Objective  Single photon ranging system is a system that uses the photon level detection sensitivity of GM-APD detector and obtains the distance information of far-field target based on the principle of photon time-of-flight ranging. The system has the advantages of long range and high ranging accuracy, and has great application potential in laser imaging, target detection and other fields. As the core module of single photon ranging system, the range and accuracy of time digital conversion circuit directly determine the working range and ranging accuracy of the ranging system. And in the field of spaceborne detection, the ranging system is required to have the ranging range of more than 100 km and centimeter-level ranging accuracy. At this time, the traditional time digital conversion circuit, limited by the counter number, in order to ensure the ranging accuracy, often lead to the limited system's measuring range, and if the system's range is increased, the system's ranging accuracy is difficult to meet the needs. In this paper, a time digital conversion circuit satisfying both wide range and high precision is designed to realize the wide range and high precision measurement of photon flight time interval. It provides technical support for single photon ranging in the field of spaceborne laser detection.

  Methods  In order to solve the contradiction between wide range and high precision, a refined time digital conversion circuit design method for long-distance single photon ranging is proposed. Firstly, based on the piecewise counting principle, a coarse and fine two-stage counting architecture is designed to ensure the wide range measurement of TDC (Fig.5, Fig.10). Secondly, for the fine counting unit, the clock equal phase shift π/N is used to generate an equivalent high-frequency pulse clock (Fig.6), which improves the timing accuracy of the fine counting unit by N times. The timing accuracy of the fine counting unit was further improved to 2N times by using the multi-counter double-edge interval counting method (Fig.7).

  Results and Discussions   Vivado software was used to verify the accuracy and range of the proposed algorithm, and the simulation results of the accuracy (Fig.11) and range (Fig.12) were given. In order to further verify the performance of GM-APD laser ranging system designed in this paper based on equivalent pulse two-stage time digital conversion circuit, the indoor and outdoor ranging experiments were carried out respectively, and the indoor ranging results (Tab.1) and outdoor ranging results (Fig.16-17, Tab.3) were given. The experimental results show that the coarse and fine two-stage counting system can effectively reduce the limit of TDC timing accuracy to its range. The timing resolution of TDC is 416.67 ps, and the ranging range is 196.608 km, which solves the contradiction between wide range and high precision.

  Conclusions  A fine time digital conversion circuit for distance single photon ranging is designed, and a TDC module with high precision, high stability and easy to expand is implemented on FPGA. By using the TDC design method of coarse and fine two-stage counting, the clock signal of the same frequency phase shift and equal phase difference, and the two-side edge precision time counting, the wide range and high precision measurement of photon flight time are realized. The structure of the proposed refined time digital converter circuit is relatively simple and flexible. The number of counter bits is 16 and the number of clocks with equal phase shift in the same frequency is 6. The range and timing accuracy of TDC conversion can be further improved by increasing the number of counter bits and the number of clocks with phase shift.