Abstract: Optical time domain reflectometry methods and apparatus are proposed in which the back-scattered optical radiation used to produce output signals is restricted to that resulting from Rayleigh scattering of light launched into a fiber 2 at a first wavelength and that in an anti-Stokes spectral band resulting from Raman or Brillouin scattering of optical radiation at the first wavelength. A first set of output signals produced in dependence upon the anti-Stokes back-scatter may be normalized to the geometric mean of a second set of output signals, produced in dependence upon the Rayleigh back-scatter at the first wavelength, and a third set of output signals, produced in dependence upon Rayleigh back-scatter resulting from light launched into the fiber at the anti-Stokes wavelength.

Abstract: An optical fibre communication network comprising one optical source (1) connected to one end of an optical fibre (3), and several optical receivers (9) for detecting light scattered to the side of the optical fibre (3), the network being characterized by the fact that each optical receiver (9) is sufficiently sensitive to require for reliable communication only light lost by the fundamental scattering of the fibre (3) during normal propagation of the light originating in said optical source (1) while travelling in the vicinity of the optical receiver (9).

Abstract: An optical time-domain reflectometry method of sensing respective values of a physical parameter, such as temperature, at different locations along an optical fibre, in which back-scattered optical radiation is used to produce output signals indicative of the values being sensed, is improved by employing optical filtering means to ensure that the back-scattered radiation used is restricted to a preselected single spectral line resulting from inelastic scattering in the fibre, for example one of the Anti-Stokes Raman lines, or to two mutually adjacent such lines.