## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Brillouin Scattering-Based Distributed Fiber Sensing
In the time-domain technologies, there is an inherent limitation for the spatial resolution. When the launched pump pulse width is, for example, one meter, the spatial resolution is one meter. In this case, the pulsed lightwave has a broadened spectrum width of around 200 MHz, which is calculated in a simple Fourier analysis. On the other hand, the intrinsic BGS linewidth is around 30 MHz, as shown in Figure 17. This means that the accuracy in measuring the BFS shift is reduced, even when only the one-meter spatial resolution is required. Then, the time-domain configuration originally has the trade-off between the spatial resolution and the strain or temperature measurement accuracy. Many studies have been accumulated to provide solution techniques to this trade-off problem in the time-domain configurations [43; 44; 45; 46; 47; 48]. Examples are pulse pre-pump (PPP) BOTDA [43], differential pulse-width pair BOTDA [44], dark-pulse BOTDA [45], Brillouin echoes BOTDA [46], and double-pulse BOTDR [47]. By using the Brillouin dynamic grating phenomenon, which is discussed in Section 6, with a time-domain configuration, the spatial resolution has been greatly improved to be in the order of centimeters [48]. However, these systems still have difficulties. Quite fast speed data acquisition electronics are required in all of the systems. Some systems require post-calculation of the system's output data or specialty fibers such as polarization-maintaining fibers. On the contrary, the time-domain technologies are suitable to elongate the measurement range. By using amplification techniques for the signal wave intensity and/or some coded pulse train techniques, such as pseudo-random sequence pulses, distributed measurements with measurement ranges longer than 100 km have already been demonstrated [12; 49]. On the other hand, the correlation-domain technologies can reduce the trade-off difficulty between the spatial resolution and the measurement accuracy, which is one of the advantages of the technologies. Of course, the BGS spectral shape, which is obtained with the position-selective measurement function of the technologies, shows a slightly broadened width compared with the natural BGS. However, it is not serious, compared with the time-domain technologies. In a higher spatial resolution case, such as in the order of centimeters, the Brillouin scattering power, which corresponds to the selected short section of the fiber, should be reduced. This results in the reduced signal to noise ratio (SNR), which deteriorates the measurement accuracy or elongates the measurement time, even in  
Figure 18: Schematic drawing of Brillouin optical time-domain reflectometry through launching a pulsed lightwave and detecting Brillouin backscattering as a function of time (**a**). Origin of trade-off between the spatial resolution (pulse width) and the measurement accuracy of strain and/or temperature (line width of the pulse) (**b**). the correlation-domain technologies. However, the SNR reduction due to the enhancement of the spatial resolution takes place also in the time-domain technologies, because the pulse width should be reduced. On the contrary, in the correlation-domain technologies, the continuous lightwave is used instead of the pulsed one. Therefore, as for the SNR aspect under the same spatial resolution condition, it is thought that the correlation-domain technologies are superior to the time domain ones. Roughly compared, it would be said that the time-domain technologies are rather suitable for the applications with requirement of a longer measurement range and a moderate spatial resolution, and that the correlation-domain technologies are rather suitable for applications that require a moderate measurement range and higher spatial resolution.