In BOTDR, the optical fiber material properties which are related to Brillouin scattering signal and Brillouin scattering light frequency are measured in BOTDR, which is mainly affected by temperature and strain. Therefore, distributed temperature and strain measurement can be realized by measuring the frequency shift of backward Brillouin scattering light of pulse light.
The continuous light from the narrow linewidth laser source is divided into two parts by the coupler. One part is modulated by an electro-optic modulator (EOM) into pulse light, which is incident to the sensing fiber, and the other part is used as the local oscillator to enter the reference optical path. The backscattered light generated by the pulse light enters the photodetector and detects the local oscillator coherent. The difference frequency component, namely Brillouin frequency shift signal, can be obtained by analyzing the Brillouin frequency spectrum, so as to obtain the temperature and strain information. The time delay between the back scattered light and the pulse light provides the measurement of optical fiber position information.