## 2 Fundamental Principles of Optical Backscatter Reflectometry  
Strain or temperature profiles along an optical fiber can be measured by Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR). OTDR commonly utilizes a laser pulse and time of flight measurements are then used to calculate the location along the fiber. OFDR however utilizes a swept frequency pulse that interacts with the fiber and a Fouriertransformation yields the location. Both methods can generally be used with sensors based on Raman, Brillouin or Rayleigh scattering. The fundamental principles of the system, which use sensors to detect Rayleigh or Brillouin scattered light, have been described in detail previously [17; 18]. OFDR utilizing Brillouin scattering has been used to monitor the strain development during cure in composite structures [19; 20]. This however was not done in pure epoxy as presented in this work and the spatial resolution of some centimeters was worse than for the results presented here. Optical Backscatter Reflectometry is an OFDR technique to locally determine strain or temperature along the entire length of an optical fiber. In principle, this is also achievable with fiber Bragg gratings but in technical applications many of those are needed and to be applied/embedded at the correct location to yield the same information as OBR. This is often not feasible in large volumes or complicated geometries. With OBR these limitations can be overcome. It especially allows long distance, one-dimensional strain measurements with high spatial resolution, which makes it possible to measure along critical points of the geometry of interest, and can thus reveal strain gradients in a product. In general, even pseudo-2D/3D measurements are possible [21]. OBR is also classified as distributed fiber-optic sensing because relative measurements of backscattered light are used [16; 22; 23; 24]. OBR allows much better evaluation of strain fields along the length of the optical fiber than other approaches. The spatial resolution is as good as 1 mm over the whole length of the fiber and strain values are determined at each point of the fiber. Thus, strain/shrinkage is determined neither as average over extended areas nor in just one point as in the methods described above. After a sufficiently small and easy to use instrument became available [25], this method became easily applicable outside of research laboratories. Each optical fiber used in OBR contains natural characteristic impurities that act as a \"fingerprint\" of each part of the fiber. When a laser light pulse is sent into the fiber, these impurities lead to Rayleigh backscattering. The arrival time and intensity of the returned backscattered light is recorded. If light is backscattered from a part of the fiber further away from the light source, it arrives later at the detector than light backscattered from parts close to the source. By using this time difference it is possible to localize the reflected signal along the length. Due to the uniqueness of the impurities, the backscattered light amplitude pattern is different for each fiber. Local temperature changes lead also to changes in the signal, due to changes in the (local) reflective index of the fiber. As long as no mechanical load is applied to the fiber, the change in temperature and in the backscattered signal are linearly correlated. However, if both kinds of loads are applied to the fiber, the signal-change is a linear combination of both mechanical and thermal load. If the thermal behavior of the fiber is known and independent temperature measurements are carried out along the fiber, one can separate the influence of these two effects onto the strain signal. However, for the here presented experiments and results, the influence of the temperature is so small that this would not change the results significantly. This is shown in Appendix A. Usually, a reference measurement is carried out before the experiment is started. This acts as a record of the locations of the impurities.