# Recent Advances in Brillouin Optical Correlation-Domain Reflectometry
## 1 Introduction  
Since the Brillouin frequency shift (BFS) in optical fibers was found to be dependent on applied strain and ambient temperature [1; 2; 3; 4], extensive research has been conducted to exploit Brillouin scattering to develop distributed fiber-optic sensors. The first demonstrations were performed by time-of-flight techniques; they are called Brillouin optical time-domain analysis (BOTDA) [5] and Brillouin optical time-domain reflectometry (BOTDR) [6]. In both techniques, optical pulses are injected into fibers under test (FUTs), and by measuring the time until the reflected signals are returned, the locations in the FUTs can be derived. In BOTDR, optical pulses are injected into only one end of the FUT, and they are partially returned via spontaneous Brillouin scattering. In contrast, in BOTDA, a continuous wave (probe) is injected into the other end of the FUTs in addition to the optical pulses (pump). The frequency of the probe light is downshifted by the BFS in advance, and then the pump and probe lights interact with each other, inducing stimulated Brillouin scattering (SBS). As SBS is much stronger than spontaneous Brillouin scattering, the signal-to-noise ratio (SNR) of BOTDA is much higher than that of BOTDR, leading to a shorter measurement time because of the reduced averaging count. However, unlike BOTDR with single-end accessibility, the two-end-access nature of BOTDA is not beneficial to users, because the degree of freedom in deploying the fiber is lowered and the measurement can no longer be continued if the fiber has one breakage point. Thus, BOTDR and BOTDA are in a trade-off relationship. The spatial resolution of these time-domain techniques is determined by the pulse width of the pump light. The basic limitation of the pulse width is known to be approximately 10 ns (corresponding to the spatial resolution of \(\sim\)1 m) because of the spectral broadening of the Brillouin gain spectrum (BGS) and reduced accuracy of BFS detection [7]. It isnoteworthy that the performance including the spatial resolution has been drastically enhanced with various schemes both in BOTDR [8; 9; 10; 11] and in BOTDA [12; 13; 14; 15; 16; 17; 18; 19; 20]; refer to each reference for the details. Researchers have also developed Brillouin optical frequency-domain analysis (BOFDA) [21; 22; 23] and Brillouin optical frequency-domain reflectometry (BOFDR) [24], which seem to suffer from their relatively slow operation speed because of the large amount of calculation; their details are also out of the scope of this paper. The other configurations of Brillouin-based distributed fiber-optic sensors are Brillouin optical correlation-domain analysis (BOCDA) [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35] and Brillouin optical correlation-domain reflectometry (BOCDR) [36; 37]. These techniques utilize not optical pulses but frequency-modulated continuous waves to achieve distributed measurements and thus possess so-called random accessibility to sensing points and high spatial resolution without specially configured implementations. BOCDA is a two-end-access system, which operates based on stimulated Brillouin scattering and thus has a relatively high SNR. The details of BOCDA will be fully described in other papers of this Special Issue. Although the SNR of BOCDR is lower than that of BOCDA, its single-end accessibility is beneficial to users as the degree of freedom in embedding the fiber is higher and distributed sensing can be continued even when the fiber is broken at a certain point during the measurement. In this paper, we review the fundamentals and recent advances in BOCDR, with an emphasis on its high-speed configuration. In Section 2, we describe the basic principle of BOCDR and clarify its difference from that of BOCDA. In Section 3, we present some configurations for improving its performance. In Section 4, we focus on slope-assisted BOCDR and present its principle along with some unique features including the beyond-nominal-resolution effect.