KEYWORDS: Analog electronics, Photon counting, Data acquisition, Signal detection, LIDAR, Sensors, Atmospheric sensing, Field programmable gate arrays, Radar, Signal processing
Using a single detector to realize the detection of wind Lidar in the upper atmosphere is the simplest signal detection mode, but the disadvantage of single detector is that it would limit the dynamic range of wind Lidar. When the detection area is 30km, the linear dynamic range of electronic system is required to be 5~6 orders of magnitude. In this case, it is difficult to satisfy the demand by using analog sampling or photon counting alone. In order to extend the dynamic range of wind Lidar based on single detector, we developed a set of wind Lidar readout electronics in the upper atmosphere based on single detector which has large dynamic range. The system of the detector used analog measurement and photon counting measurement at the same time, namely, analog sampling is adopted to measure the strong echo signals and photon counting is adopted to measure the weak echo signals. At the same time, the calibration laser energy is obtained by using the integral channel to improve the accuracy of Lidar calibration. After signal acquisition, data of analog sampling channel and photon counting channel are splicing through FPGA, and finally transmitted to the upper computer through Ethernet and UART. The results of electronics test show that the linear dynamic range of readout electronics system can reach 5~6 orders of magnitude through the combination of two channel signals, which can meet the design requirements. In addition, the high integration of this electronic readout system can meet the demand of miniaturization of current wind Lidar.
Ghost interference with entangled photon pairs are studied theoretically. The pump beam in parametric down-conversion is treated as the Gaussian profile, while for the function describing phase matching in the longitudinal direction, both a Gaussian and a sinc function are considered. The numerical results show that the transverse size of the pump beam and transverse coherence width of the parametric fluorescence strongly influence the interference pattern. With the increase of the pump transverse size and the decrease of transverse coherence width, the interference pattern becomes more and more prominent. When the transverse coherence width is small to 0.01 mm category, the Gaussian and sinc models give the same results.
The effect of pump focusing on the performance of ghost imaging is studied experimentally on an entangled source. Theoretical results show that the correlation properties of the entangled photon source are destroyed when the conversion crystal is pumped by a focused laser beam. The experiment is performed on a compact entangled source produced by type-II non-collinear degenerate SPDC, and the results demonstrate that the “walk off” effects almost have no effect on the image, while the pump focusing greatly degrades the visibility of the image. However, a sharp image could be reproduced in the configuration first proposed in Ref. [8] for the case with pump focusing.
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