Characterization of single-mode fiber coupled single photon detectors operating at telecom wavelengths (e.g. based on InGaAs SPADs). The following quantities can be measured: (1) detection efficiency at 1550nm, (2) dark count rate, (3) dead time and (4) jitter. Additionally, an estimation of the after-pulsing probability can be provided. The detector should be free-running (for gated operation contact us) and single-mode fiber coupled.