  Phase drift and random fluctuation of interference visibility in double
unbalanced M-Z QKD system are observed and distinguished. It has been found
that the interference visibilities are influenced deeply by the disturbance of
transmission fiber. Theory analysis shows that the fluctuation is derived from
the envioronmental disturbance on polarization characteristic of fiber,
especially including transmission fiber. Finally, stability conditions of
one-way anti-disturbed M-Z QKD system are given out, which provides a
theoretical guide in pragmatic anti-disturbed QKD.
