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为了能定量的评估出本技术的优势,这里将文中研究的探测技术与传统探测技术分别进行同条件分析及评估,分两项内容研究:(1)光学系统自辐射能力分析与评估;(2)灵敏度能力分析与评估。
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光学系统自辐射能力分析与评估采用单波段方式就可以验证,因此,仿真时基于单波段完成杂光分析模型设计,其中,光学自辐射仿真软件采用的是通用的光学杂光分析软件Lighttools7.0。设定的基本参数和仿真分析结果如表1所示。
从上面仿真分析可以看出采用文中研究的低温折反光学系统是传统常温的折反系统的1/4,是同轴四反光学系统自辐射的1/2,自辐射抑制效果比较明显。
表 1 光学系统自辐射分析
Table 1. Self-radiation analysis of optical system
Parameter Wave band: 7.5-10.5 μm; Optical field of view: 2.5°×2.5°; f/#=2; Optical reflectivity: 99.2%;
Lens transmission: 98.5%; Metal absorptivity: 20%; Metal reflectivity: 80%Type Ordinary temperature optical system of refraction-reflection Coaxial four anti optical system Low temperature optical system of refraction-reflection Energy of self-radiation/W 2.6×10−5 1.47×10−5 6.5×10−6 Equivalent blackbody temperature/K 223 204 183 -
对于低背景探测系统而言,探测目标在成像像面上理论上是一个点,应该用系统灵敏度的等效噪声照度(NEFD)来分析评估。
其中,探测系统的灵敏度(NEFD)由时域灵敏度(NEFDt)和空域灵敏度(NEFDs)两部分组成,表示如下:
$$NEFD = {(NEFD_{\rm{t}}^2 + NEFD_{\rm{s}}^2)^{1/2}}$$ (1) 时域灵敏度(NEFDt)表示如下:
$$NEFD_{\rm{t}} = \frac{{4(f/\# ){{(\xi \alpha \beta \Delta f)}^{1/2}}}}{{ {\text{π}} {D_0}{\tau _0}{D^*}{K_1}{K_2}}}$$ (2) 式中:D0为光学系统口径;f/#为数值孔径参数;τ0为
光学透过率;ξ 为探测器填充率;K1为光学弥散系数;K2为电路损失系数;α和β分别为俯仰方向瞬时视场、瞬时视场;D*为归一化探测率。 空域灵敏度(NEFDs)表示如下:
$$NEFD_{\rm{s}} = \frac{U}{ {\text{π}} }\xi \alpha \beta {E_{\rm{b}}}$$ (3) 这里对三种状态情况进行比较分析:(1)常温折反分光双波段系统;(2)基于叠层芯片低温折反双波段系统;(3)低温折反分光双波段探测系统。其中,常温折反分光双波段系统,是一种国内比较常用的共口径双波段系统技术途径,光学镜片和分光片都在常温环境中,图5为常温折反分光双波段示意图。
图 5 常温折反分光实现双波段系统
Figure 5. Dual-band detection system by optical splitter based on ordinary temperature refraction-reflection optical system
如前所述,低背景探测大部分以长波为主,这里分析也以长/长双波段探测系统进行比对分析,由于国内没有长/长叠层芯片,这里单波段芯片和叠层双波段芯片都以美国芯片能力为基础进行比较分析[3-5]。
首先,分析不同光学结构形式、不同探测器下的积分时间以及D*等参数进行理论推算,有些探测器指标根据国外资料推出。
(1) 积分时间理论计算
对于低背景探测系统而言,积分时间主要受到读出电路自身电荷存储能力、光电流以及器件自身暗电流决定的,具体如公式(4)和(5)所示:
$${e_{0{\text{光}}}} = \frac{{{Q_{\text{光}}}.{\tau _0}.{A_{\rm{d}}}.k}}{{4{{\left( {f/\# } \right)}^2}}}$$ (4) $${e_{\text{存储}}} = \left( {{e_{0{\text{光}}}} + {e_{0{\text{暗}}}}} \right) \times {t_{{\rm{int}}}}$$ (5) 式中:Q光为光学自辐射产生黑体辐射的光子流数;
${\tau _0}$ 为探测器的光电转换效率;Ad为探测器单位像元的面积;K 为探测器的像元填充率;f/#为系统参数;tint为系统探测积分时间;e0光为光学系统自辐射产生的光子流;e0暗为探测器自身暗电流水平;e存储为探测器电路电荷存储能力。而从国外资料可以看出长波暗电流一般都小于100 pA,基本可以忽略不计,影响探测积分时间长短主要受到光学系统自身热辐射引起的光电流影响。
因此,从国外文献上看,当光学系统达到183 K,单芯片积分时间可以达到6 ms,而如果用叠层芯片,理论上电容存储容量会被分割一半,积分时间变成3 ms;如果采用常温系统,如表1分析所示,光电流会增加4倍,单芯片积分时间理论上会变成1.5 ms;
(2) D*理论计算
美国的文献上一般用NEQ或NEI[3](噪声等效光子流数)来表征探测器能力,而国内用D*来表示,这俩之间的转换关系如公式(6)所示:
$${D^*} = \frac{{hc}}{{NEQ \times \lambda }}\sqrt {\frac{1}{{2{t_{{\rm{int}} }}{A_{\rm{d}}}}}} $$ (6) 式中:h为普朗克常数;λ为中心波长;c为光速。
通过公式(6)可以有效获得国外单波段探测器D*和叠层探测器D*,具体如表2所示。
表 2 D*分析
Table 2. D* analysis
Stacked dual FPA(183 K) Two FPA(183 K) NEQ(ph·cm2·s−1) D*(cm·Hz1/2·W−1) NEQ(ph·cm2·s−1) D*(cm·Hz1/2·W−1) Band 1: 4×1011;
Band 2: 6×1011Band 1: 4.2×1011;
Band 2: 3×1011Band 1: 1.22×1011;
Band 2: 2.15×1011Band 1: 8×1011;
Band 2: 5×1011当光学系统背景辐射提高,积分时间变短,系统的D*值会降低,具体见公式(7)所示:
$${D^*} = \frac{{\sqrt {({A_{\rm d}} \times 2\;1/{t_{{\mathop{\rm int}} }})} }}{{NEP}}$$ (7) 因此,当光子流提高4倍,积分时间降低4倍,D*值理论上降低至原来的1/2。
(3)系统灵敏度NEFD分析
根据上面的分析,可以获得系统的NEFD如表3所示。
表 3 三种不同双波段探测系统能力分析
Table 3. Ability analysis of three different dual-band detection systems
TYPE Ordinary temperature refraction-reflection dual-band detection system Dual-FPA detection system Low temperature refraction-reflection dual-band detection system Main parameter Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view:2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperature: 220 K;Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view: 2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperatur: 183 K;Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view: 2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperatur: 183 K;Theory parameter by calculation Int: 1.5 ms;
Band 1 D*: 4×1011 (cm·Hz1/2·W−1);
Band 2 D*: 2.5×1011 (cm·Hz1/2·W−1);Int: 3 ms;
Band 1 D*:4.2×1011 (cm·Hz1/2·W−1);
Band 2 D*:3×1011 (cm·Hz1/2·W−1);Int: 6 ms;
Band 1 D*: 8×1011 (cm·Hz1/2·W−1);
Band 2 D*: 5×1011 (cm·Hz1/2·W−1);NEFD(W/cm2) Band 1: 0.6×10−14 ;
Band 2: 0.85×10−14Band 1: 0.34×10−14;
Band 2: 0.6×10−14Band 1: 0.13×10−14;
Band 2: 0.26×10−14从表3可以看出:
(1) 相比常温折反分光系统,低温折反分光双波段系统由于提供较低等效背景,能获得高灵敏度(D*)和探测积分时间,从而提高系统灵敏度;
(2) 相比叠层芯片而言,由于低温折反系统单芯片有较高的填充率、电容存储能力以及较低暗电流,可以获得更高的灵敏度;
分析下来,低温折反分光系统可以充分发挥单芯片能力,也可以将辐射抑制下来,将每个多波段系统灵敏度做到与单波段系统的能力一致,具有很大的潜力和优势。
A novel technology on infrared multi-band low-background detection
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摘要: 红外低背景探测技术主要应用在深空环境,系统灵敏度与自身背景辐射关系较大,如何有效抑制探测系统自身背景辐射一直是重点研究方向;识别一直是红外领域的研究热点,增加探测谱段是提高特征获取量的最有效方式。介绍了一种局部制冷分光的红外多波段探测技术,该技术采用探测器一体化设计思路,将折反结构光学系统局部集成到探测器内部随红外芯片一起制冷;再利用分光元件进行双路分光,实现双波段能力,如果结合叠层芯片可以有效拓展到多波段能力;通过光学自辐射仿真,比较不同光学结构形式下的自辐射结果,可以看出:该技术背景辐射降低至常温折反系统背景辐射1/4,理论灵敏度可以大幅度提高,该技术优势明显,潜力巨大。Abstract: Infrared low-background detection technology is mainly used in space environment, the sensitivity of detection system has a gret relationship with the background radiation of detection system. How to effectively suppress the background radiation of detection system has always been the important research direction. And recognition has been a research hotspot in infrared field, adding the detection spectrum is the most effective way to get the features of target. An infrared multi-band detection technology based on optical lens local refrigeration and optical spiltter was introduced. Firstly, adopting the idea of integrated design with optical system and detector, integrate local optical system into the infrared detector and refrigeration with infrared FPA, then dividing the two lights by optical spiltter, which achieved dual-band ability, if the dual-band FPA was used in the future, the system can have the multi-band ability. Comparing the results of self-radiation under different optical types by the simulation of optical self-radiation, it can be seen that the background radiation of this technology is reduced to 1/4 of the ordinary temperature optical system, the sensitivity has also been greatly improved. This technology has obvious advantages and great potential.
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Key words:
- low-background detection /
- sensitivity /
- background radiation /
- infrared multi-band /
- refrigeration
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表 1 光学系统自辐射分析
Table 1. Self-radiation analysis of optical system
Parameter Wave band: 7.5-10.5 μm; Optical field of view: 2.5°×2.5°; f/#=2; Optical reflectivity: 99.2%;
Lens transmission: 98.5%; Metal absorptivity: 20%; Metal reflectivity: 80%Type Ordinary temperature optical system of refraction-reflection Coaxial four anti optical system Low temperature optical system of refraction-reflection Energy of self-radiation/W 2.6×10−5 1.47×10−5 6.5×10−6 Equivalent blackbody temperature/K 223 204 183 表 2 D*分析
Table 2. D* analysis
Stacked dual FPA(183 K) Two FPA(183 K) NEQ(ph·cm2·s−1) D*(cm·Hz1/2·W−1) NEQ(ph·cm2·s−1) D*(cm·Hz1/2·W−1) Band 1: 4×1011;
Band 2: 6×1011Band 1: 4.2×1011;
Band 2: 3×1011Band 1: 1.22×1011;
Band 2: 2.15×1011Band 1: 8×1011;
Band 2: 5×1011表 3 三种不同双波段探测系统能力分析
Table 3. Ability analysis of three different dual-band detection systems
TYPE Ordinary temperature refraction-reflection dual-band detection system Dual-FPA detection system Low temperature refraction-reflection dual-band detection system Main parameter Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view:2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperature: 220 K;Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view: 2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperatur: 183 K;Band 1: 7-8.5 μm;
Band 2: 10-11.5 μm;
Optical field of view: 2.5°×2.5°;
f/#=2;
Equivalent blackbody
temperatur: 183 K;Theory parameter by calculation Int: 1.5 ms;
Band 1 D*: 4×1011 (cm·Hz1/2·W−1);
Band 2 D*: 2.5×1011 (cm·Hz1/2·W−1);Int: 3 ms;
Band 1 D*:4.2×1011 (cm·Hz1/2·W−1);
Band 2 D*:3×1011 (cm·Hz1/2·W−1);Int: 6 ms;
Band 1 D*: 8×1011 (cm·Hz1/2·W−1);
Band 2 D*: 5×1011 (cm·Hz1/2·W−1);NEFD(W/cm2) Band 1: 0.6×10−14 ;
Band 2: 0.85×10−14Band 1: 0.34×10−14;
Band 2: 0.6×10−14Band 1: 0.13×10−14;
Band 2: 0.26×10−14 -
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