Fundamentals of Rayleigh Scatter Based Fibre Optic Sensing

   In Rayleigh scatter based distributed optical fiber cable sensing, a coherent laser pulse is sent along an optic fiber cable, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fibre. Changes in the reflected intensity of successive pulses from the same region of fibre are caused by changes in the optical path length of that section of fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made simultaneously at all sections of the optical fiber cable.