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The conventional terahertz radar suffers from limited operation range for long-distance, noncooperative target detection due to the low transmitter power and atmospheric attenuation effect, both of which pose a hindrance in meeting the requirements of warning detection applications. To improve the radar detection capability, this paper studies an ultrasensitive target detection method based on single-photon detectors to replace traditional radar receivers. The method is expected to considerably expand the operation range of terahertz radars. First, the statistical law of the number of echo photons of a terahertz single-photon radar system is analyzed, and the echo characteristics of the target are expounded from a microscopic perspective. Furthermore, a terahertz single-photon target detection model, incorporating the characteristics of a quantum capacitor detector, is established. In addition, the mathematical expression of the target detection performance is derived, and the performance is evaluated via simulations. Further, a target detection performance curve is obtained. Finally, a time-resolved terahertz photon-counting mechanism experiment is performed, wherein we realize high-precision ranging by counting echo pulses. This work can provide support for the research and development of ultrasensitive target detection technologies and single-photon radar systems in the terahertz band.

The paper analyzed influence of dead time on single sub-module of modular multilevel converter and reached conclusions that the sub-module will produce positive dead voltage from inserted state to cut-off state when arm current is positive and negative dead voltage from cut-off state to inserted state when arm current is negative. It analyzed influence of dead time on ideal fundamental voltage of modular multilevel converter, and deduced formulas of equivalent amplitude of output fundamental wave of dead voltage, actual amplitude of fundamental wave and offset of angle of output voltage, so as to reach conclusions that when voltage of DC side is fixed value, influence of dead time on modular multilevel converter relates to carrier wave ratio, dead time and phase-difference between output voltage and output current and unrelates to numbers of input module, improving modulation depth of voltage and reducing dead time can reduce influence of dead time on modular multilevel converter effectively. Simulation result validates effectiveness of the above conclusions.