We found that background radiation limits the dark count rates of superconducting single photon detectors coupled to standard single mode optical fibers to a minimum level when the source temperature of the photons is close to 300 K. We measured this level to be 103 cps, which was confirmed by a theoretical analysis of the background radiation influence. We also investigated the filtering-effect of cooled single mode optical fibers with different bending diameters and showed that for superconducting photon receivers with operating wavelengths below 2 µm the minimum dark count rate can be significantly decreased down to 0.1 cps.
Studies of electronic transitions in the photoconverters with In0.4Ga0.6As quantum well-dots (QWD) layers have been carried out. It is shown that the quantum yield and electroluminescence spectral peaks are well described by e1-lh1 and e1-hh1 optical transitions in the quantum well with the same average composition and thickness. The energy of the optical transitions shifts toward longer wavelengths with an increase in the number of QWD layers. The calculated shifts of electron and hole levels due to the redistribution of elastic strain between In0.4Ga0.6As QWDs and GaAs spacer layers demonstrated a very good agreement with the experimental data.