Photon number adaptive single-photon detection with time-walk passive compensation

In long-distance laser time transfer, such as satellite-to-satellite and satellite-to-ground, the accuracy of flight time measurement for photon pulses is significantly compromised by the time walk phenomenon, stemming from variations of the incident photon numbers. In this paper, we propose a single-photon detection method, which is adaptive to the photon number, and passively compensates for the time walk effect. This method utilizes a fiber ring to divide an incident photon pulse into a series of photon pulse trains, with each pulse in the train separated by equal time intervals and subject to equal attenuation. The last detected photon pulse in the pulse train can always be attenuated to a single-photon level sufficient to render the time walk effect negligible. The experimental results demonstrate that the proposed method effectively mitigates the time walk effect across a range of average photon numbers, spanning from 1 to 100. It provides high-precision single-photon detection for laser time transfer in complex and variable environments.