  Spectral loopholes in the 1000-2100 nm range have been the focus of attention
of quantum hacking research aimed at searching and elimination of
vulnerabilities in quantum key distribution systems. In this work, we
concentrate on shorter wavelengths ranged from 400 nm up to 800 nm and
experimentally study spectra of insertion losses for a number of fiber optical
components to assess potentials for their implementation as countermeasures. We
show that efficiency of the elements commonly used as countermeasures against
hacking attacks in the telecom range can be significantly impaired in the
visible range. It is found that the vulnerabilities detected in this range play
an increasingly important role for hacking strategies such as the
induced-photorefraction attack whose efficiency improves when the wavelength
decreases.
