## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Basic BOCDA System  
A BOCDA configuration was proposed in 1999, in which the stimulated Brillouin scattering was excited position-selectively at one specific position along a sensing optical fiber, by synthesizing the delta function-like optical coherence peak between the pump and the probe lightwave in the system. This is not the configuration using the pulsed lightwave, but rather that with continuous lightwaves. By manipulating the correlation, or the interference between the pump and the probe lightwave through applying the SOCF technology [14], position-selective measurement can be realized. So far, superior functions have been realized with the BOCDA configurations, such as 1.6-mm spatial resolution, a BGS shaping speed of 5000 times/s, a 7000-\(\upmu\)c strain dynamic range, and so on, individually. Of course, these superior measurement characteristics cannot be realized at the same time. These factors have relationships with each other, which creates limitations regarding the total performance of the BOCDA systems. This will be briefly discussed later. BOCDA has another specific feature called \"random accessibility\". On the contrary, in the time-domain configurations, the measurement is sequentially done from one fiber end to the other. That means that only the distributed measurement mode can be provided. In BOCDA configurations, the measurement is done at a specific position, which is the basic measurement mode of the BOCDA. Therefore, we can select multiple points arbitrarily along the sensing fiber and perform the measurements at only the selected multiple points. Of course, the ordinary distributed measurement mode can also be realized. The random accessibility is the most specific feature of the BOCDA, which cannot be done by the time-domain configurations. In the random access mode, the measurement parameters, such as the spatial resolution, measurement range, measurement speed, and measurement accuracy, are restricted in relation to each other. As for the total limitation, we shall discuss that later in Section 4.4.3. Figure 19a shows the first setup of the BOCDA [8]. The frequency difference between the pump and the probe is generated by an optical intensity modulator driven by a microwave with around 11 GHz of frequency. The carrier-suppressed condition is satisfied, and the first lower sideband is used as the probe wave. The SOCF was performed simply with a sinusoidal frequency modulation to the LD by utilizing its direct frequency modulation nature with the input current modulation. By adjusting the FM frequency and the sensing fiber length, which is the fiber length between the isolator and the 10-dB fiber coupler in Figure 19, only one correlation peak position appears within the sensing fiber portion. Then, the position-selective measurement can be realized. The measurement position, or the correlation peak position, can easily be changed with the FM frequency change. Both the distributed measurement and the random-access measurement can easily be selected. As schematically shown in Figure 19b, the BOCDA output spectrum that is obtained at the end of the fiber has the total shape of a hill-like background and a tower-like signal spectrum. The tower-shaped component at the top of the hill corresponds to the BGS at the correlation peak position. In Figure 20, the experimental results and simulation results are compared in a distributed measurement of BGS [8]. The fiber under test is composed of an ordinary single mode fiber with 10.83 GHz BFS and a 150-cm section with 10.56 GHz BFS. Figure 20a,c,e are for the experiments,and Figure 20b,d,f are for the simulation. Figure 20a,b show the BGS distribution, while Figure 20c,d show the BFS distribution. Figure 20e,f show the intensity distributions for the two BFS of 10.83 GHz and 10.56 GHz, respectively [8].