In the measurement of high-order photon correlation in light fields, high-precision time-to-digital converters (TDCs) are required to accurately measure the photon arrival time. The purpose of this study is to develop a multichannel high-precision TDC acquisition system that can accurately measure the photon arrival time using a field-programmable gate array (FPGA) hardware platform to support the high-order coherence of the light field, which can be accurately obtained via high-order photon correlation measurements of the light field.

In this study, a multichannel photon-arrival-time measurement design was implemented using XC7A100TFGG484 of the Artix-7 series. By combining “coarse counting” and “fine measurement” and developing the CARRY4 two-tap (CO0, CO3) structure, we constructed a tap delay chain for fine measurement, which improves the resolution of the TDC while overcoming the limitations of overfeeding. The dead time of the TDC was reduced to one system clock cycle using a switching input stage and dual-mode single-counter coding. The nonlinear delay of the delay cell was calibrated using the code-density calibration method, and the data were transmitted over gigabit ethernet. Subsequently, a test platform was constructed to continue implementing testing and data analysis for the TDC. Finally, the TDC device was verified using a time-correlated single-photon counting measurement system.

Accuracy was tested using a signal generator to generate a set of fixed time intervals from small to large to obtain the variation in the accuracy with the time interval. The maximum and minimum accuracies are approximately 25.6 and 20.1 ps, respectively (Fig. 10). The accuracy of the system was tested and counted for each of the eight channels, and the accuracy of the system was obtained as 26.3 ps (Fig. 11). The accuracy at different temperatures was measured at a time interval of 90.91 ns, and the variation in the accuracy error over the temperature range was 2.6 ps (Fig. 12). The results clearly indicate the low temperature sensitivity of the TDC designed in this study. The dead time was reduced to 5 ns in one system clock cycle by adding a switching input stage structure. The CARRY4 two-tap (CO0, CO3) structure was used to construct the tap delay chain, and a resolution of 34.7 ps was achieved. The differential nonlinearity (DNL) and integral nonlinearity (INL) of the system were obtained via code-density tests. For the 1-mode, the DNL and INL ranges are (-0.75t_LSB, 1.5t_LSB) and (-2t_LSB, 2.8t_LSB) , respectively. For the 0-mode, the DNL and INL ranges are (-0.76t_LSB, 1.32t_LSB) and (-1.2t_LSB, 1.9t_LSB), respectively (Fig. 13).

In this study, a TDC system for multichannel photon-arrival-time measurement was implemented based on an FPGA. The over-advanced feed of the delay unit and the issue of its nonlinear time delay being affected by temperature and voltage were addressed by performing calibration using a single CARRY4 two-tap (CO0, CO3) structure and the code-density calibration method, which in fact resulted in fine counting. An 8-bit 200 MHz system clock was used for coarse counting, and a combination of coarse and fine counting was performed to achieve high-precision measurements. The experimental results show that the developed eight-channel TDC system exhibits an average resolution of 34.7 ps, a timing accuracy of 26.3 ps, a dead time of 5 ns, as well as DNL and INL ranges of (-0.75t_LSB, 1.5t_LSB) and (-2t_LSB, 2.8t_LSB) for the 1-mode, respectively. For the 0-mode, the DNL and INL ranges are (-0.76t_LSB, 1.32t_LSB) and (-1.2t_LSB, 1.9t_LSB) , respectively. The TDC system implemented in this study combines the advantages of multichannel, high accuracy, and short dead time. Experimental validation was performed via time-correlated single-photon counting measurements, which indicated the practical requirement for higher-order photon-correlation measurements of the light field.