Multimode fiber influence on single-photon avalanche diode timing jitter

The timing jitter of a single-photon avalanche diode (SPAD) can be a limiting factor for the performance of quantum key distribution (QKD). Within QKD receiver modules, single mode fiber coupling has been extensively studied due to optical fiber-based demonstrations. However, multimode fiber, which is utilized for free-space demonstrations because it reduces the coupling losses, has been less extensively studied, but it is understood that multimode fibers will increase the timing jitter and decrease the performance of the detector as the fiber core diameter increases. Here, we present a collection of experimental analysis over the relationships between characteristics of multimode fiber such as length, core diameter and incident spot size and how this impacts the timing jitter of the detector. Additionally, we present analysis on the use of graded-index multimode fibers, mitigating for a portion of the impact on quantum bit error rate (QBER) due to multimode characteristics which can improve implementation of these receiver systems.