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偏振光的状态可以采用Stokes矢量[14]进行表达,设偏振角度分别是0°、45°、90°和−45°,则光强表示为I0°、I45°、I90°和I−45°。总光强为S0,与之对应的存在0°与90°的光强差S1、45°与−45°的光强差S3,左旋圆偏振Il与右旋圆偏振Ir的光强差S3,则有:
$$\left\{ \begin{array}{l} \overrightarrow {{S_0}} {\rm{ = }}\overrightarrow {{I_{{0^ \circ }}}} + \overrightarrow {{I_{{{90}^ \circ }}}} = \overrightarrow {{I_{{\rm{4}}{{\rm{5}}^ \circ }}}} + \overrightarrow {{I_{ - {{45}^ \circ }}}} = \overrightarrow {{I_l}} + \overrightarrow {{I_r}} \\ \overrightarrow {{S_1}} = \overrightarrow {{I_{{0^ \circ }}}} - \overrightarrow {{I_{{{90}^ \circ }}}} ,\overrightarrow {{S_2}} = \overrightarrow {{I_{{\rm{4}}{{\rm{5}}^ \circ }}}} - \overrightarrow {{I_{ - {{45}^ \circ }}}} , = \overrightarrow {{I_l}} - \overrightarrow {{I_r}} \\ \end{array} \right.$$ (1) 式中:S0、S1、S2和S3为4个Stokes分量,分别表示光强、两个相互垂直的偏振分量以及圆偏振分量[15]。
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对沃拉斯顿棱镜组1和2的分析是一致的,只是在角度上旋转了90°,为避免赘述本节只讨论沃拉斯顿棱镜组1的传输表达。当入射光经过相位调制模块时,产生相位延迟量δ1和δ2,可表达为:
$$\left\{ \begin{array}{l} {\delta _1}(\upsilon ) = 2\pi \upsilon {L_1} \\ {\delta _2}(\upsilon ) = 2\pi \upsilon {L_2} \\ \end{array} \right.$$ (2) 式中:v表示波数;L1和L2分别表示经过相位调制模块的光程差。
则对于沃拉斯顿棱镜组1(M1)和沃拉斯顿棱镜组2(M2)而言,其传递函数为:
$$\left\{ \begin{array}{l} {M_1} = \dfrac{1}{2}\left[ {\begin{array}{*{20}{c}} 1&{\cos 2\theta }&{\sin 2\theta }&0 \\ {\cos 2\theta }&{{{\cos }^2}2\theta }&{\sin 2\theta \cos 2\theta }&0 \\ {\sin 2\theta }&{\sin 2\theta \cos 2\theta }&{{{\sin }^2}2\theta }&0 \\ 0&0&0&0 \end{array}} \right] \\ {M_2} = \dfrac{1}{2}\left[ {\begin{array}{*{20}{c}} 1&{\cos 2\phi }&{\sin 2\phi }&0 \\ {\cos 2\phi }&{{{\cos }^2}2\phi }&{\sin 2\phi \cos 2\phi }&0 \\ {\sin 2\phi }&{\sin 2\phi \cos 2\phi }&{{{\sin }^2}2\phi }&0 \\ 0&0&0&0 \end{array}} \right] \\ \end{array} \right.$$ (3) 式中:θ 为线偏振光的快轴和光学系统中y 轴的夹角;ϕ 为线偏振光的快轴和光学系统中x 轴的夹角。由公式(3)可知,传递函数仅与夹角θ 有关,故通过设计合适的沃拉斯顿棱镜组夹角就能获得合适的传输函数,由此再将沃拉斯顿棱镜组2引入后,就能得到一组相关的传输函数,从而使偏振与光谱信息同时被解调。
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为了保证传感器能接收到足够的光能量,子棱镜的个数不宜设置过多,文中系统采用三个,由于单个沃拉斯顿棱镜的光谱分辨能力由静态干涉棱镜的最大光程差决定[16],故沃拉斯顿棱镜组1和沃拉斯顿棱镜组2叠加形成的光谱分辨率由两组最小光程差与两组最大光程差的范围决定,即:
$$\left\{ \begin{array}{l} \Delta (h,\alpha ) \approx 2h\left( {{n_{\rm o}} - {n_{\rm e}}} \right){\rm{tan}}\alpha \\ R = {\left( {3{\Delta _1}} \right)^{ - 1}}{\left( {3{\Delta _2}} \right)^{ - 1}}{\rm{ = }}{\left( {9\Delta } \right)^{ - 1}} \\ \end{array} \right.$$ (4) 式中:h表示两束光之间的距离(指当光从第一级沃拉斯顿棱镜出射为o光和e光时,两束光之间的距离);α表示沃拉斯顿棱镜分束面夹角;no和ne为o光与e光在棱镜中的折射率;Δ1和Δ2分别为沃拉斯顿棱镜组1和2的子棱镜的最大光程差,由于两个结构一致,故其相等表示为Δ。
Target polarization detection method of combined Wollaston prism group
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摘要: 为了提高静态偏振光谱成像系统光谱分辨率并获得更好的目标识别能力,设计了正交组合沃拉斯顿棱镜组结构,配合静态相位调制技术完成了目标的偏振光谱成像。该技术采用了多级棱镜组合的方法在不扩大原有静态干涉棱镜尺寸的条件下扩大了空间光程差变化范围,从而提升了静态光谱分辨率。通过相位调制与图像周期性匹配的方法完成了二维图像与光谱分离。仿真分析了结构尺寸与调制度对光谱分辨能力的函数关系。实验采用计算模拟验证了二维图像与光谱分离的可行性。在晴天与阴天两种不同状态下,测试了30.0 cm铝板目标的信号对比度,采用偏振光谱成像并完成数据提取的测试结果均值为53.4%和49.3%,而基于强度图像的测试结果均值为24.4%和14.1%。文中设计可以改善目标识别效果,提高光谱分辨精度。Abstract: In order to improve the spectral resolution of the static polarization spectrum imaging system and obtain better target recognition ability, the orthogonal combined Wollaston prism group structure was designed, and the static phase modulation technology was used to complete the polarization spectrum imaging of the target. This technology used a multi-stage prism combination method to expand the range of spatial optical path difference without expanding the size of the original static interference prism, thereby improving the static spectral resolution. The two-dimensional image and the spectrum were separated by the method of phase modulation and image periodic matching. The function relationship between the structure size and the modulation degree on the spectral resolution was analyzed by simulation. The feasibility of the separation of the two-dimensional image and the spectrum were verified by computational simulation. The signal contrast of the 30.0 cm aluminum plate target was tested under two different conditions: sunny and cloudy. The polarization spectrum imaging was used and data extraction were 53.4% and 49.3%, while the mean of test result based on intensity image was 24.4 % and 14.1%. This design can improve the target recognition effect and increase the spectral resolution accuracy.
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Key words:
- spectral imaging /
- polarization spectrum /
- Wollaston prism /
- contrast
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