The technology of Polarization Optical Time Domain Reflectometer (POTDR) can be used to obtain the external
events' information by measuring the change of state of polarization (SOP) of the Rayleigh backscattering in fiber. When
the fiber is disturbed at two different positions simultaneously, we analyze the frequency spectrums of the change of
Rayleigh backscattering light which are obtained by POTDR theoretically for ideal fiber, and by numerical simulation
for single mode fibers. We find that the frequency spectrums between the first and second events contain the first
vibration's frequency and its frequency multiplication. The frequency components of the spectrums after the second
event are the linear combination of the first and the second events' frequencies. So we can obtain the location and the
frequency information of the two events by analyzing the frequency spectrums. In addition, the frequency distribution in
the frequency spectrums from different positions are different because of the different initial SOPs at different positions.
So all the actual frequency information can not be obtained from only one frequency spectrum. We add up the frequency
spectrums from the positions within a beat length to obtain the perturbation's frequency and the method can reduce the
misdiagnosis rate because the sum of the frequency spectrums contains all the initial SOP within a beat length.
The residual axial strain induced in the cabled fiber is a vital factor to evaluate the reliability of optical fiber cables.
However, little is known about the thermal effect on the fiber residual strain, especially on the particular short section of
the fiber. In order to precisely determine the thermal effects on fiber residual strain of optical cables, a measurement
method based on Brillouin Optical Time-Domain Reflectometry (BOTDR) system is proposed. Thermal characteristics
of fiber residual strain of optical fiber cables are investigated theoretically and experimentally. Measurements of the
residual strain in particular short section of fiber are made with the distributed fiber sensing system based on BOTDR.
Experimental results for a double-coated fiber in loose structure cable are in good agreement with those predicted from
the theory. It has been found that the fiber residual strain increases linearly with the decrease of the temperature in the
range from 50°C to -50°C.