  The maintenance and repair of optical fiber networks often requires the
discrimination of underground cables among a group of buried ones. Presently,
the methods commonly used are either inefficient or harmful to the fiber cable.
In this paper, we evaluate the effectiveness of fiber optic vibration sensing
method on underground fiber cable discrimination. We find that the typical
vibration sensing method-distributed acoustic sensing (DAS) is not suitable for
fiber cable discrimination. Especially for long distance scenario, due to
"frequency grafting" effect, the DAS method will give a false response on the
spectrum of knock events during cable discrimination process. In response, we
propose an underground fiber cable discrimination method based on laser
interferometer, and demonstrate it on the 40-km urban fiber cable connecting
Tsinghua University and Yongding Road. It can give an obvious and real response
to the knock event, and can be used in practical applications.
