ABSTRACT
The simplest approach to distributed vibration sensing (DVS) using coherent Rayleigh backscatter is to use a highly coherent source in an otherwise conventional single-mode Optical Time-Domain Reflectometry (OTDR) arrangement. Rayleigh backscatter is rather different from Raman and Brillouin scattering. It relies on subtle variations of the refractive index that are fixed rather than transient, as is the case for inelastic scattering. When Rayleigh backscatter is used for distributed vibration sensing, this pseudo-grating provides a pair of reflectors on either side of the disturbance that is measured. The main difference between the needs of OTDR and distributed vibration sensing is, of course, that in DVS, the backscatter data from each probe pulse must be acquired and processed location by location as a function of pulse number. Heterodyne DVS (VS) differs from the approaches previously discussed in that the phase comparison is not carried out directly in the optical domain.
