# Recent Advances in Brillouin Optical Correlation-Domain Reflectometry
## 4 Development of Slope-Assisted BOCDR
### Detection of Extremely Short Hot Spots  
Here, we performed more detailed characterization of the beyond-nominal-resolution effect of slope-assisted BOCDR. First, using a silica SMF, we found that a strained section which is over 50 times  
Figure 11: Deduced strain distributions with a spatial resolution of 9.5 cm: (**a**) example measurement along the whole length of the FUT when a 1500 \(\mu\varepsilon\) strain was applied to a 20-cm-long section, and magnified views when strains of (**b**) 750 \(\mu\varepsilon\) and (**c**) 1500 \(\mu\varepsilon\) were applied to 5-cm- to 100-cm-long sections. In (**b**,**c**), each distribution was shifted by 3000 \(\mu\varepsilon\), and the dotted lines indicate the simulated data. Reproduced with permission from [60]. Copyright (2016) OSA. Figure 12: Magnified views of the deduced strain distributions with a spatial resolution of 19.6 cm; a 1500 \(\mu\varepsilon\) strain was applied to 5-cm- to 20-cm-long sections. Each distribution was shifted by 3000 \(\mu\varepsilon\), and the dotted lines indicate the simulated data. Reproduced with permission from [60]. Copyright (2016) OSA. shorter than the nominal spatial resolution can be detected. Then, we showed the usefulness of the beyond-nominal-resolution effect by detecting a 2-mm-long strained section along a silica SMF and a 5-mm-long heated section along a POF. These values are smaller than the world records previously reported, which are 3 mm [66] and 100 mm [67] for a silica SMF and a POF, respectively. First, to characterize the beyond-nominal-resolution effect, we employed a 14.0-m-long silica SMF as an FUT. The output light from a laser was amplified to \(\sim\)25 dBm using an EDFA and injected into the FUT. The BFS of the FUT was 10.89 GHz at 1.55 \(\upmu\)m at 26 \({}^{\circ}\)C (room temperature). The open end of the FUT was bent to suppress the Fresnel reflection. The spectral power-change at 10.87 GHz (determined by differentiating the BGS) was monitored using an OSC. The modulation amplitude \(\Delta f\) and the modulation frequency \(f_{\mathrm{m}}\) were set to 0.19 GHz and 5.10-5.26 MHz, respectively, leading to the nominal spatial resolution of 1.01 m and the measurement range of 20.1 m. A strain of 0.1% was applied to 0.01-, 0.02-, 0.05-, 0.10-, 0.20-, 0.50-, and 1.00-m-long sections of the FUT, as shown in Figure 13a. When the length of the strained section was changed, the position of its proximal end was fixed. Averaging was performed 512 times on the OSC. Subsequently, exploiting the beyond-nominal-resolution effect, we attempted to detect the shortest-ever strained and heated sections along a silica SMF and a POF. In this experiment, we injected \(\sim\)28 dBm light to the FUTs to enhance the measurement sensitivity, and averaging was performed 1024 times. To begin with, we employed a 2.0-m-long silica SMF as an FUT, the BFS of which was 10.85 GHz at room temperature. We applied strains of 0.10, 0.15, and 0.20% to a 2-mm-long section (\(\sim\)55 times shorter than the nominal spatial resolution; see below), as depicted in Figure 13b. The nominal spatial resolution and the measurement range were 0.11 m and 7.64 m (\(\Delta f\): 0.67 GHz; \(f_{\mathrm{m}}\): 13.45-13.54 MHz), respectively. The change in the spectral power at 10.84 GHz was observed using the OSC. Then, a 2.0-m-long perfluorinated graded-index POF [39; 68; 69] (core diameter: 50 \(\upmu\)m; propagation loss at 1.55 \(\upmu\)m: 0.25 dB/m; BFS at room temperature: 2.75 GHz) was used as an FUT. A 5-mm-long section was heated to 35, 45, and 55 \({}^{\circ}\)C (see Figure 13c). The modulation amplitude \(\Delta f\) and the modulation frequency \(f_{\mathrm{m}}\) were set to 0.69 GHz and 24.70-24.84 MHz, respectively, resulting in the nominal spatial resolution of 0.21 m and the measurement range of 4.5 m. Note that the length of the heated section was over 40 times shorter than the nominal spatial resolution. The change in the spectral power at 2.78 GHz was monitored.