A DAQ system with low-dead-time, high-precision TDC for APD detection efficiency calibration

Correlated photon calibration based on spontaneous parametric down-conversion (SPDC) provides a highly precise means to calibrate the detection efficiency of avalanche photodiode (APD). During the calibration process of detection efficiency via SPDC, precise measurement of the arrival time of correlated photons and accurate photon counting are essential. To achieve this goal, a acquisition (DAQ) system with low-dead-time, high-resolution Time-to-Digital Converter (TDC) designed in this paper. This TDC is designed based on tapped delay line (TDL), which is implemented on Xilinx Kintex-7 series field programmable gate array (FPGA). This TDC can accurately measure the time of arrival of input signals. Upon arrival of a photon signal, the TDC rapidly generates timestamp to record the arrival time of the photon signal. Utilizing these timestamps, time delay accurate measurements of time intervals can be achieved. The specially designed TDC input structure and encoding algorithm enable the alternating propagation and sampling of '01' and transitions on TDL. Coupled with a pipelined architecture, the TDC's dead time is close to one cycle, which is 2.33 ns in the current implementation version. The structure of mode recognition triggering ensures that the TDC can correctly calibrate time measurements and count photon counts. Experimental results show that the TDC achieves an RMS precision of better than 11.08 ps, and measurement precision is maintained over long time intervals (0-10 us). The reliability of this for photon counting has been verified through standard signal sources.