【作者】 尹丽菊；

【导师】 陈钱；

【作者基本信息】 南京理工大学 ， 电子科学与技术， 2012， 博士

【摘要】 为了在更低照度下,微光成像系统能够获取反映目标特征的高质量图像,主要要求实现光电转换和倍增的光电器件具有更高的探测灵敏度和更高的信噪比。为此,各种类型的光电器件层出不穷。工作在盖革模式下的雪崩光电二极管(Avalanche Photodiode in Geiger Mode, GM-APD)以其单光子探测能力、高信噪比、全固态结构、快速响应、低磁场敏感度、低功耗和独特的脉冲式输出等特点,成为微光探测领域研究的热点。本文围绕GM-APD的特性和它在光子计数成像中的应用展开研究,从理论上分析了GM-APD的电气特性和光学特性,建立了GM-APD的等效电路和探测电路模型并对模型进行电气特性的仿真研究。计算了夜天光环境下不同材料的GM-APD产生的光电子分布。基于统计光学理论和蒙特卡洛方法建立了GM-APD成像仿真模型。以理论研究和仿真模型为依据,设计并建立了GM-APD光子计数成像实验平台,实现了微光环境下目标的成像探测,得到了质量良好的光子计数图像。同时,借助平台进行了大量的实验,实验结果验证了论文中理论分析和仿真模型的正确性。GM-APD独具的优良性能与构成器件的材料和结构密不可分,半导体材料的受光激发导致共价键断裂机制、雪崩倍增机制和GM-APD具有浅结工艺的全固态结构,为GM-APD的电气和光学特性奠定了基础。微光环境下,以光的量子本性和GM-APD的工作过程,建立了GM-APD的等效电路模型和由其构成的探测电路仿真模型。通过改变模型中入射光强度和频率、反向偏置电压、负载电阻和分布电容的设置,分析了电路中输出电流、输出电压和死时间的变化情况。提高入射光强度和频率, GM-APD的输出电流也随之增大。反向偏置电压越大,产生相同倍增电流输出所需要的时间越短,雪崩倍增的强度也越大。分布电容、负载电阻越大,输出的雪崩信号也越大,雪崩信号脉冲宽度加宽。但是分布电容、负载电阻的提高,将会导致死时间的延长,从而降低GM-APD探测电路捕获光子的数量。本文研究了满月光、浓云满月光、晴朗星光和浓云无月光环境下夜天光辐射的光子数分布,得到了单位面积单位时间内夜天光单色辐射光子数范围为2.53×1012至6.96×1015。不同天气条件下,夜天光总的辐射光子数变化范围从3.76×1015到5.75×1016。根据GM-APD的像元面积、量子效率,计算了Si GM-APD和InGaAs GM-APD在单位时间内接收的夜天光辐射光子经光电转换后产生光电子数的分布。Si GM-APD能充分利用月光下的辐射光子。器件在满月光、浓云满月光、晴朗星光和浓云无月光环境下产生的总的光电子数分别为1016338、254084、17303和3460。InGaAs GM-APD在四种天气条件下对0.8μm-1.7μm波段的辐射光子都有较强响应,产生的总的光电子数分别为602325、150581、362302和72460；Si GM-APD在该波段产生的光电子数分别为80584、20146、5556、1111。GM-APD单光子计数器工作于门控方式,根据统计光学理论,确定GM-APD单光子计数器输出的光子计数值为离散型随机变量。光子计数平均值与探测点处的入射光强、探测时间、探测器光敏面的面积和量子效率成正比,与入射光的频率和普朗克常数成反比,光子计数平均值可以反映入射光场特性。采用蒙特卡洛方法建立成像仿真模型,通过仿真获得了光子计数图像,分析了时间、光照强度和器件量子效率对成像结果的影响。仿真结果为基于GM-APD的光子计数成像系统的搭建打下了理论基础。在理论分析和仿真研究的基础上,设计并建立了基于GM-APD的光子计数成像实验平台。利用平台进行了GM-APD光子计数值与光照关系标定和分辨率测试。测试数据显示在不同光照环境下光子计数值与照度呈分段线性关系,平台最小分辨角为2.0608°。在2.3×10-5lx照度下,对目标进行成像探测,得到了质量良好的光子计数图像。研究了实验平台参数设置的变化对光子计数图像的影响,对绿色植物、枯草和混凝土在夜天光光谱范围和单色光谱进行成像研究。实验结果证明,基于GM-APD的光子计数成像探测方案切实可行,它为微光成像探测领域提供了一种提高成像系统探测灵敏度和信噪比的有效方法。更多还原

【Abstract】 In order to obtain the high quality image that presents the features of the target in low-light condition, low-light imaging system requires photoelectric devices which achieve photoelectric conversion with multiplication demand higher sensitivity and higher singal to noise ratio. Therefore, a various kinds of photoelectric devices have been developed. Among them, avalanche photodiodes in Geiger mode have gained significant interests in the field of low-light detection due to its single photon detection capability with high SNR, all-solid-state structure, rapid response, insensitive to magnetic field, low power consumption and unique pulse outputs. The characteristics of GM-APD and its application in photon counting imaging were studied in this work. The electrical characteristics and optical properties of GM-APD were analyzed in the theory. The electrical characteristics were simulated by an equivalent circuit model and a detection circuit model. The distributions of the photonelectron generated by the different GM-APDs were calculated in the various spectral conditions of night sky radiation. An imaging simulation model based on GM-APD was established according to statistical optics theory and Monte Carlo method. A photon counting imaging test platform based on GM-APD was designed and setup. The imaging detection of the target can be achieved on the platform in the low-light environment. The good quality photon counting images were obtained. At the same time, a large number of experiments on the platform verified the results of theoretical analysis and the simulation results validity of the model.The excellent performances of GM-APD come from the material and structure of the device. The electrical and optical properties of GM-APD base on covalent bonds rupture mechanism, avalanche multiplication mechanism, as well as all-solid-state structure of GM-APD with a shallow junction. In the low-light environment, combining quantum nature of photons and the work process of GM-APD, an equivalent circuit model and a detection circuit model were put forward. The output current, output voltage and dead time were analyzed with respect to the change of incident light intensity and frequency, reverse bias voltage, load resistance and parasitic capacitance. The output current of GM-APD increased with increasing the intensity and frequency of incident light. The Larger the reverse bias voltage, the shorter the time required to produce the same output current; the larger the avalanche multiplication. With increasing the parasitic capacitance or load resistance, the amplitude and wildth of the GM-APD output pulse increased. However, as the value of the parasitic capacitance and load resistance increased, the dead time increased correspondingly, which resulted in reducing detectability of GM-APD on photons.The photon distributions of night sky radiation were studied in full moonlight, heavy cloud full moonlight, clear starlight and heavy cloud no moonlight conditions. The density of of the photon ranged from 2.53×1012/m2·s·μm to 6.96×1015/m2·s·μm under monochromatic radiation. The number of the total photon was from 3.76×1015/m2·s to 5.75×1016/m2·s in the night sky spectrum on different weather. The number of the photoelectron per second generated by photoelectric conversion in Si GM-APD and InGaAs GM-APD were calculated according to GM-APD’s pixel size, quantum efficiency. Si GM-APD made good use of the photons radiated in moonlight condition, the number of the photoelectron was 1016338, 254084,17303 and 3460 per second in night sky spectrum in full moonlight, heavy cloud full moonlight, clear starlight and heavy cloud no moonlight conditions, respectively. InGaAs GM-APD had a strong response from 0.8μm to 1.7μm. For InGaAs GM-APD (Si GM-APD), the number of the photoelectron was 602325,150581,362302, and 72460(80584,20146, 5556 and 1111) per second on the above four weather, respectively.GM-APD single photon counter was operated in gated mode. The photon counting values were discrete type random variables according to statistics optical theory. It could be concluded that the average photon counting values were proportional to the incident light, detection time, photosensitive surface area and quantum efficiency of GM-APD and inversely proportional to the frequency and Planck’s constant. The characteristics of incident light field can be reflected by the average photon counting values. The Monte Carlo method was adopted to develop an imaging model. The photon counting images were obtained by the simulations. Furthermore, the time, light intensity and quantum efficiency of the device were changed to observe the effects on the simulation results. The theoretical foundation was supplied for the design of the photon counting imaging system based on GM-APD.The GM-APD-based photon counting imaging test platform was designed and established based on the theoretical analysis and simulation studies. The photon counting values and illumination calibration and resolution test were achieved on the platform. The testing data showed that the relationship between photon counting values and illumination was piecewise linear in different illumination conditions, the minimum resolution angle was 2.0608°. With the image printed as the target, the good quality photon counting images were obtained at the illumination of 2.3×10-5 lx. The illumination and sampling time were changed to observe the influence of these parameters on the imaging quality. The hay, green plants and concrete were imaged in the night sky spectrum or at a certain wavelength. The experimental results showed that the scheme of GM-APD-based photon counting imaging was practicable. It provided an effective method to improve detectivity and SNR for the low-light imaging.更多还原