长波红外制冷杜瓦组件杂散光分析【增强内容出版】
Infrared detection technology is widely used in the aerospace industry owing to its strong anti-interference capabilities and wide detection ranges. This enables comprehensive and large-scale continuous monitoring of land and oceans. Notable examples include the successful launch of the Ocean No.1 (HY-1A) and Fengyun sequence satellites, both of which are equipped with an infrared payload. However, infrared detection is challenging because of weak target signals and the need for increased detector sensitivity. To address this issue, infrared focal plane detectors such as mercury cadmium telluride (MCT) operate at lower temperatures, necessitating sufficient cooling capacity provided by refrigerators. Currently, most MCT detectors used in space applications employ mechanical refrigeration. These detectors are typically packaged in a metal Dewar to meet low-temperature working requirements and are coupled with the refrigerator through the Dewar cold finger. Hence, the Dewar design directly affects the refrigeration efficiency. Additionally, strict stray light analysis and suppression are essential for satisfying the high-performance requirements of infrared remote-sensing loads. This is crucial for ensuring the accurate quantitative inversion and image quality of infrared remote sensing data. A notable example is the temporary shutdown of the EU Meteosat-5/7 series imager owing to stray light interference. Infrared remote sensing detection systems, particularly those operating at long wavelengths, rely heavily on the suppression of internal stray light radiation to enhance image quality. To reduce the infrared component background radiation, the infrared component barrel, lens, and Dewar window are often low-temperature optically processed, ensuring the infrared optical component cooling capacity in a limited space becomes the key to component design.
To suppress stray light in the system, the veiling glare index (VGI) is calculated based on the functional relationship between the point source transmittance (PST) and the VGI (Fig.2). In addition, the VGI and noise signal ratio (NSR) are used as indicators to investigate methods for suppressing external stray light and background radiation. This study aims to optimize the design of a Dewar cold screen and window. Initially, the effectiveness of the cold optical design for reducing the background radiation of the system is compared (Figs.3 and 4). Refrigeration is essential for the cold optical design and proper functioning of the Dewar detector. The impact of the Dewar cold screen and window design parameters on the Dewar heat loss is examined (Tables 1 and 3). By fitting the experimental data, the relationship between the Dewar heat leakage and chiller power consumption is defined (Fig.6). Subsequently, a cold-screen design with low cooling power consumption is proposed (Fig.8), and the values of the system VGI and Dewar NSR are calculated through simulations.
The results indicate that the cold optical design effectively reduces stray light from background radiation in the system. The NSR of the three bands operating in the system decreases significantly from above 4.5 to below 0.35 (Fig.4). Theoretical calculations demonstrate that as the distance between the cold screen and window decreases, there is an increase in the Dewar heat leakage and power consumption of the refrigerator (Table 1). Furthermore, the relationship between the Dewar heat leakage and power consumption follows an E-exponential function (Fig.6). By utilizing the new cold-screen design (Fig.8), it is possible to reduce the power consumption of the refrigerator while simultaneously improving the external and Dewar background radiation stray lights (Figs.9 and 10).
With the improvements in the detection accuracy of space remote sensing loads, stray light suppression design has become a key technology in space remote sensing. The infrared optical load benefits from the low-temperature optical design, which effectively suppresses the background radiation of the infrared system. The refrigeration of the refrigerator is crucial for the normal operation of the detector and the low temperature optical design. Therefore, it plays a key role in ensuring an efficient stray-light-suppression design within the limited refrigeration resources of refrigerators. This study utilizes the functional relationship between the PST and VGI to calculate the VGI. It investigates the impact of the Dewar cold screen and window design on the external stray light, system background radiation, and power consumption of the refrigerator using the VGI and NSR as indicators. This study establishes a preliminary system for a Dewar cold screen and window design, encompassing stray light suppression and Dewar heat leakage designs. A cold screen design that achieves both low cooling power consumption and high stray light suppression is proposed. Under these design conditions, the Dewar heat leakage is 1.7 W, the chiller power consumption is 103.72 W, VGI is reduced from 1.95% to 1.92%, and the energy proportion of the window radiation stray light is reduced by 60%, meeting the project design requirements. This study addresses issues related to low-temperature optical design, refrigerator power consumption, and stray light suppression, providing valuable insights for the design and engineering applications of infrared Dewar refrigeration components.
1 引言
随着红外探测技术的快速发展,红外遥感技术在航空航天领域中得到了广泛的应用[1-3]。红外探测技术是将辐射信号转换为电学信号,具有高分辨率、高探测能力的红外遥感仪可以对陆地和海洋进行全方位、大范围的连续监测,如我国2002年成功发射的海洋一号(HY-1A)卫星和风云序列卫星,都搭载了红外载荷,其主要用途是监测海洋海域环境气候、陆地生态环境气候和全球自然灾害等[4]。因此,获得高光谱分辨率、高空间分辨率、高灵敏度和宽幅的红外遥感卫星载荷成为关键[5-8]。
随着探测目标信号的微弱化和探测器探测灵敏度的提高,一方面,以碲镉汞(MCT)为代表的红外焦平面探测器需要在更低的温度下工作,这需要制冷机提供更好的制冷效果。机械制冷具有结构紧凑、体积小、重量轻、制冷量大、制冷时间短、制冷温度可控范围大等优点,目前MCT探测器在空间应用中大多采用机械制冷方式。为满足红外焦平面探测器芯片的深低温工作要求,探测器芯片大多采用金属杜瓦形式封装,形成红外焦平面杜瓦组件,再通过杜瓦冷指与制冷机耦合,制冷机能够提供芯片所需的温度。因此杜瓦成为制冷机与红外焦平面探测器耦合的重要部件,杜瓦的结构设计将直接影响制冷机的使用效率。另一方面,为了满足红外遥感载荷高性能的技术要求,需要对红外遥感载荷进行严格的杂散光分析与抑制。杂散光会影响红外遥感载荷的探测能力,如果红外遥感载荷的杂散光抑制不足,轻则影响探测器像面均匀性,重则红外遥感载荷的输出信号会被杂散光湮没,从而导致探测失效。如欧盟Meteosat-5/7系列成像仪曾由于杂散光影响而暂时关机[9]。杂散光通常指光学系统中除成像光线外通过折射、反射和散射等方式到达探测器像面的非预期光线。按照来源可以将杂散光分为三类[10]:第一类,成像光线通过鬼反射等方式到达像面。第二类,光学系统外部的杂散源通过反射、折射和散射等方式经过光学系统到达像面,如地气杂散光、太阳光等。第三类,光机系统自身的红外背景辐射。第一类成像光线产生的杂散光常采用在透镜和反射镜等光学元件表面镀制抗反射或增透射膜层来抑制。在红外遥感探测系统中,尤其是工作波段为长波的红外探测系统,第二类光学系统外部杂散光和第三类内部背景辐射杂散光的抑制往往对红外遥感载荷成像质量的提高起到关键性作用。而为了降低红外组件背景辐射杂散光,常针对红外组件的镜筒和镜片等元件进行冷光学设计,因此在有限空间下保证红外光学组件的制冷量成为组件设计的关键。
以某项目用长波红外杜瓦制冷组件为研究对象,研究了长波红外杜瓦制冷组件的窗口和冷屏设计对光学系统外部杂散光、内部辐射杂散光和制冷机功耗的影响。为了降低制冷机热载荷和杜瓦漏热,提出了一种低热损的冷屏设计,组件外部杂散光和内部背景辐射杂散光都得到改善,并成功研制出长波红外杜瓦制冷组件,为航天用红外长波杜瓦组件的设计提供了参考。
2 红外组件杂散光分析
2.1 红外组件模型
图 1. 红外组件模型。(a)红外光学系统;(b)红外杜瓦组件
Fig. 1. Infrared component model. (a) Infrared optical system; (b) infrared Dewar component
2.2 杂散光系数
杂散光系数(VGI)和点源透过率(PST)是评价光学系统抑制外部杂散光能力的主要指标。其中点源透过率(
式中:
PST反映的是光学系统对点源杂散光的抑制能力,与杂散光源的强度无关,PST值越小,表明光学系统对杂散光源的抑制能力越强。当太阳和行星作为杂散光源时,PST通常作为光学系统杂散光强弱的评价指标。当环境背景和大气背景作为杂散光源时,需要引入杂散光系数(
式中:
目前仿真计算杂散光系数的方法主要有两种[12]。一种方法是通过黑斑法测试光路,即正向追迹法。该方法需要评估不同路径的杂散光占比,只适用于反射式光学系统。对于折射式和折反射光学系统,因无法准确辨别相应类别的光线,该方法的适应性不足。另一种方法是通过辐射热理论得到VGI与PST之间的函数关系,即VGI是PST在w~π/2范围内的积分[13],即
式中:F为成像光学系统的F数;
图 2. PST仿真结果。(a)0°≤θ≤90.00°;(b)0°≤θ≤3.55°;(c)3.55°<θ≤66.00°;(d)66.00°<θ≤90.00°
Fig. 2. PST simulation results. (a) 0°≤θ≤90.00°; (b) 0°≤θ≤3.55°; (c) 3.55°<θ≤66.00°; (d) 66.00°<θ≤90.00°
当0°≤θ≤3.55°时,PST曲线和拟合曲线如
该拟合多项式的拟合精度为7.17×10-5,将
当3.55°<θ≤66.00°时,PST曲线和拟合曲线如
该拟合多项式的拟合精度为6.46×10-6,将
当66.00°<θ≤90.00°时,PST曲线和拟合曲线如
该拟合多项式的拟合精度为1.84×10-8,将
根据VGI与PST的关系式[
2.3 背景辐射分析
在红外光学系统中,仪器内部热辐射产生的杂散光对系统的成像质量的影响不可忽略。尽管光机系统的内部杂散辐射可以在冷空观测中通过校对去除,但如果杂散光在信号中的占比过大,将会影响探测器信号的响应范围,降低成像质量。根据黑体辐射公式可知,一定温度下元件会产生热辐射,并且随着温度的升高,能量辐射中心向短波方向漂移,辐射能量增大。在红外系统众多表面中,可以被探测器直接观测到的表面称为关键面[15],它们的表面辐射可能通过折射、反射或散射直接到达探测器。在抑制仪器自身辐射杂散光时,需要重点关注这些关键面。在本红外光学系统中,主要关键面分别为镜筒、镜片、窗口和窗口帽。
图 3. 关键面的辐射杂散光。(a)8.0~10.5 μm;(b)10.3~11.3 μm;(c)11.5~12.5 μm;(d)总NSR
Fig. 3. Radiation stray lights at key surfaces. (a) 8.0‒10.5 μm; (b) 10.3‒11.3 μm; (c) 11.5‒12.5 μm; (d) total NSR
在红外光学系统中,需要独特的内部热辐射抑制方法,其中温度控制法作为一种最有效的内辐射抑制方法,被多个红外探测器系统使用[13]。其基本原理是:系统内部辐射强度与温度和波长相关,适当降低系统表面温度可有效降低辐射强度,且根据维恩光子位移定律可知,随着温度的降低,短波波段将成为辐射能量中心区域。低温光学设计是一种降低光机自身辐射的有效手段,
图 4. 关键面的辐射杂散光。(a)8.0~10.5 μm波段;(b)10.3~11.3 μm波段;(c)11.5~12.5 μm波段;(d)总NSR
Fig. 4. Radiation stray lights at key surfaces. (a) 8.0‒10.5 μm wave band; (b) 10.3‒11.3 μm wave band; (c) 11.5‒12.5 μm wave band; (d) total NSR
3 杜瓦组件杂散光抑制设计
3.1 冷屏杂散光抑制设计
热传导方式分为传导漏热、辐射换热和对流换热。其中对流换热是指杜瓦内的气体由于温度不同而发生迁移,热量从温度较高的部分被传递到温度较低的部分,因此又可称之为气体分子传导漏热。由于杜瓦工作时内部真空度保持在10-7 Pa量级,因此杜瓦内部对流很小,可以忽略不计。杜瓦的漏热包含引线传导漏热、芯柱传导漏热、杜瓦窗口与冷屏的总辐射漏热、杜瓦窗口帽与冷屏的总辐射漏热、芯柱与柱壳辐射漏热等。杜瓦内部热网络模型如
任意两个表面之间的辐射换热(Q12)公式为
式中:Eb1和Eb2分别为相同温度下表面1和表面2的发射功率;
表 1. 杜瓦寄生热负载
Table 1. Parasitic heat loads of Dewar
|
由
表 2. 杜瓦寄生负载与制冷机功耗
Table 2. Dewar parasitic load and chiller power consumption
|
由
杜瓦封装结构中的冷屏作为最靠近探测器的光阑,冷屏的设计结构对探测器的像面均匀性和杂散光都有着直接的影响。杂散光强弱与表面的双向反射分布函数(BRDF)有关,通过降低表面的BRDF,可以较好地消除杂散光。降低表面BRDF的思路为:1)通过机械加工改变通光表面的形貌;2)改善表面吸收光线的能力。利用导热性好的镍钴材料,通过电铸工艺加工制得冷屏,冷屏外表面采用抛光工艺降低窗口帽辐射,内表面采用低反射率涂层工艺抑制杂散光。为了更好地抑制杂散光,在不影响成像光线的情况下,在冷屏端面加10 mm高的竖直挡板,厚度为0.2 mm,如
加了10 mm高的挡板后系统的PST曲线如
图 9. 增加挡板后的PST仿真结果。(a)0°≤θ≤90.00°;(b)0°≤θ≤3.55°;(c)3.55°<θ≤66.00°;(d)66.00°<θ≤90.00°
Fig. 9. PST simulation results after adding baffle. (a) 0°≤θ≤90.00°; (b) 0°≤θ≤3.55°; (c) 3.55°<θ≤66.00°; (d) 66.00°<θ≤90.00°
当0°≤θ≤3.55°时,PST曲线拟合的函数表达式为
该拟合多项式的拟合精度为7.18×10-5,将
当3.55°<θ≤66.00°时,PST曲线拟合的函数表达式为
该拟合多项式的拟合精度为6.68×10-6,将
当66.00°<θ≤90.00°时,PST曲线拟合的函数表达式为
该拟合多项式的拟合精度为2.023×10-8,将
根据杂散光系数VGI与PST的关系式[
图 10. 杜瓦关键面的NSR。(a)窗口;(b)窗口帽
Fig. 10. NSRs of Dewar key surfaces. (a) Window; (b) window cap
3.2 杜瓦窗口杂散光抑制设计
杜瓦窗口作为探测器与光学系统接口需要同时满足光学性能和高气密性的双重要求,它作为气密元件保证探测器工作在高真空环境下,也作为光学系统元件参与光学成像。为了抑制成像光线通过鬼反射产生的杂散光,在杜瓦窗口上镀制增透膜,保证探测器在工作波段8.0~12.5 μm内的透过率大于94%,如
长波红外制冷杜瓦组件随载荷工作在太空中时,杜瓦窗口内外都为真空状态,窗口温度保持在200 K低温状态。杜瓦窗口在低温载荷下产生形变,而杜瓦窗口的曲率不为0,这将导致光线穿过窗口产生不可预期的变化,从而给光学系统成像和杂散光分析带来困难。杜瓦窗口尺寸一般由红外光学系统决定。然而,在工程中,考虑窗口帽与窗口之间的气密焊接和装配带来的误差,窗口通光尺寸往往大于光学系统设计值[17]。
表 3. 杜瓦窗口余量与制冷机功耗
Table 3. Dewar window margins and chiller power consumptions
|
图 12. 不同窗口余量下的杂散分析。(a)8.0~10.5 μm波段的镜筒NSR;(b)10.3~11.3 μm波段的镜筒NSR;(c)11.5~12.5 μm波段的镜筒NSR;(d)光学系统的VGI
Fig. 12. Spurious analysis under different window margins. (a) NSR for lens cone at 8.0‒10.5 μm; (b) NSR for lens cone at 10.3‒11.3 μm; (c) NSR for lens cone at 11.5‒12.5 μm;(d) VGI of optical system
3.3 组件性能
杂散光的存在不仅会影响探测器像面的非均匀性,也是限制相机灵敏度的一个重要因素。合理的杂散光抑制设计可以降低杂散光对探测器成像质量的影响,提高光学系统的信噪比,从而提高相机的灵敏度。红外光学组件的杂散光抑制设计需要考虑制冷成本等因素。
表 4. 主要性能指标
Table 4. Main performance indicators
|
4 结论
随着空间遥感载荷的探测精度的逐渐提高,杂散光抑制设计已成为空间遥感关键技术之一。在红外光学载荷中,低温光学设计是抑制红外系统背景辐射的最有效手段之一。制冷机制冷是探测器正常工作和低温光学设计的保障,因此在有限的制冷机制冷资源下保证高效的杂散光抑制设计成为关键。利用点源透过率与杂散光系数的函数关系计算了VGI,并分别以VGI和杂散比作为指标,研究了杜瓦冷屏和窗口设计对系统外部杂散光、系统背景辐射和制冷机功耗的影响,建立了杜瓦冷屏和窗口设计的初步体系,包括杂散光抑制设计和杜瓦漏热设计。并提出了一种具有低制冷功耗和显著杂散光抑制效果的冷屏设计,此时杜瓦漏热为1.7 W,制冷机功耗为103.72 W,VGI从1.95%降低至1.92%,窗口的NSR下降了60%,满足项目设计要求。研究结果对红外杜瓦制冷组件的设计和工程化应用具有借鉴意义。
[1] Chander G, Markham B L, Helder D L. Summary of current radiometric calibration coefficients for Landsat MSS, TM, ETM+, and EO-1 ALI sensors[J]. Remote Sensing of Environment, 2009, 113(5): 893-903.
[2] Montanaro M, Gerace A. Performance of the proposed stray light correction algorithm for the Thermal Infrared Sensor (TIRS) onboard Landsat 8[J]. Proceedings of SPIE, 2016, 9972: 99720F.
[3] Xiong X X, Sun J Q, Wu A S, et al. Terra and Aqua MODIS calibration algorithms and uncertainty analysis[J]. Proceedings of SPIE, 2005, 5978: 59780V.
[4] 胡秀清, 张里阳, 郑照军, 等. FY-3A中分辨率光谱成像仪热红外通道的多探元辐射定标[J]. 光学 精密工程, 2010, 18(9): 1972-1980.
[8] 陈世涵, 李玲, 蒋弘凡, 等. 基于高空间分辨率卫星遥感数据的探测环境变化对气温的影响[J]. 光学学报, 2020, 40(10): 1028001.
[9] Ghaffarian B, Sprunger K. Solar intrusion thermal analysis[J]. Proceedings of SPIE, 1996, 2812: 251-259.
[10] Wang J Y, Chen Y W. Study on the stray light suppression of fundus camera[J]. Applied Mechanics and Materials, 2012, 271/272: 1664-1668.
[11] Sun L, Cui Q F. Analysis and calculation of the veiling glare index in optical systems[J]. Laser Physics, 2018, 28(11): 115302.
[12] 李艳杰, 钟兴, 杨成龙. 一种快速获取杂散光来源和杂散光系数的仿真建模方法: CN112485901A[P]. 2021-03-12.
LiY J, ZhongX, YangC L. A simulation modeling method for quickly obtaining sources and coefficients of stray light: CN112485901A[P]. 2021-03-12.
[13] 王虎, 陈钦芳, 马占鹏, 等. 杂散光抑制与评估技术发展与展望(特邀)[J]. 光子学报, 2022, 51(7): 0751406.
[14] 徐亮. 大口径光学系统杂散光测试关键技术研究[D]. 北京: 中国科学院大学, 2019.
XuL. Research on key technologies of stray light measurement for large aperture optical system[D]. Beijing: University of Chinese Academy of Sciences, 2019.
[15] FestE C. Stray light: analysis and control[M]. YuQ H, YuK, LiuX, B, Transl. Wuhan: Huazhong University of Science and Technology Press, 2019.
FestEric C.. 杂散光抑制设计与分析[M]. 于清华, 俞侃, 刘祥彪译. 武汉: 华中科技大学出版社, 2019.
[16] 朱海勇, 陈俊林, 曾智江, 等. 用于冷光学长波红外杜瓦组件杂散光分析与抑制[J]. 红外与激光工程, 2023, 52(7): 20220823.
[17] 朱海勇, 曾智江, 李雪, 等. 冷光学大口径长波红外探测器杜瓦窗口应用设计[J]. 中国激光, 2023, 50(14): 1410003.
Article Outline
朱海勇, 陈俊林, 曾智江, 徐勤飞, 王小坤, 李亚冉, 李雪. 长波红外制冷杜瓦组件杂散光分析[J]. 中国激光, 2024, 51(8): 0810005. Haiyong Zhu, Junlin Chen, Zhijiang Zeng, Qinfei Xu, Xiaokun Wang, Yaran Li, Xue Li. Stray Light Analysis of Long‐Wave Infrared Refrigeration Dewar Components[J]. Chinese Journal of Lasers, 2024, 51(8): 0810005.