# Recent Progress in Fast Distributed Brillouin Optical Fiber Sensing
## 5 Challenges and Future Perspectives  
A comparison between the traditional frequency-sweeping method and the above four improved schemes is shown in Table 2. Since the length \(L\) of the FUT is the inherent limitation and the code length \(N_{code}\) have the same influence on these schemes, the two factors haven't been listed in Table 2. For the averaging time \(N_{ave}\), the OFC + OFDM scheme can eliminate the polarization noise while the OCC scheme can enhance it through a cross-correlation algorithm [1; 48; 49]. Since all the frequency segments are injected into FUT at the same time for the OFC + OFDM scheme, its frequency-switching time is ideally zero. Compared to the \(\sim\)ms frequency-switching time for the traditional scheme, the frequency-switching time of the other three improved schemes can be dropped down to ns thanks to the OFA technique. The OFA technique is unable to optimize the averaging time \(N_{ave}\) and the effective frequency-sweeping number \(N_{f}\). For the effective frequency-sweeping number \(N_{f}\), the OFC + OFDM scheme multiplexes Brillouin signals with all frequencies in frequency-domain and demultiplexes the BGS by the FFT while the OCC scheme directly demultiplexes the BGS through the optical chirp segment piece-by-piece in the time domain, so that their effective frequency-sweeping number is \(1\). The effective frequency-sweeping number for the SA + OFA scheme is dependent on the dynamic range. However, the OFC + OFDM scheme has an inferior spatial resolution of about tens of meters and the SA + OFA scheme has a nonlinear sensing curve, which will influence the demodulation accuracy while the OCC scheme cannot provide the symmetrical BGS and the absolute BFS value. In the future, a \"cocktail therapy\" that combines two or more techniques may be a good solution for distributed ultra-fast measurements with a proper spatial resolution. For example, combining the DPP technique, a high spatial resolution may be obtained for the OCC scheme and the OFC scheme. As the development of the fast BOTDA schemes and electrical signal processing, a real-time, ultra-fast, and distributed Brillouin-based optical fiber sensing will be realized, which has a wide range of applications such as the earth activity detection, the health monitoring of large civil infrastructure (transportation system, bridge, dam, oil, and gas pipeline), border security, the capturing of the reaction process, and more.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Averaging Time** & **Frequency-Switching** & **Effective Frequency-Sweeping** \\  & \(\mathbf{N_{ave}}\) & **Time \(\mathbf{T_{switch}}\)** & **Number \(\mathbf{N_{f}}\)** \\ \hline Frequency-sweeping & \(\sim\)100 & \(\sim\)ms & \(\sim\)100 \\ OFC + OFDM & \(\geq\)1 & 0 & 1 \\ OFA & \(\sim\)100 & \(\sim\)ns & \(\sim\)100 \\ SA + OFA & \(\geq\)1 & \(\sim\)ns & \(\geq\)1 \\ OCC & \(\geq\)1 & \(\sim\)ns & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of the proposed BOTDA schemes in this work.  
Figure 16: The dynamic measurement [1] for (**a**) a periodic mechanical vibration, (**b**) a mechanical shock, and (**c**) a switch event.