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我国在天文紫外探测领域起步较晚,目前仅开展了紫外成像仪(CE-3、CE-7、FY-3E、SATech-01、YW-01等)的在轨应用、紫外光谱仪的研制等工作,在紫外偏振领域仍处于关键部组件研制阶段,目前暂无我国紫外天文偏振载荷的研制以及在轨应用信息。
自20世纪以来,欧美等国在空间紫外天文偏振探测领域进行了大量研究,其中NASA在规划和研制众多紫外偏振探测载荷方面扮演了主导角色。表1总结了国际上典型的空间天文紫外偏振载荷。
以下将针对不同的探测任务(太阳、行星、星际物质等)分别介绍其中一些典型的紫外偏振载荷。
表 1 典型空间天文紫外偏振载荷
Table 1. Typical space astronomy UV polarization loads
Mission Main
countryLaunch
timeInstrument Target Interkosmos 16 Sweden 1976 UVSP VUV linear polarization in Solar transition zone, etc.[8, 35] SMM USA 1980 UVSP Solar flares, solar active regions, solar corona, etc.[36–37] SUMI USA 2012 SUMI Chromosphere, solar transition region magnetic field, etc.[19] CLASP Japan 2015 CLASP Upper solar chromosphere, transition region, etc.[13, 15] CLASP-2 Japan 2019 CLASP Upper solar chromosphere, transition region, etc.[38] CLASP-2.1 Japan 2021 CLASP Upper solar chromosphere, transition region, etc.[38] Pioneer Venus Orbiter USA 1978 OCPP Venus cloud layers, physical characteristics of aerosol particles in the atmosphere, and vertical distribution of aerosols, etc.[39] Hubble telescope USA 1990 FOS Active galaxies, quasars, and planetary nebulae, etc.[40–43] ASTRO-1 USA 1990 WUPPE Interstellar dust, hot stars, material in the solar system, active galaxies, etc.[44] ASTRO-2 USA 1995 WUPPE Interstellar dust, hot stars, material in the solar system, active galaxies, etc.[44] WISP USA 1997 WISP Diffuse nebula, dust nebula, Crab Nebula , etc.[1, 45] CUPID USA Unknown CUPID Cosmic background , etc.[1] FUSP USA Unknown FUSP Hot star systems, circumstellar disks , etc.[1] SUNRISE III Germany Pending SUSI Solar atmosphere, etc.[46] ARAGO Europe Under planning ARAGO Stellar-planetary interactions, etc.[47–48] LUVOIR USA Under planning POLLUX Circumstellar disk, interstellar magnetic fields, etc.[49–52] SolmeX Europe Under planning EIP; SUSP;
ChroME; CUSPThe structure of the solar outer atmosphere magnetic field, magnetic field variations, and magnetic field-derived effects, etc.[53] Polstar USA Under planning Polstar Circumstellar disk, interstellar magnetic fields, etc.[54-55] -
1) Interkosmos 16任务
Interkosmos 16是Interkosmos DS-U3-IK系列的子任务之一,该项目由社会主义国家国际合作组织Interkosmos发起,旨在测量太阳辐射及其对大气的影响。Interkosmos 16搭载了一台由瑞典隆德大学天文学研究所研制的紫外偏振载荷(UVSP),用于对太阳过渡区中的温度梯度和磁场进行测量[35]。该项目于1976年发射升空,在轨运行了4个月,成功获取了少量太阳紫外偏振数据[8]。
2)太阳极大期任务
太阳极大期任务(SMM)于1980年发射,恰逢当时太阳黑子周期的活动高峰期。其搭载的紫外光谱偏振探测载荷(UVSP)由马里兰州格林贝尔特的戈达德太空飞行中心和阿拉巴马州亨茨维尔的马歇尔太空飞行中心开发[36]。该任务的科学目标旨在研究紫外线辐射,尤其是来自活动区域、耀斑和日冕的辐射。UVSP除了具备基本仪器的高分辨率光谱学能力外,还能够测量全偏振信息,有助于研究太阳磁场在色球层、过渡区和日冕中的分布;以及由共振散射和/或冲击偏振产生的线偏振[37]。然而,受地磁风暴影响,该任务于1989年被提前终止。
3)太阳紫外线磁力仪调查
太阳紫外线磁力仪调查(SUMI)是一项由NASA马歇尔飞行中心主导的探空火箭任务,于2012年7月成功发射。SUMI是继SMM-UVSP后又一台用于研究太阳过渡区紫外偏振信号的天基载荷。该任务的主要目标是通过观测太阳辐射中Mg II和C VI辐射线,获取太阳磁场强度和方向,从而建立该区域的三维磁图[19]。SUMI作为一次试飞任务,其主要目的是确保仪器的正常工作,并评估可能的改进措施,为未来的探空项目提供基础。
4) CLASP系列
CLASP系列是由日本国家天文台主导的一项国际合作探测火箭实验,共同研制参与的国家包括美国NASA马歇尔太空飞行中心、法国天文研究所和西班牙天文研究所。截至目前,CLASP系列实验已经经历了3个阶段:2015年发射的CLASP、2019年发射的CLASP2,以及2021年发射的CLASP2.1。CLASP主要针对太阳色球上层和过渡区的Ly-α谱线进行探测;CLASP2在对CLASP任务中存在的问题进行改进的基础上,开展了对太阳色球上层和过渡区Mg II谱线的全Stokes参数探测研究[38];CLASP2.1在探测系统上与CLASP2相同,其主要任务是获取完整的色球层磁场的三维图。
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1)先锋金星轨道器
先锋金星轨道器,也被称为先锋金星1号或先锋12号,是NASA进行的一项前往金星的任务,是先锋金星项目的一部分。该航天器于1978年被送入围绕金星的椭圆形轨道。 先锋号金星轨道器携带了17项实验,其中云层光偏振仪(OCPP)由NASA和休斯飞行器公司共同研制[39]。OCPP在近紫外、可见光和近红外区域有4个光谱带,用于对金星大气进行偏振测量和辐射测绘,以获取金星详细的大气组成与结构信息[9]。
2)哈勃望远镜
暗天体摄谱仪(FOS)由加利福尼亚大学圣地亚哥分校以及马丁•玛丽埃塔公司(MMC)联合研制而成,是哈勃太空望远镜(HST)上4个原始观测载荷之一[40-41]。FOS的设计目的是为了对暗的、通常是非常遥远的天体的物理条件进行光谱分析,谱段从近紫外延伸到近红外。FOS在轨期间主要执行以下科学任务,包括对活跃星系(Active galaxies)、类星体(Quasars)、星系化学丰度分析、行星状星云等宇宙物质的研究[12, 42-43]。
3) ASTRO系列
威斯康辛紫外光偏振仪实验(WUPPE)是目前探索偏振和紫外光谱光度测定能力最全面的载荷之一,20世纪80年代由美国威斯康辛大学空间天文实验室设计和建造。WUPPE于1990年、1995年分别搭载ASTRO-1哥伦比亚号和ASTRO-2奋进号航天飞机入轨探测[44]。在ASTRO-1任务和ASTRO-2任务期间,WUPPE获得了各种天体的光谱偏振数据,包括星际介质的遥远恒星、热恒星、具有周围物质的恒星、相互作用的双星、新星、太阳系天体和活动星系等[18]。
4)宽视场成像测偏仪
宽视场成像测偏仪(WISP)是一项NASA资助项目,由美国威斯康星-麦迪逊分校空间天文实验室主导的亚轨道火箭有效载荷望远镜,其目的是为了获得第一幅紫外光下的天文广域偏振图像而建造的[1]。该项任务共发射4次,先后对昴宿星团、大麦哲伦星云、M81/M82等目标进行了偏振观测,获取了大量有关反射星云、漫射星系光、系外行星的观测数据[45]。
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1)日出三号(SUNRISE III)
SUNRISE是一项由德国马克斯普朗克研究所牵头的探空气球太阳观测站任务,该项任务已于2009、2013年先后发射两次,计划于2023年发射SUNRISE III。该气球搭载紫外光谱偏振载荷SUSI,将探索太阳300~410 nm近紫外辐射,为研究太阳大气各层之间的耦合提供独特的机会[46]。
2) ARAGO
ARAGO是欧空局M7任务候选项目,其探测任务将包括对数千颗恒星的大型调查和对60颗精心挑选的恒星的详细3D绘图。ARAGO配备紫外、光学偏振模块,将实现119~888 nm波段范围内的光谱偏振探测功能[47–48]。该项目已于2022年通过了欧空局第一阶段项目筛选,计划于2025年进行最终确认。若通过,Arago将在2037年发射。
3)大型紫外/光学/红外巡天望远镜
大型紫外/光学/红外巡天望远镜(LUVOIR)是NASA在2016年征集到的未来新一代太空天文台的项目之一[49–50]。该天文台计划在21世纪30年代中期发射,并将驻留在日地拉格朗日L2点,其中,POLLUX紫外光谱偏振载荷由欧洲科研机构提供,用于研究星周盘、星际磁场等现象中存在的极化、去极化现象[51–52]。
4) SolmeX
SolmeX是ESA 一项拟议的面向太阳上层大气磁场探测的综合探测卫星项目,其科学目标主要针对外大气磁场结构、磁场变化以及磁场衍生效应进行研究。SolmeX卫星计划搭载五台科学载荷,其中具备紫外偏振探测能力的载荷包括日冕紫外光谱偏振仪(CUSP)、极紫外成像偏振仪(EIP)和扫描式紫外光谱偏振仪(SUSP)[53]。
5) MIDEX-Polstar
Polstar是NASA一项拟议的中级探险家(MIDEX)空间探测任务,由威斯康辛麦迪逊大学、NASA戈达德太空飞行中心等研究机构提出,拟使用高分辨率光谱偏振测量技术研究恒星和行星的磁场[54]。该任务旨在为天体磁场结构、动力学和演变提供新的发现,将对理解恒星和行星的形成、空间天气和系外行星的可居住性等科学研究起到至关重要的作用[55]。
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基于上述内容,表2对目前已有和规划中的天文紫外偏振载荷性能指标进行了归纳总结。清晰可见,当前紫外偏振探测主要以偏振+光谱探测相结合为主,单一紫外偏振信息已无法满足天文观测的需求。随着观测需求的不断提高,需要更高精度的光谱分辨率和偏振测量精度,因此对于紫外偏振器件也提出了更高的要求。此外,由于宇宙中的紫外辐射信号主要集中在极远紫外波段,辐射强度相较于可见、红外波段较弱,因此对系统整体透过率和探测效率的要求也极为高。基于上述分析,可以提炼出3个关键技术方面的应用于紫外偏振探测载荷的要点,下面将对其进行详细介绍。
表 2 典型空间天文紫外偏振载荷指标参数
Table 2. Parameters of typical space astronomy UV polarization load
Parameter instrument Instrument type Wavelength/nm Spectral resolution/nm Stokes parameter Polarimetry precision Interkosmos 16 Spectro-polarimeter 120-150 3.9 I Q U 10−2 SMM-UVSP Spectro-polarimeter 115-360 0.002-0.004 I V 10−2 SUMI Spectro-polarimeter C IV(155)
Mg II(280)0.05@ C IV
0.08@ Mg III Q U V 10−3 CLASP Spectro-polarimeter Ly-α(121.6) 0.01 I Q U 10−3 CLASP2/2.1 Spectro-polarimeter Mg II(280) 0.01 I Q U V 10−3 Pioneer Venus Orbiter-OCPP Photo-polarimeter 270, 365 ~18 I Q U 10−3 HST-FOS Spectro-polarimeter 115-850 >0.25 I Q U V 10−2 ASTRO1/2-WUPPE Spectro-polarimeter 135-330 0.6 I Q U V 10−3 WISP Photo-polarimeter 135-260 40 I Q U - CUPID Photo-polarimeter 118-161 20 I Q U 10−2 FUSP Spectro-polarimeter 105-150 0.07 I Q U 10−3 SUNRISE III-SUSI Spectro-polarimeter 300-410 0.001 I Q U V 10−3 ARAGO Spectro-polarimeter 119-888 >0.009 I Q U V - LUVOIR-POLLUX Spectro-polarimeter 90-400 0.002 I Q U V 10−4 SolmeX-CUSP Spectro-polarimeter 95-125 0.009 I Q U 10−2 SolmeX-EIP Photo-polarimeter Fe X(17.4) 0.35 I Q U 10−3 SolmeX-SUSP Spectro-polarimeter 115-155 0.0066 I Q U V 10−3 SolmeX-ChroME Spectro-polarimeter Mg II(279) 0.005 I Q U V 10−3 MIDEX-Polstar Spectro-polarimeter 122-320 >0.005 I Q U V 10−3 -
鉴于紫外波段可使用的光学折射材料较为有限,且在真空紫外波段难以应用,因此目前天文紫外偏振探测系统主要采用反射式光学设计。为了实现系统在紫外波段的高光学透过效率和宽波段响应,需要在光学镜面上涂覆高性能的紫外反射薄膜。通过对上述紫外偏振载荷的调研,总结了不同偏振载荷所采用的薄膜类型及其响应波段,如表3所示。所使用的薄膜大致可以分为三类:金属、金属+保护性外涂层、多层介电膜[56]。图1展示了一些目前常用的紫外反射薄膜的反射率曲线。
表 3 典型空间天文紫外偏振载荷工作谱段及关键部组件
Table 3. Typical spectral range of space astrophysical UV polarization payloads and key components
Parameter instrument Wavelength/nm UV coating Detector Interkosmos 16 120-150 Al+MgF2
AuPMT SMM-UVSP 115-360 Al+MgF2 PMT SUMI C IV(155)
Mg II(280)HfO2/SiO2/MgF2/LaF3
Dielectric coatingBICCD CLASP Ly-α(121.6) Al+MgF2 Frame-transfer CCD CLASP2/2.1 Mg II(280) Al+MgF2 CCD Pioneer Venus Orbiter-OCPP 270, 365 SiO2/MgF2+Al UV-enhanced silicon photodiode HST-FOS 115-850 Al+Al2 O3 Digicon photon detector ASTRO1/2-WUPPE 135-330 Al+MgF2 Reticon photodiode array WISP 135-260 Al+MgF2 Thinned CCD CUPID 118-161 Al+MgF2 Thinned CCD FUSP 105-150 Al+LiF Thinned CCD SUNRISE III-SUSI 300-410 Al BICCD ARAGO 119-888 Al+MgF2 δ-CMOS LUVOIR-POLLUX 90-400 Al+MgF2+SiC δ-CCD SolmeX-CUSP 95-125 - ICCD SolmeX-EIP Fe X(17.4) Al+B4 C+Mo BTCCD SolmeX-SUSP 115-155 - ICMOS SolmeX-ChroME Mg II(279) - - MIDEX-Polstar 122-320 Al +MgF2 CCD 在常见的金属镜面涂层(金、银、铝)中,只有铝能够在波长小于90 nm的极紫外波段提供高反射率[57],然而,由于铝金属化学特性较为活泼,当暴露在空气中(即便是在非常低的压力下),在涂层表面会形成一个氧化膜,大大降低其在紫外波段的性能,因此单质铝薄膜在实际工程中需要对载荷进行真空设计,通过在光机腔体充氮、氦等惰性气体来防止铝膜层的氧化,但是此类系统设计难度大、成本高。
目前,最常见的紫外薄膜是在金属膜层表面涂覆保护性膜层[58](通常采用氟化物,如LiF、MgF2、AlF3)[59−61]来阻止金属膜层的氧化,使得金属膜层的适用范围和场景大大拓宽,减低了载荷的设计难度。然而,保护性膜层的使用带来了问题,即会降低金属膜层在短波段的反射率。图2展示了几种常见的紫外薄膜在波段小于200 nm的反射率曲线。根据曲线趋势,可以明显看出金属+保护性外涂层在120 nm以上的波段提供了出色的反射率(大于85%),而在120 nm以下时,反射率急剧下降到20%,远低于金属单质薄膜。这就需要在设计载荷时权衡考虑,根据具体的波段要求来选择适当的薄膜设计。
针对200 nm以下的真空波段,除了使用常规的Al或Al+氟化物膜之外,科学家们针对不同特定谱段探测需求(如C IV、Fe X、Ly-α等),研制出了适用于该谱段的高性能多层介电反射膜,例如10~20 nm下采用Mo/Si多层膜[62];17~20 nm下采用Al/Zr多层膜;17~34 nm采用Al/Mo/B4C多层膜[63];25~40 nm下采用Mg基多层膜;105~130 nm下采用LaF3/MgF2多层膜等,如图3所示[64]。这些多层介电反射膜能够在特定波段下提供高反射率,从而提高整体探测系统的光学效率,对于偏振测量精度和探测效率的提升具有显著作用[65]。
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偏振系统是系统实现获取偏振光以及检测偏振光的关键。在太阳观测、行星大气等天文研究领域,需要对目标光线特性进行测量,改变偏振态,以及利用其偏振特性进行一些物理量(如磁场、元素组成等)的测量[3, 5]。
偏振系统按照测量所得Stokes参数的个数可划分为一维、二维、三维和四维Stokes偏振系统。一维、二维偏振系统主要用于提高目标与背景之间的对比度,携带的目标有用信息较少,因此在天文偏振观测中很少应用[6]。在绝大多数被动测量应用中,很少会用到圆偏振分量,而且圆偏振参数的测量最为困难,因此在早期紫外天文偏振载荷设计中,通常以三维偏振系统设计为主。随着天文学研究不断深入,发现地外生物分子、太阳活动等天文观测目标中蕴含大量的圆偏振光,这些信息可用于揭示地外生命、太阳活动起源等未解之谜,配合其他偏振态信息可获取目标更为详尽的辐射信息,因此,四维偏振系统成为目前国际上最为主流的偏振测量设备,文中也将着重对四维偏振系统的组成和发展进行介绍和分析。
四维偏振系统在设计上主要由偏振光相位调制器(Modulator)和偏振分析仪(Analyzer)两部分组成。其中,根据不同的相位调制方法,偏振系统可划分为时间调制型(Temporal modulation)、空间调制型(Spatial modulation)、时空调制型(Spatiotemporal modulation)以及光谱调制型(Spectropolarimeteric modulation)四类Stokes偏振系统[66],如图4所示。
1)时间调制型偏振系统
时间调制型偏振系统通过引入动态元件来采集不同时刻下,不同偏振方向的强度图像,然后反解出偏振信息。目前适用性较广的动态元件包括旋转波片(RWP)、电光调制器[67–68](LCVR、FLC、DFLC)以及弹光调制器[69](PEM),如表4所示。其中,由于紫外谱段的光学特殊性,电光调制器和弹光调制器在紫外波段的研究和应用较少,因此大多紫外天文偏振系统采用旋转波片[70−72]。
图 4 不同相位调制方法偏振系统工作原理图
Figure 4. Working principle diagram of polarization system with different phase modulation methods
表 4 常用偏振调制方式优缺点
Table 4. Advantages and disadvantages of common polarization modulation methods
Polarization modulation Modulation device Advantages Disadvantages Mechanical
ModulationRWP Broader working
spectral range, suitable for most UV regime; Stable in timeMoving parts increase mass, power consumption and vibrations; Modulation frequency is mechanically limited by the retarder rotating speed Electro-optic modulation LCVR, FLC, DFLC High modulation
frequency; Large aperture; Low power consumptionLimited in-orbit application;
Not suitable for most UV regimeAcousto-optic modulation PEM Broader working
spectral range(VUV-IR)No full-Stokes modulator has been developed; Small aperture Reflective
modulationBrewster angle reflectors Suitable for FUV/EUV regime Complex structure, hard to install;
large weight and volume在波片材料选择上,根据探测波段需求,多采用MgF2、CaF2、LiF等紫外光透材料,上述材料所制旋转波片在>110 nm波段有着较好的透过效率,而<110 nm的极远紫外光无法透过旋转波片,因此基于旋转波片的偏振系统不适用于110 nm以下谱段[73–74]。
在选择波片材料时,通常根据探测波段的需求进行考虑。常用的材料包括MgF2、CaF2、LiF等紫外光透明材料。这些材料制成的旋转波片在波长大于110 nm的范围内表现出较好的透过效率。然而,在波长小于110 nm的极远紫外光下,旋转波片失去透过的能力。因此,基于旋转波片的偏振系统在110 nm以下的谱段并不适用[73–74]。
在LUVOIR-POLLUX任务中,欧美等国科学家针对远紫外谱段的观测选择了一种先进的技术方案,即旋转式三反射相位调制器(K镜)。该系统采用了3个镀有FUV膜的反射镜,通过它们的巧妙组合,能够在光束不偏离光轴的情况下实现最佳强度[51],具体结构见图5。在该技术方案中,每次反射时,由于入射角度的变化,光束会在P偏振和S偏振之间发生相移。通过将这3个反射镜整体旋转,可以实现对光束的时间调制。这一创新技术方案旨在满足对极远紫外偏振的探测需求的同时,最大限度地减少反射镜的数量,从而提高光学系统的效率。这种方法不仅使得光学系统更加精细和高效,同时也为极/远紫外波段观测提供了一种更为先进的技术手段。
2)空间调制型偏振系统
空间调制型偏振系统以光楔、Savart偏光镜等光学元件作为空间调制模块,将Stokes矢量的4个参数(I、Q、U、V)信息调制到不同频率的载波上,用于实现对同一目标不同偏振分量的同时探测。空间调制型系统相较于其他偏振系统来说,由于系统中不包含机械转动部件,无需分时多次采集来获取目标完整的Stokes矢量信息,因此可对运动目标偏振信息进行精细观测[6]。空间调制元件除了常规的Savart偏光镜外,科学家还尝试使用基于方解石楔形晶体的傅里叶变换调制器、沃拉斯顿棱镜组,但由于通光口径小、光学效率低、质量体积大等因素并未应用于空间探测。
当前空间调制型偏振系统发展以片上偏振光探测器研制为主,通过在焦平面前端放置4个方向(0°、45°、90°、135°)的线栅偏振片,让在垂直于偏振片的方向振动的光透过,同时阻断在水平方向上振动的光[75],见图6。借此,偏振图像传感器会根据每个方向的偏振强度计算出探测所需的Stokes矢量信息。由于目前偏振探测器的研制主要以可见光和红外波段为主,因此对于紫外波段的使用需做进一步研究。
3)时空调制型偏振系统
时空调制型偏振系统由时间调制模块和空间调制模块构成,入射的偏振光线经旋转波片等动态组件调制后,通过偏振分束器将光束分成两个正交偏振分量,由两个探测器或同一探测器的两个部分对两个正交光束进行成像[76]。
在空间紫外天文偏振测量中使用的大多数分束器主要为以下几种不同的实现形式[77−79],包括偏振分束立方体(Beam Splitter Cubes)[46]、沃拉斯顿棱镜(Woll-aston Prisms)[80]、线栅偏振器(Wire-grid Polarizer)[81–82]、布儒斯特平面(Brewster Plane)[13]等,且上述器件针对不同特定载荷的需求进行了不同程度的优化,其中包括分光程度、波长范围、消光比,表5 列出了紫外波段最常用的偏振分束器以及定制设计或组件。
4)光谱调制型偏振系统
前文介绍的三类偏振设计主要针对准单色光进行探测的。近年来,偏振探测技术与光谱探测技术的融合成为了未来空间天文光学探测手段的发展趋势。光谱调制型偏振系统结合上述三类载荷设计,将时空调制与光谱分析相融合,这种多维信息获取方法不仅提高了光学遥感探测获取的信息量,在针对动态变化目标的测量时,还能够克服传统分时偏振测量和分孔径偏振测量的时间匹配和空间匹配问题[83−84]。
目前,在紫外波段常用的光谱调制型偏振系统为:1)光谱+偏振双通道系统(独立获取偏振或光谱信息,例如CLASP系列、Polstar等);2)偏振光谱单通道系统(获取不同偏振状态下光谱信息,例如SUMI、SUNRISE III-SUSI等),如图7所示。
表 5 紫外波段常用偏振分束器
Table 5. Commonly used polarizing beam splitters in the UV band
Device Discription Working diagram Application Beam splitter cube Two beams output at a large angle and imaged
in two separate detectorsSUNRISE III-SUSI, SolmeX-ChroME Wollaston prism Output beams are refracted at nearly opposite angles HST-FOS, ASTRO1/2-WUPPE, ARAGO, LUVOIR-POLLUX(MUV/NUV),
MIDEX-polstarDouble Wollaston prism Two Wollaston prisms are combined, the first one acts as beam splitter while the second one produces two parallel
beams at the outputSUMI, Pioneer venus orbiter-OCPP Brewster plane Beamsplitting is accomplished by two reflections on two
MgF2 birefringent plates placed at the Brewster angleInterkosmos 16, SMM-UVSP, SolmeX-SUSP, LUVOIR-POLLUX(FUV), CLASP Wire-grid polarizer By employing a structure with fine metal grids, selectively allow light of specific orientations to pass through CLASP2/2.1, SolmeX-EIP 在过去的10年,基于光谱调制型偏振系统的探测载荷在太阳观测和深空探测等天文学领域得到了快速地发展与应用,是目前紫外偏振天文观测领域中应用最广、观测效果最好的偏振探测系统[66],具体应用载荷见表2。
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紫外探测器在宇宙天文偏振信号的可视化中扮演着关键的角色,其性能直接影响着紫外偏振载荷的优劣。根据工作原理的不同,紫外探测器可以划分为真空型紫外探测器和固态紫外探测器,表3列举出了当前紫外偏振载荷所用的探测器。
1)真空型紫外探测器
目前,应用于天文紫外偏振探测载荷的真空型紫外探测器主要有紫外像增强器、紫外像增强型CCD/CMOS等。这类探测器由于具有电子倍增器件,如像管、微通道板(MCP),因此探测灵敏度高,可实现对微弱信号放大检测[85]。通过选择合适的光电阴极材料(碘化铯(CsI)、碲化铯(Cs2Te)等),可满足极紫外到近紫外宽波段响应。其缺点在于,受到电子加速轰击影响,真空型紫外探测器寿命约10 000 h;且由于探测器中存在高压模块,导致太空中的微粒、尘埃等通过静电作用吸附在探测器内,导致探测器件降效或失效,严重影响真空器件的使用寿命;此外,体积和成本也是制约真空器件使用的因素之一。
2)固体紫外探测器
随着天文观测载荷逐渐向轻小型、高鲁棒性趋势发展,体积小、寿命长、可靠性高的紫外固态探测器随着半导体技术的出现得以发展。目前空间紫外常用的是硅基半导体,其中以像元规模大、量子效率高、读出噪声低、动态范围宽、像质均匀性高[86]的紫外型CCD和读出噪声低、帧频快、功耗低、量子效率高、低成本[87]的CMOS器件为主。
传统设计的CCD和CMOS探测器件在紫外波段使用时存在几大问题,导致其紫外探测效能较差,例如:1)由于表面的多晶电极、金属电极和钝化层对紫外辐射产生强烈吸收或散射,阻碍紫外辐射到达光敏二极管;2)硅材料对于紫外波段吸收深度浅,在表面电势作用下光生电荷更易被俘获或复合;3)硅在紫外波段的折射率比可见光波段更高且变化大[88]。
针对上述问题,科学家们对CCD和CMOS进行了紫外改进设计,按照技术路线分为两类:1)通过背照射探测器[89]设计和紫外增透窗口来避免电极和钝化层对于紫外光子的吸收,具体应用包括SUMI、SUNRISE;2)通过表面浅层离子注入和外延生长原子级厚度的δ掺杂层技术在硅材料表面形成新的内建电场,收集近入射面的光生电荷[90],该技术原理图如图8所示,具体应用包括WISP、CUPID、FUSP、ARAGO等。图9比较了不同类型探测器紫外波段的量子效率,经δ掺杂技术处理后的探测器相较于传统前照式设计在400 nm以下波段具有更高的响应效率。
图 9 不同类型探测器紫外波段量子效率对比
Figure 9. Comparison of quantum efficiency in the UV band for different types of detectors
在偏振探测应用中,CCD存在一些不足之处。由于CCD的工作特性,多个像元共用一个电荷读出放大器来实现信号读出,导致CCD难以实现高帧频工作,并且随着探测器面阵规模的扩大,其帧频呈指数下降[66]。因此,在时间调制型偏振系统中存在CCD与偏振调制器速度不匹配的问题。此外,随着大口径空间望远镜的不断发展,CCD器件已经无法满足天文载荷对于探测器高帧频、高性能、低成本等要求。CMOS在实际测量中也存在一些问题,因为CMOS器件在设计上将电荷阱与放大器集成在同一个像元上,导致探测器响应呈现较高的非线性度,使得在高偏振精度探测(<10−4)时CMOS的探测效能难以保证,从而限制了其在某些高精度天文探测任务中的应用[66]。
除此之外,电子倍增CCD(EMCCD)、科学级CMOS(sCMOS)等器件的快速发展以及在轨应用也为未来空间天文紫外偏振载荷设计提供了更多地选择[91],文中在表6中总结了各类固体探测器的优缺点。
表 6 各类固体紫外探测器件优缺点
Table 6. Advantages and disadvantages of different solid UV detectors
Device Advantages Disadvantages EMCCD High sensitivity; low noise; high frame rate;
high dynamic rangeHigh cost; limited dynamic range compared to CCD and sCMOS; limited lifespan; limited resolution sCMOS High sensitivity; high frame rate;
high dynamic range; large FOVHigh cost; limited lifespan; limited resolution; sensitivity to cosmic rays, leading to potential image artifacts or data loss CCD High sensitivity; high dynamic range; low noise Low readout speed; high power consumption; high cost;
limited resolutionCMOS Low power consumption; low cost; high frame rate;
high resolution; large FOVLow sensitivity; low dynamic range; high noise 鉴于当前空间天文学的研究热点以及中国在深空探测方面的布局,文中对空间天文紫外偏振探测技术的研究进展进行了概述,针对其科学意义、载荷发展以及载荷关键技术组成进行剖析,针对紫外偏振探测器件未来发展方向提出以下建议:
Advances in ultraviolet polarization detection for space astronomy
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摘要: 新型空间天文探测技术在科学进步和国家发展方面起着至关重要的作用。其高精度的观测提供了独特的机遇,深化了人们对宇宙的理解,同时推动了天体物理学和宇宙学的前沿研究。这些探测不仅为理论模型的验证与发展提供了不可或缺的数据,还在基础物理学领域催生了新理论。在国家层面,拥有先进的空间天文探测技术不仅凸显了国家在科研领域的实力,同时为培养高水平的科研人才提供了重要平台,有助于国家在全球科研舞台上赢得竞争优势。因此,对新型空间天文探测技术的研究和发展显得十分紧迫。天文紫外偏振作为一种创新的空间天文探测手段,相较于传统探测手段,紫外偏振测量技术能够提供多维度的观测数据,进而实现全方位的宇宙感知能力,具备巨大地应用潜力。鉴于目前尚未有关于空间天文紫外偏振探测的系统性综述报告和文章,因此,文中从紫外偏振科学研究的角度出发,总结已有的紫外偏振载荷,挖掘这一领域技术发展的思路和途径,最后对该技术未来发展方向进行展望与总结,为我国在该领域的未来研究提供有益参考。Abstract:
Significance In the realm of scientific advancement and national development, emerging space-based astronomical detection technologies play a pivotal role. Their high-precision observations afford unique opportunities, deepening our comprehension of the universe and propelling the forefronts of astrophysics and cosmology. These detections not only furnish indispensable data for the validation and development of theoretical models but also instigate the emergence of novel theories in fundamental physics. On the national scale, possessing advanced space-based astronomical detection capabilities not only underscores a nation's prowess in the scientific domain but also provides a crucial platform for nurturing high-caliber research talent. This, in turn, contributes to the nation's competitive edge on the global scientific stage. Therefore, the imperative nature of researching and developing novel space-based astronomical detection technologies is evident. Progress This article delivers a comprehensive examination across three dimensions: the advancement of ultraviolet polarization space observations, the current global landscape of ultraviolet polarization payloads in both domestic and international contexts, and the pivotal technologies associated with ultraviolet polarization payloads. Addressing the progress in ultraviolet polarization space observations, the study elucidates the significance of ultraviolet polarization within the domains of solar physics, planetary science, and interstellar matter research. Furthermore, the article provides an overview of the prevailing global scientific research developments in this field. Concerning the development status of ultraviolet polarization payloads both at home and abroad, given the absence of relevant payloads in China, the emphasis is placed on introducing typical international space-based astronomical ultraviolet polarization payloads, elucidating their detection targets, and summarizing their prospective development directions. Regarding the key technologies associated with ultraviolet polarization payloads, the article synthesizes the performance indicators of both existing and planned astronomical ultraviolet polarization payloads. It is evident that contemporary ultraviolet polarization detection primarily hinges on the fusion of polarization and spectroscopic detection, and a singular ultraviolet polarization datum falls short of meeting the demands of astronomical observations. With escalating observational requisites, the necessity for heightened precision in spectral resolution and polarization measurement accuracy is underscored, thereby imposing heightened demands on ultraviolet polarization devices. Furthermore, given the concentration of ultraviolet radiation signals in the far-ultraviolet wavelength range in the cosmos, which exhibits weaker intensity compared to the visible and infrared bands, there exists a stringent requirement for high overall transmittance and detection efficiency of the system. Building upon these considerations, the article furnishes a forward-looking and succinct perspective on specific key technologies and future directions across three focal areas of ultraviolet optical coatings, ultraviolet polarization systems, and ultraviolet detectors. Conclusions and Prospects Following an in-depth analysis and synthesis of advancements in ultraviolet polarization space observations both at home and abroad, this review delineates current challenges in ultraviolet polarization detection. These challenges encompass suboptimal optical detection efficiency, subpar polarization measurement accuracy, and the high complexity and cost associated with device development. In response to these challenges, this study puts forth future development directions for space-based ultraviolet polarization detection technology. These directions encompass the exploration of cutting-edge coating technologies, such as Atomic Layer Deposition (ALD), advancements in the high-reflectance performance of multilayer reflective films, the application of emerging dynamic components like electro-optic modulators in the UV spectrum, the development of on-chip ultraviolet-sensitive polarization detectors, the expansion of ultraviolet solid-state detectors into the EUV wavelength range, enhancements in detector sensitivity, and the exploration of innovative ultraviolet detector technologies. This forward-looking perspective is geared towards not only addressing existing challenges but also propelling significant advancements in space-based ultraviolet polarization detection technology. -
Key words:
- UV polarization /
- space polarization detection /
- space astronomy
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表 1 典型空间天文紫外偏振载荷
Table 1. Typical space astronomy UV polarization loads
Mission Main
countryLaunch
timeInstrument Target Interkosmos 16 Sweden 1976 UVSP VUV linear polarization in Solar transition zone, etc.[8, 35] SMM USA 1980 UVSP Solar flares, solar active regions, solar corona, etc.[36–37] SUMI USA 2012 SUMI Chromosphere, solar transition region magnetic field, etc.[19] CLASP Japan 2015 CLASP Upper solar chromosphere, transition region, etc.[13, 15] CLASP-2 Japan 2019 CLASP Upper solar chromosphere, transition region, etc.[38] CLASP-2.1 Japan 2021 CLASP Upper solar chromosphere, transition region, etc.[38] Pioneer Venus Orbiter USA 1978 OCPP Venus cloud layers, physical characteristics of aerosol particles in the atmosphere, and vertical distribution of aerosols, etc.[39] Hubble telescope USA 1990 FOS Active galaxies, quasars, and planetary nebulae, etc.[40–43] ASTRO-1 USA 1990 WUPPE Interstellar dust, hot stars, material in the solar system, active galaxies, etc.[44] ASTRO-2 USA 1995 WUPPE Interstellar dust, hot stars, material in the solar system, active galaxies, etc.[44] WISP USA 1997 WISP Diffuse nebula, dust nebula, Crab Nebula , etc.[1, 45] CUPID USA Unknown CUPID Cosmic background , etc.[1] FUSP USA Unknown FUSP Hot star systems, circumstellar disks , etc.[1] SUNRISE III Germany Pending SUSI Solar atmosphere, etc.[46] ARAGO Europe Under planning ARAGO Stellar-planetary interactions, etc.[47–48] LUVOIR USA Under planning POLLUX Circumstellar disk, interstellar magnetic fields, etc.[49–52] SolmeX Europe Under planning EIP; SUSP;
ChroME; CUSPThe structure of the solar outer atmosphere magnetic field, magnetic field variations, and magnetic field-derived effects, etc.[53] Polstar USA Under planning Polstar Circumstellar disk, interstellar magnetic fields, etc.[54-55] 表 2 典型空间天文紫外偏振载荷指标参数
Table 2. Parameters of typical space astronomy UV polarization load
Parameter instrument Instrument type Wavelength/nm Spectral resolution/nm Stokes parameter Polarimetry precision Interkosmos 16 Spectro-polarimeter 120-150 3.9 I Q U 10−2 SMM-UVSP Spectro-polarimeter 115-360 0.002-0.004 I V 10−2 SUMI Spectro-polarimeter C IV(155)
Mg II(280)0.05@ C IV
0.08@ Mg III Q U V 10−3 CLASP Spectro-polarimeter Ly-α(121.6) 0.01 I Q U 10−3 CLASP2/2.1 Spectro-polarimeter Mg II(280) 0.01 I Q U V 10−3 Pioneer Venus Orbiter-OCPP Photo-polarimeter 270, 365 ~18 I Q U 10−3 HST-FOS Spectro-polarimeter 115-850 >0.25 I Q U V 10−2 ASTRO1/2-WUPPE Spectro-polarimeter 135-330 0.6 I Q U V 10−3 WISP Photo-polarimeter 135-260 40 I Q U - CUPID Photo-polarimeter 118-161 20 I Q U 10−2 FUSP Spectro-polarimeter 105-150 0.07 I Q U 10−3 SUNRISE III-SUSI Spectro-polarimeter 300-410 0.001 I Q U V 10−3 ARAGO Spectro-polarimeter 119-888 >0.009 I Q U V - LUVOIR-POLLUX Spectro-polarimeter 90-400 0.002 I Q U V 10−4 SolmeX-CUSP Spectro-polarimeter 95-125 0.009 I Q U 10−2 SolmeX-EIP Photo-polarimeter Fe X(17.4) 0.35 I Q U 10−3 SolmeX-SUSP Spectro-polarimeter 115-155 0.0066 I Q U V 10−3 SolmeX-ChroME Spectro-polarimeter Mg II(279) 0.005 I Q U V 10−3 MIDEX-Polstar Spectro-polarimeter 122-320 >0.005 I Q U V 10−3 表 3 典型空间天文紫外偏振载荷工作谱段及关键部组件
Table 3. Typical spectral range of space astrophysical UV polarization payloads and key components
Parameter instrument Wavelength/nm UV coating Detector Interkosmos 16 120-150 Al+MgF2
AuPMT SMM-UVSP 115-360 Al+MgF2 PMT SUMI C IV(155)
Mg II(280)HfO2/SiO2/MgF2/LaF3
Dielectric coatingBICCD CLASP Ly-α(121.6) Al+MgF2 Frame-transfer CCD CLASP2/2.1 Mg II(280) Al+MgF2 CCD Pioneer Venus Orbiter-OCPP 270, 365 SiO2/MgF2+Al UV-enhanced silicon photodiode HST-FOS 115-850 Al+Al2 O3 Digicon photon detector ASTRO1/2-WUPPE 135-330 Al+MgF2 Reticon photodiode array WISP 135-260 Al+MgF2 Thinned CCD CUPID 118-161 Al+MgF2 Thinned CCD FUSP 105-150 Al+LiF Thinned CCD SUNRISE III-SUSI 300-410 Al BICCD ARAGO 119-888 Al+MgF2 δ-CMOS LUVOIR-POLLUX 90-400 Al+MgF2+SiC δ-CCD SolmeX-CUSP 95-125 - ICCD SolmeX-EIP Fe X(17.4) Al+B4 C+Mo BTCCD SolmeX-SUSP 115-155 - ICMOS SolmeX-ChroME Mg II(279) - - MIDEX-Polstar 122-320 Al +MgF2 CCD 表 4 常用偏振调制方式优缺点
Table 4. Advantages and disadvantages of common polarization modulation methods
Polarization modulation Modulation device Advantages Disadvantages Mechanical
ModulationRWP Broader working
spectral range, suitable for most UV regime; Stable in timeMoving parts increase mass, power consumption and vibrations; Modulation frequency is mechanically limited by the retarder rotating speed Electro-optic modulation LCVR, FLC, DFLC High modulation
frequency; Large aperture; Low power consumptionLimited in-orbit application;
Not suitable for most UV regimeAcousto-optic modulation PEM Broader working
spectral range(VUV-IR)No full-Stokes modulator has been developed; Small aperture Reflective
modulationBrewster angle reflectors Suitable for FUV/EUV regime Complex structure, hard to install;
large weight and volume表 5 紫外波段常用偏振分束器
Table 5. Commonly used polarizing beam splitters in the UV band
Device Discription Working diagram Application Beam splitter cube Two beams output at a large angle and imaged
in two separate detectorsSUNRISE III-SUSI, SolmeX-ChroME Wollaston prism Output beams are refracted at nearly opposite angles HST-FOS, ASTRO1/2-WUPPE, ARAGO, LUVOIR-POLLUX(MUV/NUV),
MIDEX-polstarDouble Wollaston prism Two Wollaston prisms are combined, the first one acts as beam splitter while the second one produces two parallel
beams at the outputSUMI, Pioneer venus orbiter-OCPP Brewster plane Beamsplitting is accomplished by two reflections on two
MgF2 birefringent plates placed at the Brewster angleInterkosmos 16, SMM-UVSP, SolmeX-SUSP, LUVOIR-POLLUX(FUV), CLASP Wire-grid polarizer By employing a structure with fine metal grids, selectively allow light of specific orientations to pass through CLASP2/2.1, SolmeX-EIP 表 6 各类固体紫外探测器件优缺点
Table 6. Advantages and disadvantages of different solid UV detectors
Device Advantages Disadvantages EMCCD High sensitivity; low noise; high frame rate;
high dynamic rangeHigh cost; limited dynamic range compared to CCD and sCMOS; limited lifespan; limited resolution sCMOS High sensitivity; high frame rate;
high dynamic range; large FOVHigh cost; limited lifespan; limited resolution; sensitivity to cosmic rays, leading to potential image artifacts or data loss CCD High sensitivity; high dynamic range; low noise Low readout speed; high power consumption; high cost;
limited resolutionCMOS Low power consumption; low cost; high frame rate;
high resolution; large FOVLow sensitivity; low dynamic range; high noise -
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