An analysis of the most critical issues in quantum
key distribution systems with polarization encoded photons is
presented. Effects like the fiber birefringence, fiber losses and
some technical aspects can difficult the state of polarization
control in such kind of systems, leading to an undesired quantum
bit-error-rate increment. It is shown that a system with a long
fiber, presenting a typical PMD value, only assures a strong
correlation between two Stokes vectors for narrow wavelength
separations. In terms of time correlation, it is shown that the
typical drift times are much longer than delays between reference
and data signals. The problematic of the reference pulse leakage
to the data detector is also analyzed; a good agreement between
experimental and theoretical data is observed