  Quantum key distribution (QKD) at telecom wavelengths (1260-1625nm) has the
potential for fast deployment due to existing optical fibre infrastructure and
mature telecom technologies. At these wavelengths, indium gallium arsenide
(InGaAs) avalanche photodiode (APD) based detectors are the preferred choice
for photon detection. Similar to their silicon counterparts used at shorter
wavelengths, they exhibit fluorescence from recombination of electron-hole
pairs generated in the avalanche breakdown process. This fluorescence may open
side channels for attacks on QKD systems. Here, we characterize the breakdown
fluorescence from two commercial InGaAs single photon counting modules, and
find a spectral distribution between 1000nm and 1600nm. We also show that by
spectral filtering, this side channel can be efficiently suppressed.
