# A Novel Fiber Optic Sensor for Microparticle Velocity Measurement Using Multicore Fiber
## 1 Introduction  
In many research-based and industrial situations, the velocity measurement of microparticles is necessary and promising [1; 2; 3; 4; 5; 6; 7], such as air pollution monitoring (Particulate Matter (PM) 2.5 or dust) [1], biological and medical applications (human blood flow measurement) [2; 3], two-phase flow analyses [4; 5; 6] and combustion research [7]. Nowadays a variety of techniques is used for measuring particle motion, such as particle image velocimetry (PIV) and high speed photography (HSP). While these techniques are well established, they are very expensive; for example, Dantec, one of the commercially available digital stereoscopic systems, whose price exceeds US $150,000. Besides, these systems are difficult to implement due to the size of their lenses. Spatial filtering velocimetry (SFV) has shown great usefulness for measuring particle velocity. It has advantages of flexibility and cost effectiveness. Different structures of SFV systems have been studied. In 1983, [PERSON] and [PERSON] used a transmission grating as a spatial filter to obtain flow velocity by measuring particle velocity in flow in a microscopic region [8]. In 2007, [PERSON] and [PERSON] used a Complementary Metal Oxide Semiconductor (CMOS) camera based on SFV to research human blood flow rate in the capillaries by measuring the velocity of blood particles [3]. By contrast, optical fibers sensing techniques have advantages in flexibility, cost effectiveness, corrosion resistance and electromagnetic noise tolerance. Optical fiber probes with different structures have been studied. [PERSON] et al. designed a probe composed of three optical fibers arranged linearly to measure the one-dimensional velocity of particles. In the probe, the optical fiber at the center was used to receive the reflected light of the particles, and the two adjacent optical fibers were used to emit light [9]. [PERSON] et al. used a probe composed of five silicon optical fibers to measure the one-dimensional velocity of vertically moving particles in a circulating fluidized bed. The five fibers were linearly arranged, two fibers were used to emit light, and the other three fibers, which were connected to three separate photo-multipliers, were used to collect reflected light from the particle. The probe could minimize errors caused by light reflected to transmitting fibers from different particles, which is a problem encountered in a three-fiber probe [10]. In order to solve the problem of measuring the two-dimensional velocity of particles, [PERSON] et al. used a probe composed of thirty-seven ordinary optical fibers to measure the two-dimensional velocity of particles. In their design, 37 fibers formed the measurement plane, of which seven fibers were arranged in a regular hexagon to receive the light signal reflected by the particle, and the other 30 fibers surrounded the seven fibers for light emission [11]. Due to the inherently small size of optical fibers, they are easier to place in confined spaces for particle movement monitoring. Therefore, in some studies, researchers have gradually used tiny optical fiber probes to replace traditional large-volume imaging lenses, and PIV systems were combined. For instance, [PERSON] et al. combined an imaging fiber bundle (consisting of 31,300 fibers) with a PIV system to measure classic jet flow [12]. In 2019, we reported a one-dimensional microparticle velocity sensor with a conical lens fiber array, which has a high signal-to-noise ratio with a smaller blind area and wider detection area [13]. Typical structures of optical fiber probes [9; 10; 11; 12; 13] for particle velocity measurement reported by previous researches are shown in Figure 1.  
However, for manual-assembled fiber arrays, a non-uniform arrangement of fibers in manual fabrication is prone to make the standard deviation of the spacing between adjacent cores larger, which will lead to inaccuracy in the velocity measurement according to the principle of SFV.