## 5 Brillouin Optical Correlation Domain Reflectometry
### BOCDR Systems with Performance Improvement Schemes  
Figure 32 shows simulations and experiments regarding the intensity modulation scheme for reducing the background spectrum in the BOCDR system [82]. In BOCDR, the IM modulator for the scheme is set just after the laser source, for example, in Figure 31. In Figure 32, the FUT was formed by an ordinary SMF fiber with a short-length DSF section between the SMFs. In Figure 32, four cases are described [82]. These are no IM modulation, IM by a sinusoidal waveform where both edges of the LD power spectrum are reduced to be zero, IM by a Gaussian waveform in which both edges of the LD power spectrum are also reduced to be zero, and IM by a Gaussian waveform with peaks to recover the power at both edges, which is called the \"optimized modulation\" in Figure 32. Figure 32b,c shows, respectively, the BGS measurement at the DSF section, and that at the SMF section. In the no-IM case, of course, the mountain-shaped background spectrum fully remains, as was shown also for the BOCDA in Section 4.4.1. Thus, the DSF section cannot be recognized both in the simulation and in the experiment. On the contrary, in the three other cases,  
Figure 31: Basic BOCDR system [9]. Figure 32: Experimental and simulation verification of the intensity modulation scheme in the BOCDR system [82]. **(a)** Four cases are shown: no IM, sinusoidal waveform IM, Gaussian waveform IM, and IM of Gaussian waveform and peaks to recover the power at both edges of the time-averaged LD power spectrum (optimized modulation). **(b,c)** show, respectively, the BGS measured at the DSF section and that at the SMF section. the mountain-shaped background spectrum is clearly reduced. Then, the DSF section can clearly be recognized, as shown in Figure 32b. However, in Figure 32b, the dip that was caused at the BFS frequency of 10.85 MHz corresponded to the BFS frequency of the single mode fiber (SMF) or no strained portion. When we measure the SMF section, the dip reduces the Lorentz-shaped signal spectrum, as shown in Figure 32c. Therefore, we need to recover the power at both edges of the LD power spectrum through using the \"optimized IM modulation\". Figure 33 shows the experimental results of the distributed measurement of the BFS at the five-cm DSF section with one-cm spatial resolution [83]. The DSF section could not be recognized without the IM scheme due to the background spectrum, but it can be recognized through using the IM scheme. Another scheme to reduce the background spectrum is the \"differential modulation scheme\", which was originally proposed and demonstrated for the BOCDA system by Professor [PERSON] et al. in Chung Ang University, Korea. This scheme has been also tried for use in the BOCDR system [84; 85]. In Ref. [84], a phase modulator is set at the reference lightwave arm of a BOCDR system. Then, the BOCDR output spectrum is measured with and without the PM modulation. Schematic drawings for the two cases are shown in Figure 34a,b, respectively [84]. In the situation where the Brillouin scattering and the reference are highly correlated with each other, which corresponds to the correlation peak position, the PM distorts the Lorentz-shaped signal spectrum, as shown in Figure 34b. By subtracting the spectrum with the PM from the spectrum without the PM, the signal spectrum is featured, as shown in Figure 34c. This is the mechanism to reduce the background spectrum in the \"differential modulation scheme\" [52] or the \"phase modulation scheme\" [84]. In Figure 35, experimental verification of the \"phase modulation scheme\" for the BOCDR system is shown [84]. The red and the blue curve correspond to the measurement at the DSF section and the SMF section, respectively. Figure 35a,b show the BOCDR spectrum without the PM, and with the PM, respectively. Figure 35c shows the subtracted spectra, in which the signal spectrum for the DSF and the SMF portion is clearly shown by the red and the blue curves, respectively.