Subnanosecond single-photon timing with commercially available germanium photodiodes

We demonstrate that commercially available germanium avalanche photodiodes can achieve single-photon sensitivity and subnanosecond time resolution at 77 K. Experiments show that carrier trapping phenomena give the main contribution to the detector noise. Therefore technological efforts will be welcome to overcome the present limitations that are due to the material quality. These devices are suitable for the detection of short optical pulses in the near-infrared range and in optical time-domain reflectometry characterization of short optical fibers with centimeter resolution at 1.3-1.5 microm. Futher improvements in the timing performance can be obtained by suitable device design.