ABSTRACT
Brillouin scattering in glasses is sensitive to strain and temperature in its frequency and intensity, whereas the Raman strain-dependent frequency shift is only readily detected in crystalline or reasonably ordered solid materials such as polymer fibres with aligned molecular chains. The spontaneous Brillouin backscatter signal is substantially stronger than the Raman anti-Stokes backscatter, although that advantage is mitigated by the weaker sensitivity of its intensity to temperature. The term Brillouin optical time-domain reflectometer (BOTDR) is used for systems based on time-domain interrogation of spontaneous Brillouin scattering (SpBS). The spectral width of the spontaneous Brillouin backscatter is dictated by the lifetime of the thermal phonons. High-spatial-resolution BOTDR sensors, where the probe power is limited by the stimulated Raman scattering threshold, can benefit from pulse-compression coding. Associated with the SpBS process is a stimulated process that results in gain at the Stokes wavelength.
