# Recent Advances in Brillouin Optical Correlation-Domain Reflectometry
## 4 Development of Slope-Assisted BOCDR
### Principle and Fundamental Operation
The experimental setup of slope-assisted BOCDR is schematically shown in Figure 7, which is basically the same as Figure 1; the only essential difference lies in the final signal processing. The output light from a distributed-feedback laser diode at 1.55 \(\upmu\)m was divided into two light beams, pump and reference. The pump light was amplified to \(\sim\)24 dBm using an EDFA and injected into the FUT. After passing through a \(\sim\)1-km-long delay line and another EDFA (amplified to \(\sim\)5 dBm), the reference light was used for heterodyne detection with the Stokes light, which was amplified to \(\sim\)2 dBm. The polarization-dependent fluctuations were suppressed using a polarization scrambler (PSCR). The heterodyned optical signal was converted into an electrical signal using a PD and was guided to an ESA. Using the narrowband-pass filtering function of the ESA, the \(P_{\text{B0}}\) change at a fixed frequency \(\
u_{\text{B0}}\) (=10.85 GHz) was sequentially output to an oscilloscope (OSC). Figure 6: (**a**) Measured BGS and (**b**) its differential coefficient at each frequency. The linear region of the lower-frequency side is indicated. Reproduced with permission from [59]. Copyright (2016) IEEE. Figure 7: Experimental setup of slope-assisted BOCDR. AMP, amplifier; EDFA, erbium-doped fiber amplifier; ESA, electrical spectrum analyzer; FUT, fiber under test; LD, laser diode; OSC, oscilloscope; PD, photodiode; PSCR, polarization scrambler. Reproduced with permission from [59]. Copyright (2016) IEEE. In the initial experiment, a 0.1-m-long section around the distal open end of the FUT was bent to suppress the Fresnel reflection. The modulation frequency \(f_{\mathrm{m}}\) and amplitude \(\Delta\)/ were set to 7.975-8.055 MHz and 1.4 GHz, respectively, corresponding to the measurement range of 12.9 m and the theoretical spatial resolution of 88 mm. The repetition rate was 100 Hz, and averaging was performed 16 times on the OSC. The room temperature was 26 \({}^{\circ}\)C. The measured results of the fundamental characterization, i.e., the \(P_{\mathrm{B0}}\) change dependencies on strain, temperature, and loss, are presented in Reference [59] (the data are not presented here because of the low SNR). The strain-, temperature-, and loss-dependence coefficients of the output signal were 1.95 \(\times\) 10\({}^{-4}\) dB/\(\mu\epsilon\), 4.42 \(\times\) 10\({}^{-3}\) dB/K, and 0.191, respectively. A proof-of-concept demonstration of slope-assisted BOCDR was then performed by simultaneous measurement of strain, temperature, and loss. The structure of the 5.0-m-long FUT is shown in Figure 8a; the ambient temperature was changed to 55 \({}^{\circ}\)C along the 1.9-2.1-m section, a 0.64-dB loss was applied at the midpoint, and a 550 \(\mu\epsilon\) strain was applied to the 3.5-3.7-m section. Figure 8b shows the measured \(P_{\mathrm{B0}}\) change distribution along the FUT. The \(P_{\mathrm{B0}}\) changes corresponding to the temperature change, loss, and strain were observed at the expected sections. The amounts of the \(P_{\mathrm{B0}}\) changes for the temperature change, loss, and strain were approximately 0.16, 0.14, and 0.12 (=0.26-0.14) dB (corresponding to the temperature change, loss, and strain of \(\sim\)62 \({}^{\circ}\)C, \(\sim\)0.73 dB, and \(\sim\)615 \(\mu\epsilon\), respectively). The measurement errors probably originate from the signal fluctuations and the resultant low SNR.