率f0为[11]:
$$ {{{f}}_0} = \frac{{15}}{w}(\rm{GHz}) $$ (1) 但是,这种夹层金属网丝会对光学探测器的成像质量有一定的影响,影响程度的大小与夹层金属网丝的宽度和目数有关。以屏蔽性能要求为不小于60 dB夹层金属网丝光学玻璃计算:采用等效电路的方法分析金属网栅的特性。设 g 为金属网栅的线周期,2a为金属网栅的线宽,厚度为t的金属网栅,等效为简单的电感电容并联振荡电路,并且其导纳是电感性的。
在自由空间垂直入射时,当t ≤ 2a ≤ g,电感性网栅具有归一化导纳:
$$ {\rm{y}} = {f_0}\left[\ln \left(\sin \frac{\text{π}} {g}\right)\right]{\left(\frac{f}{{{f_0}}} - \frac{{{f_0}}}{f}\right)^{ - 1}} $$ (2) 式中:f=g/λ,f为归一化谐振波长,λ为波长,当λ≥g时,归一化导纳近似等于:
$$ {\rm{y}} = - \frac{g}{\lambda }{\left[\ln\left(\sin \frac{{{\text{π}} a}}{g}\right)\right]^2} $$ (3) 对于2.4 GHz的波段,y2≤1,垂直入射的金属网栅的透过率T近似为:
$${{T}}(0,0) \approx 4\frac{{{g^2}}}{{{\lambda ^2}}}{\left[\ln\left(\sin \frac{{{\text{π}} a}}{g}\right)\right]^2}$$ (4) 则定义的电磁屏蔽效率为:
$$ {{s}} = - 10\lg (T) $$ (5) 计算取λ=0.125 m,用光学点扩展函数(PSF)可推导出中心级的透过率:
$$ {T_0}(0,0) \approx k\frac{{{{(g - 2a)}^4}}}{{{g^4}}} $$ (6) 从上述分析可以看出,k与垂直入射的玻璃透过率有关,由于所用光学玻璃材料厚度不大,光学玻璃本身透过率影响不大。如图1所示为线周期不同g为50 μm时对应的透过率和屏蔽效能,此时取金属线宽为9 μm时,可达到60 dB的屏蔽效能,则透过率下降到了0.4左右。
图 1 不同金属丝网线宽度对应的屏蔽效能和透过率。(a)屏蔽效能;(b)透过率
Figure 1. Shielding property and transmittance of different width of metal line. (a)Shielding property; (b)transmittance
从理论计算分析可得,两块光学玻璃之间的金属丝宽度设计为9 μm,可以满足电磁屏蔽性能不小于60 dB的指标要求,此时透过率计算为0.4,需要进一步验证该透过率下光窗对探测图像目标的影响。
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对已经制备的85目、100目和165目三种不同目数的屏蔽光窗,以及无屏蔽金属丝夹层的普通光学玻璃和镀有ITO膜的光学玻璃五种成品,开展电磁屏蔽性能和光学透过率的测试比较。采用通过金属屏蔽壳体加载不同的光窗进行测试。其中,输入输出信号采用光缆通信,避免了其他干扰进入金属壳体。
选取测试频率为1 GHz±100 MHz和2.4 GHz±100 MHz两个频段的范围内,测试结果如表1所示。对制备的上述五光学元件光谱特性测试,测得光谱特性曲线如图2所示。
表 1 几种光窗屏蔽性能测试结果
Table 1. Test results of shielding property of different optical windows
No. Type of optical window Shielding property
/dB(2.4 GHz±
100 MHz)Shielding property
/dB(1 GHz±
100 MHz)1 165 mesh 65 48.5 2 100 mesh 51 59 3 85 mesh 60 59.5 4 ITO coating 32 32 5 General window 2 0 在2.4 G±100 MHz和1 GHz±100 MHz的频段内,从几种光窗屏蔽性能测试和结果可以看出,未做屏蔽处理的普通光窗,基本没有电磁屏蔽性能,但在500~700 nm波长范围内,透过率较好,达到了90%;ITO膜光窗的屏蔽性能小于金属丝网夹层的屏蔽光窗的屏蔽性能,且随着金属丝网目数的增多屏蔽性能增大,但透过率却逐渐降低,加载到165目的屏蔽光窗时,屏蔽性能为65 dB,满足指标要求,但在可见光波段内,透过率约为40%,成像较暗,为了提高光通量,一方面,可以在屏蔽光窗表面镀涂高透过率光学膜层,透过率可以提升10%~20%;另一方面,用于图像探测跟踪的系统,可以适当地在软件处理上采用暗目标算法进行补偿等。开展了对运动目标的探测试验,目标图像如图3所示。经过滤波等算法处理后的探测图像如图4所示。
图 3 3.4 km距离无人机纯天空背景探测目标图像(视场:8°×5.3°)
Figure 3. Target image of UAV with clear sky background at 3.4 km (FOV: 8°×5.3°)
根据实测数据推算探测距离, 图4中加屏蔽窗口后的目标的局部信噪比PSNR为3.8左右,探测距离为3.4 km左右。
$$PSNR = (I - mean(s))mean(im)/std(s)$$ 式中:PSNR为局部信噪比;I为目标像素的灰度均值;mean(s)为局部背景区域的均值;mean(im)为图像区域的均值;std(s)为局部背景区域的均方差。局部背景是以目标质心为中心,4倍目标尺寸的区域中去除目标像素后剩下的像素。探测距离主要影响的是大气传输效率,且直接为正比关系,根据Modtran软件计算,3.4 km距离(20 km能见度)可见光平均透过率为0.546 2,4 km计算的平均大气透过率为0.4924。利用可见光探测光照度与F数关系,此时推算出的信噪比大于3,能够较好的与背景进行区分,即在4 km远距离,图4中的目标仍能够被探测到。
从电磁屏蔽性能测试和探测试验研究发现,实际研制的电磁屏蔽光窗与设计理论分析结果基本一致,在2.4 GHz±100 MHz频带内电磁屏蔽效能达到了65 dB,且在能见度20 km条件下,在4 km处可探测到运动目标。
Design and application of electromagnetic shielding of optical detection system
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摘要: 光学探测系统在高功率微波系统运行造成的强辐射、强电磁干扰环境下工作,HPM产生的强电磁脉冲会通过后门耦合的方式由探测器前端光学镜头进入内部的电路系统造成光学探测系统瞬间黑屏、图像抖动、器件毁坏等现象,通过采用光学玻璃金属丝夹层的方法研制光窗,在2.4 GHz±100 MHz频段内电磁屏蔽性能达到了65 dB,同时满足探测目标的光窗透过率的要求。经过应用后表明:加载了这种电磁屏蔽光窗的光学探测系统在HPM工作时图像稳定,未受到干扰。Abstract: IR optical detector system can work at strong radiation and electromagnetic interference caused by the operation of high power microwave(HPM) system. The strong electromagnetic pulse generated by HPM will enter the inner circuit system through the front-end optical lens of the detector with the back-door coupling, resulting in the phenomenon of instant black screen, image jitter, electronic device destruction and so on. The optical window was developed by using embedding metal lines into optical materials. These metal lines were made to be a net and to be implanted in the middle of optical glass. The performance of electrical shielding was up to 65 dB in 2.4 GHz±100 MHz frequency band. It can meet the requirement of the optical window transmittance of the detection target. The application shows that the image of the optical detection system loaded with the electromagnetic shielding optical window is stable and undisturbed when the HPM is working.
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Key words:
- optical detector system /
- HPM /
- electromagnetic shielding /
- IR optical window
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表 1 几种光窗屏蔽性能测试结果
Table 1. Test results of shielding property of different optical windows
No. Type of optical window Shielding property
/dB(2.4 GHz±
100 MHz)Shielding property
/dB(1 GHz±
100 MHz)1 165 mesh 65 48.5 2 100 mesh 51 59 3 85 mesh 60 59.5 4 ITO coating 32 32 5 General window 2 0 -
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