Research On Photon Counting Imaging Technology Based On GM-APD

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.