A sensor using the backward stimulated Brillouin scattering (SBS) of an optical fiber. The pumping pulse light is launched at one end of the fiber and propagates in the fiber, while the CW light is launched at the opposite end of the fiber and propagates in the opposite direction. In this configuration, the pump pulse generates backward Brillouin gain in a single-mode fiber.
When the CW light frequency is different from the pump pulse light as same as the Brillouin frequency of the optical fiber, then the CW light is amplified through Brillouin interaction with the pump pulse. Also, the Brillouin frequency of a fiber is changed by the strain or temperature applied
on the fiber. Therefore, the Brillouin frequency shift can give the temperature or strain information as the following equation.

v(T)= v(0)(1 + C*T)

where T is temperature and CT is the temperature coefficient, which is known to be about 1 MHz/°C for conventional single mode optical fibers used at the 1.5
wavelength range of optical communication.

Fig. 1 fiber optic BOTDA sensor.