  The temperature of the semiconductor diode increases under strong light
illumination whether thermoelectric cooler is installed or not, which changes
the output wavelength of the laser (Lee M. S. et al., 2017). However, other
characteristics also vary as temperature increases. These variations may help
the eavesdropper in practical quantum key distribution systems. We study the
effects of temperature increase on gain-switched semiconductor lasers by
simulating temperature dependent rate equations. The results show that
temperature increase may cause large intensity fluctuation, decrease the output
intensity and lead the signal state and decoy state distinguishable. We also
propose a modified photon number splitting attack by exploiting the effects of
temperature increase. Countermeasures are also proposed.
