# Recent Advances in Brillouin Optical Correlation-Domain Reflectometry
## 5 Conclusions  
We reviewed the BOCDR technology from the basic principle to the recent advances. In Section 1, a variety of Brillouin-based distributed sensing techniques including BOCDR and BOTDA were introduced. In Section 2, the basic operating principle of BOCDR was presented. In Section 3, some configurations for improving the performance of BOCDR were described. In Section 4, the slope-assisted BOCDR was detailed; the operating principle and some unique features including the beyond-nominal-resolution effect were presented. Thus, significant technical advances have been achieved to enhance the performance of BOCDR. However, there still remain some important issues to be tackled. One unclarified point of this system is the measurement accuracy, which should be in a trade-off relationship with some parameters such as an average count (or measurement time), spatial resolution, incident optical power, etc. Full investigation of the measurement accuracy is extremely important in order for users to judge whether BOCDR can be employed or not for their purposes. Another point is the clarification of the maximal sampling rate, which is currently limited by the electronic devices used in the setup (in any configuration of BOCDR). The measurement instability caused by the power fluctuations is also a problem in the slope-assisted configuration, which could be mitigated using the ratio between the spectral powers at two frequencies; this push-pull scheme will provide another configuration for loss-insensitive operation. In addition, the dynamic range of slope-assisted BOCDR currently limited by the BGS shape could be extended by low-pass filtering the spectrum (at the cost of the sensitivity). Brillouin-based distributed sensing is, in general, constantly confronted with a problem of discriminative sensing of strain and temperature, and BOCDR is no exception. Combined use of some other sensing mechanisms [75; 76; 77; 78; 79; 80; 81; 82; 83; 84] will be necessary. Finally, the current setup is not suitable for field tests outside the laboratory. The system portability should be improved by implementing the functions of the relatively large devices, such as the ESA, the EDFAs, the PSCR, and the frequency-modulated laser, using local electronic circuits and/or module-type devices. These replacements will also lead to reduction in the system cost. We hope that BOCDR will, in the near future, serve as one of the most promising Brillouin-based distributed sensing techniques for practical applications because of its single-end accessibility and many other advantages.  
[PERSON] and [PERSON] performed the experiments, and [PERSON], [PERSON] and [PERSON] analyzed the data. [PERSON] wrote the review paper. This work was supported in part by JSPS KAKENHI (17H04930, 17J07226) and the research grants from the Japan Gas Association, the ESPEC Foundation for Global Environment Research and Technology, the Association for Disaster Prevention Research, the Fujikura Foundation, and the Japan Association for Chemical Innovation. The authors are indebted to [PERSON], the Toyota Technological Institute (formerly, the University of Tokyo), Japan, for his insightful comments and suggestions regarding the future prospects of BOCDR. The authors declare no conflicts of interest.