## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Examples of Performance-Improved BOCDA Systems
#### 4.4.3 BOCDA System with High-Speed Random Accessibility  
The feature of the BOCDA systems, compared with the time-domain technologies, is the random accessibility. Additionally, the BOCDA can realize a fast measurement speed, because the system utilizes a continuous lightwave. The CW lightwave, of course, has a 100% duty ratio, which means that the lightwave energy is used most efficiently. This is suitable for enhancing the SNR and measurement speed. The factors related to the individual measurement performance, including the spatial resolution, measurement range, measurement speed, measurement accuracy, strain dynamic range, and so on, have a mutual relationship with each other. Therefore, it is not possible to enhance all of the performance factors at the same time. This aspect shall be discussed briefly later in this section. Before that, we shall show BOCDA systems that have solved practical limitations mainly due to the performance of the equipment. Figure 29 shows a BOCDA system that was designed to realize a fast random accessing speed [67]. A high-speed change of the measurement point, by changing the SOCF frequency, can be realized by simply using a waveform generator with a high-speed frequency changing function. The frequency difference between the pump and the probe lightwave, which is around 10 GHz, must be also swept in a high speed for measuring the BGS shape. The frequency difference has been generated by the SSBM modulator, which is driven by a microwave synthesizer with a high frequency accuracy. However, in this device, the frequency cannot be swept at a high speed, for example, in kHz order. Then, in Figure 29, a voltage-controlled oscillator is adopted, whose oscillation frequency can be changed quickly by simply changing the input voltage. The frequency stability is enough for application of the BOCDA. Figure 30 demonstrates a 5000-point high-speed random accessing measurement performed by the system that was shown in Figure 29. Multiple measurement points can be selected arbitrarily along the sensing fiber. In the demonstration shown in Figure 30, five points are selected arbitrarily, and each point is measured at a sampling speed of 1000 samples/s. The measurement points along the fiber are hopped in the sequence of Point A, B, C, D, E, A, B, C, D, E,..., which corresponds to changing the SOCF frequency stepwise at the rate of 5000 times/s. During one step, the frequency difference between the pump and probe lightwave is linearly swept by driving the voltage controlled oscillator (VCO) with a saw-tooth voltage waveform of 5000 Hz. Then, the time-division multiplexed measurement for the selected five points can be demonstrated. Due to the high-speed hopping among the multiple measurement points, simultaneous dynamic strain measurements are realized equivalently, as shown in Figure 30[67]. Figure 29: BOCDA system to realize a fast random accessibility speed [67]. In this experiment, the spatial resolution was three cm, and the measurement range was only six meters. Then, elongation of the measurement range was tried by using the temporal gating scheme [68]. In the SOCF technology, which is the principle of the position-selective measurement in the correlation-domain technologies, the system output must be integrated, at least, for one period of the LD frequency modulation in order to realize the position selectivity. Then, the intensity modulation frequency for the lock-in detection should be set at exactly half the SOCF frequency to enhance the random access speed. This means that the lock-in frequency must be changed rapidly when we change the measurement position by the SOCF frequency. However, in an ordinary lock-in amplifier, the reference frequency cannot be changed at a high speed, such as five kHz. Therefore, in Figure 30, the lock-in frequency was set to be constant, which reduced the BFS measurement accuracy. In this experiment, the standard deviation in the BFS measurement was 4.13 MHz. This corresponds to a strain accuracy of about 80 \(\mu\)s. In Ref.