## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Examples of Performance-Improved BOCDA Systems
#### 4.4.2 BOCDA System with Temporal Gating Scheme and Time Division Pump and Prove Generation Scheme  
Figure 27 shows a BOCDA system that has the \"temporal gating scheme\" and the \"time-division pump and probe generation scheme\" [54].  
Figure 26: Demonstration of quite a high spatial resolution in the distributed BFS measurement by BOCDA with the IM scheme and the beat lock-in scheme [66]. (**a**) BFS distributions when a three-mm length of the fiber is elongated; (**b**) BFS distribution when the two three-mm short sections of a dispersion-shifted fiber (DSF) are fusion-spliced into an ordinary single-mode fiber. A 300-MHz BFS difference (6000 \(\upmu\upvarepsilon\) strain) is clearly measured due to the IM scheme and the beat lock-in scheme.  
Figure 27: BOCDA system that has the temporal gating scheme and the time-division pump and probe generation scheme [54].  
The direct frequency modulation nature of the LD is also used to set the pump and the prove frequency difference in Figure 27. As shown in an inset at the upper portion, a modified square-waveform current is injected into the LD [59]. The modified waveform shape is synthesized through measuring the frequency response of both the LD and the driving circuits. Figure 27a in the inset is an example of the synthesized current waveform, which can realize a nice square shape of the output laser frequency modulation, as shown in Figure 27b in the inset [59]. The upper and the lower flat portions correspond to the pump and the probe frequency, respectively. The modified square-waveform current for the pump and probe generation and the sinusoidal current for the SOCF are totally injected into the LD, as shown in a circle at the left side of Figure 27.  
In the fiber paths for the pump and the probe, EOM1 and EOM2 are set, respectively, to select one of the two frequency portions. This is the principle for the \"time-division pump and probe generation scheme\" [53]. In this scheme, an expensive SSBM is not used to reduce the cost.  
In this system, EOM1 and EOM2 have an additional function, which is the \"temporal gating scheme\" function. Besides the selection of one frequency portion, the IM modulators select only one FM modulation period and block the following n-1 times FM periods. This realizes one correlation peak position selection among the periodically generated n-times correlation peaks. Then, this scheme elongates the total measurement range by n times, as shown in the lower inset of Figure 27.  
Figure 28 shows a result of the distributed BFS measurement that was obtained by the BOCDA system, as shown in Figure 27[54]. The spatial resolution is set to be seven cm, and the measurement range is elongated by the \"temporal gating scheme\" to be 1030 m. The ratio between the range and the resolution is 14,570 [54]. In Figure 28b, the BFS distribution is shown at a strain-applied portion, and Figure 28c shows BGS distribution at another strain-applied portion.  
Figure 28: Experimental results of the distributed measurement by the BOCDA system shown in Figure 27. (**a**) Total distribution of the BFS; the spatial resolution is set to be seven cm, and the measurement range is elongated by the temporal gating scheme to be 1030 m. The ratio between the range and the resolution is 14,570; (**b**) BFS distribution at a strain-applied portion. (**c**) BGS distribution at another strain-applied portion [54].