仪器偏振对空间干涉仪高对比度成像的影响

Effect of instrumental polarization on high contrast imaging of space interferometer

为实现基线运动和相位控制,空间干涉仪中会包含有大量的折轴光路,它们往往具有很强仪器偏振效应。当各干涉支路的仪器偏振不一致时,系统中出现偏振像差,它会劣化干涉仪的成像质量和干涉条纹对比度,因此对干涉仪的光学设计进行偏振分析与优化非常必要。为此,发展了考虑仪器偏振效应的综合孔径系统干涉成像模型,开发了基于偏振光线跟踪技术和矢量傅里叶变换方法的偏振条纹模拟方法。以一个三孔径菲索干涉仪为例,讨论了折轴光路几何结构和镀膜对干涉仪成像特性和消零干涉对比度的影响。结果表明,例子干涉仪的仪器偏振主要来自折轴光路中的延迟线,它由4个共入射面的45°镜构成。在可见光波段,干涉条纹的轮廓和对比度对折轴光路的几何设计非常敏感。将各干涉臂延迟线的反射面设计为相互平行能够降低偏振像差,使合成光束的成像特性基本达到衍射极限的成像质量。但在可见光波段,该方法难以实现地外行星探测所要求的高对比度。综上,仪器偏振是影响空间干涉仪探测性能的重要因素,空间干涉仪必须采用偏振优化设计以达到期望的高分辨和高对比度观测能力。

To realize the baseline motion and phrase control,many folding mirrors are used in optical trains of space stellar interferometers.The folding optical trains usually have very strong instrumental polarization.It causes polarization aberrations and thus decrease of imaging quality and fringe contrast of interferometers when the polarization properties for each arm aren′t identical to each other.So it is very necessary to analyze the polarization properties and optimize the polarization design of interferometers.A model of interferometric imaging of synthesis aperture system was built,considering the effect of instrumental polarization.The simulation method based on polarization ray tracing and the vectorial Fourier transform was developed to calculate the polarized interference fringe.An example Fizeau interferometer with three apertures was defined to implement the simulation.The result shows that the instrumental polarization of the given interferometer comes mainly from the delay line in each arm,which is composed of four diagonal mirrors.The profile and contrast of fringe are sensitive to the azimuthal angle of the incident plane of delay line.Setting the incident planes parallel to each other can decrease the polarization aberrations significantly,and making the imaging quality much closer to the ideal case.But in the visible band,this configuration can′t improve the fringe contrast to the required level for exoplanet detection.Generally,the instrumental polarization is an important factor of optical design of space interferometer.The polarization-optimized optical design is very necessary for these missions which pursue the astronomical high contrast imaging.