## 5 Brillouin Optical Correlation Domain Reflectometry
### Discriminative and Distributed Measurement of Strain and Temperature with BDG-BOCDA Systems
In the simulations, the correlation peak position is set at the center of the strain-applied section, and \(d_{z}\) is set to be eight centimeters. Figure 44(b-1) shows the BDG reflection spectrum when the strain-applied section is five times longer than the BOCDA spatial resolution. The two peaks are observed in the figure. The left and the right peak correspond to the unstrained portion and the strained portion, respectively. The height for the unstrained portion is higher than that for the strained portion, which means that the effective spatial resolution is worth more than five times the length of the resolution of \(d_{z}\). When the length of the strained portion is 15 times \(d_{z}\), the two peaks are almost the same, which indicates that the equivalent spatial resolution is almost \(15d_{z}\). This coincides well with the experiments. When the strain-applied length is elongated more, as shown in Figure 44c, the peak for the strained portion becomes larger. From the simulations and experiments, it is known that the equivalent spatial resolution for the BDG distributed measurement by the BOCDA is worth more than that of the BFS measurement [108]. The reason for the degradation of the spatial resolution was thought to be attributed to the side lobes existing on both the sides of the correlation peak position. We assumed that the side lobes do not form the mountain-shaped background spectrum, but rather form a single peak-shaped one, due to the FM modulation onto the reading lightwave. If this assumption is fine, we have the possibility of improving the spatial resolution by reducing the effect of the side lobes through using the IM scheme, which is mentioned above for BOCDA and BOCDR. Figure 45 shows an experimental setup of the BDG-BOCDA system, in which an IM modulator is set to apodize both the x-probe and the x-pump wave [109]. Figure 46a shows the experimental results for the sample fiber, which is the same as the fiber shown in Figure 44 [109]. The stain-applied portion in the simulation is realized to put the portion into the cooled water, and  
Figure 43: Experimental demonstration of distributed and discriminative measurement of strain and temperature [18]. (**a**) Measured distribution of BFS and birefringence-induced Bragg frequency difference. Blue data; B, D, E, F, and H sections are heated, but no strain is applied. Red data; temperature distribution is the same, but strain is applied at only the E section; (**b**) Strain and temperature distribution calculated by Equation (5). the portion length is changed in the experiments. The correlation peak is set at the center of the cooled section. Figure 46a shows the BDG reflection spectrum, which is the same as that in Figure 44. The two peaks' power are plotted in Figure 46b. The cross point of the two curves shows the equivalent spatial resolution. Without the IM, the resolution is 75 cm. On the contrary, when introducing the IM, the resolution is improved to be 17 cm, as shown in Figure 46c,d. An improvement by a factor of around five has been realized with the IM scheme [106]. Recently, much improvement has also been shown by applying the IM onto the reading wave [110]. Besides the BOCDA-BDG system shown in Figure 42, other configurations have also been constructed, including the system with only one LD, which can form all the lightwaves of the X-pomp, X-probe, and the Y-read. An example is shown in Figure 45.  
As for the BDG-BOCDA system, performance improvement schemes have already been introduced [111; 112; 113]. Additionally, a prototype model has already been fabricated for the application of the aircraft SHM. In flight conditions, the strain and temperature discriminative and distributed measurement have been successfully demonstrated [74]. The spatial resolution for the BFS measurement is three cm, and the total measurement range is 500 m when using the temporal gating scheme. The sampling rate for the random access  
Figure 44: Simulation results of the BDG reflection spectrum shape.