# Recent Progress in Fast Distributed Brillouin Optical Fiber Sensing
## 1 Introduction  
By combining the laser and optical fiber, optical fiber sensing technology has been developed rapidly, which is widely used in the fields of civil structure health monitoring, modern industrial controlling, and national defense security. Compared with the traditional resistance and capacitance-based electronic sensors, optical fiber sensing has clear advantages [1]: optical fiber is not only the low-loss optical transmission medium but also the continuously deployed sensors. The optical fiber is easy to access to the all-optical network in adverse working conditions thanks to its lightweight and compact structure while maintaining high resolution, high sensitivity, and a wide measurement range. Then, physical parameters outside of the fiber can also be obtained by measuring the acoustic impedance along the fiber, which has been demonstrated in recent reports [2, 3]. Because of the above merits, a variety of optical fiber sensors are developed for modern industry. Among them, the distributed ultra-fast BOTDA, which has more practical application value is urgently needed.  
From the 1990s, the Brillouin-based optical fiber sensing [4, 5, 6, 7] has drawn much interest since it can offer an attractive solution to the societal concern for distributed dynamic measurement. Two optical waves are launched into the fiber under test (FUT) in the opposite direction. When theirfrequency-detuning is close to the Brillouin frequency shift (BFS) of the FUT, the power will be dramatically transferred from the high-frequency optical wave to the low-frequency optical wave via the stimulated Brillouin scattering (SBS) effect [8; 9]. By scanning the frequency-detuning, the distributed Brillouin gain spectrum (BGS) can be obtained and its central frequency (i.e., BFS) computed by curve-fitting has a linear relationship [1; 4; 5; 10; 11] with an environmental strain or temperature.  
Up to now, there are several Brillouin-based sensing schemes, which are shown in Table 1. For one end schemes, Brillouin reflectometers are achieved by only injecting an optical pump wave into the FUT and detecting the backscattering Brillouin signal to demodulate the spontaneous BGS. To enhance the Brillouin signal, two end schemes (i.e., analysis method) are obtained by injecting a counter-propagating probe wave resulting in an enhanced SBS effect. Considering the interaction domains for the optical waves, three subcategories can be classified as the time-domain, the frequency-domain, and the correlation domain. For the time-domain approach, a high signal-to-noise ratio (SNR), a long sensing distance [12], and a high sampling rate can be provided but the spatial resolution is restricted in \(\sim\)1 m [13]. For the frequency-domain approach, high sensitivity and a wide dynamic range can be obtained but the sampling rate is limited. As for the correlation-domain approach, a high spatial resolution can be achieved but it is suitable for points sensing with a high sampling rate. In addition, some schemes based on the Brillouin dynamic grating [10; 14; 15; 16; 17] is also restricted in the static measurement.  
In this paper, we mainly focus on the BOTDA scheme. By analyzing the limiting factors of the measurement speed for classical BOTDA scheme, several improved BOTDA schemes proposed in recent years are summarized for distributed dynamic measurements.