# A Novel Fiber Optic Sensor for Microparticle Velocity Measurement Using Multicore Fiber
## 3 Results  
An example of the PDs' output signals is illustrated in Figure 6, where the \(V_{a}\) signal, \(V_{b}\) signal and \(V_{c}\) signal are outputs of PDa, PDb and PDc, respectively. We can see that when there is no particle passing through the sensor probe, the signals received by each PD are almost unchanged. Once a particle passes through the sensor probe in the direction at the angle \(\alpha=30^{\circ}\), some light is blocked by the particle so that the signals received by PDs are fluctuated. Note that as EDFA is used, signals are strengthened. The particle velocity and its direction can be acquired from two of the three velocity components. In practice, which signals will be chosen for velocity calculation is up to the signal quality, so here we only focus on the signals of \(V_{a}\) and \(V_{b}\). In order to verify the ability of the probe to measure the movement direction of the particle, we measured the velocity components of the particle at different angles and summarize the data in Table 1. Based on the data in Table 1, Figure 7 illustrates the trend of the particle velocities with different direction angles. Note that the theoretical speed component is calculated by giving different angles to Equations (1) and (2). It can be seen that as the incident angle of the particle increases from 0 to \(30^{\circ}\), the measured velocity \(V_{a}\) decreases, but at same time, the measured velocity \(V_{b}\) increases. There is only a small fluctuation of the measured velocity \(V\). Observing the relationship between the measured velocity and the theoretical velocity, it is found that the change of the measured velocity is the same as the trend of the theoretical velocity, that is, the theoretical velocity \(V_{a}\) decreases and the measured velocity \(V_{a}\) also decreases. The theoretical velocity \(V_{b}\) increases, and the measured velocity \(V\) also increases. The theoretical velocity \(V\) remains unchanged, and the measured velocity \(V\) fluctuates only slightly. To sum up, the measured 2D movement velocity of the particles is consistent with the theoretical trend. The above data can reflect the change of the speed value caused by the change of  
Figure 5: Photo of the major part of the experimental setup. An enlargement of the sensor probe is shown in the picture. Figure 6: Example of the measured temporal signals of \(V_{a}\), \(V_{b}\) and \(V_{c}\). the movement angle, so the sensor has the ability to measure particle velocity in two dimensions. Error bars for \(V_{a}\) and \(V_{b}\) at different particle angles are also shown in Figure 7. Error bars are calculated by standard deviation and based on three repeated measurements of each velocity component. Measured \(V_{a}\) and \(V_{b}\) with error bars at different actual velocities of the moving particle are shown in Figure 8a,b. Error bars were calculated by standard deviation and are based on three repeated measurements of each velocity component. According to the measured \(V_{a}\) and \(V_{b}\), the measured particle velocity \(V\) can be calculated by Equations (1) and (2). Figure 8c shows the measured velocity and actual velocity of the moving particle. The measured velocity is consistent with actual velocity within an error of 2%. In fact, the size of the particle matters because the size of the particle affects whether it is suitable for the sensor. The geometric distribution of seven-core end defines the size of the measurable particles as shown in Figure 9. Considering a spherical particle, theoretically, for the upper limit of particle size, the particle should not completely block the hexagon formed by seven cores as the sensor signal may not contain enough valid details of velocity information, so the diameter of the particle should not be larger than the diagonal length (91 \(\upmu\)m) of the hexagon formed by the seven cores.