Avalanche photodiodes (APD) are the most common devices to detect single photon
states - in most quantum key distribution (QKD) systems, they are an essential
component. For operation at telecom-compatible wavelengths, APDs based in
InGaAs are commonly used. In their silicon-based counterparts, it is known for
a long time that with detection of single photons in an Geiger-mode
operation, light is emitted in during the detection process as a consequence
of carrier recombination in the junction. Such a light emission may provide a
vulnerability for a QKD system that uses the telecom wavelength range.

To our knowledge, no systematic investigation on this problem has been carried
out, even though the direct bandgap character of the InGaAs devices would
suggest this to be an issue. In this work, we determine the spectral character
of this effect, and provide a countermeasure for information leakage to a
potential eavesdropper in a QKD scenario.