# Recent Advances in Brillouin Optical Correlation-Domain Reflectometry
## 4 Development of Slope-Assisted BOCDR
### Detection of Extremely Short Hot Spots
The power-change distributions measured when the length of the strained section \(L_{\varepsilon}\) was reduced from 1.00 m to 0.01 m are shown in Figure 14a. Note that the nominal spatial resolution was 1.01 m  
Figure 13: **(a)** Structure of a silica SMF under test. The length of the strained section \(L_{\varepsilon}\) was set to 0.01, 0.02, 0.05, 0.10, 0.20, 0.50, and 1.00 m. **(b)** Structure of a silica SMF under test, which was locally strained for 0, 0.10, 0.15, and 0.20%. **(c)** Structure of a POF under test, which was locally heated to 26, 35, 45, and 55 \({}^{\circ}\)C. Reproduced with permission from [61]. Copyright (2016) JSAP. throughout the measurement. The vertical axis was normalized so that the maximal power-change became 1 when \(L_{\varepsilon}\) was 1.00 m. Each distribution was displayed with a vertical interval of 0.5. When \(L_{\varepsilon}\) was 1.00 m, the power-change distribution showed a triangular shape. As \(L_{\varepsilon}\) decreased, the maximal power-change also decreased. When \(L_{\varepsilon}\) was 0.02 m, the power-change was still clearly detected, but it became almost comparable to the signal fluctuations when \(L_{\varepsilon}\) was 0.01 m. Thus, in this experimental condition, a strained section, which was over 50 times shorter than the nominal spatial resolution, was shown to be detectable. Figure 14b shows the normalized maximal power-change plotted as a function of \(L_{\varepsilon}\). The error bars were standard deviations of the signal fluctuations at non-strained sections. The maximal power-change decreased almost linearly with decreasing \(L_{\varepsilon}\), which agrees well with the theory. The observed length of non-zero-power-change section was almost equal to the sum of the strained length \(L_{\varepsilon}\) and the nominal spatial resolution, which also agrees with the theory. Finally, exploiting the beyond-nominal-resolution effect verified above, we attempted to detect the shortest-ever strained and heated sections along a silica SMF and a POF. Figure 15a shows the normalized power-change distributions measured when the strain of 0.10, 0.15, and 0.20% were applied to a 2-mm-long section of the silica SMF. The local power-changes corresponding to the strain were observed at the correct position. The lengths of the power-changed sections appeared to be much longer than 2 mm; in theory, they should be almost the same as the nominal resolution (0.11 m), which moderately agrees with the measured results. The maximal normalized power-change of each distribution in Figure 15a was plotted as a function of applied strain in Figure 15b. With increasing strain, the power-change also increased almost linearly, which indicated the magnitude of the strain can be correctly measured in this strain range. Then, we performed a similar experiment using the POF. The normalized power-change distributions measured when the 5-mm-long section was heated to 26 (room temperature), 35, 45, and 55 \({}^{\circ}\)C are shown in Figure 16a. The abrupt power-changes were observed at the correct position along the POF. The lengths of the power-changed sections moderately  
Figure 14: (**a**) Normalized power-change distributions measured when the length of the strained section \(L_{\varepsilon}\) was reduced from 1.00 m to 0.01 m; each distribution was shifted by 0.5. Dotted lines indicate the theoretical trends. (**b**) Maximal power-changes plotted as a function of \(L_{\varepsilon}\). The error bars were calculated as standard deviations of the signal fluctuations at non-strained sections. The dotted line is a linear fit. The inset shows the magnified view at \(L_{\varepsilon}\) shorter than 0.1 m. Reproduced with permission from [61]. Copyright (2016) JSAP. agreed with the nominal resolution (0.21 m). The magnitude of the maximal power-change was in proportion to the temperature change, as shown in Figure 16b, indicating the potential feasibility of distributed temperature sensing.