# Research Progress in the Flight Characteristics of Flyers Driven by Different Technologies
## 3 Research Progress in Flyer Velocity
### Two-Dimensional (2D) VISAR
#### 3.1.4 FPI  
Figure 13 illustrates a schematic overview of the Fabry-Perot system. A multi-mode fiber is employed to guide laser light to the moving object. The interferometer is composed of a Fabry-Perot etalon with coated mirrors, and the interference pattern is captured by a streak camera. The wavelength of the incoming light determines the diameter of the interference fringe. As a result, the projectile velocity may be expressed in terms of the Fabry-Perot fringes' instantaneous diameter. The temporal resolution is usually at the nanosecond level; however, for some studies, it is still limited by etalon settings and camera resolution.  
[PERSON] et al. developed an FPI system with nanosecond temporal resolution, a velocity resolution of 10 m/s, and a fringe constant of 1000 m/s. Based on this technology, velocity measurements for explosive-driven and electric gun-driven flyers were implemented [42]. [PERSON] et al. investigated the feasibility of employing a single Fabry-Perot velocimeter for two surface velocity measurements in an experiment. This approach has been used to measure equations of state [43].  
As shown in Figure 14, [PERSON] et al. designed a multi-beam Fabry-Perot velocimeter and used it to obtain high-resolution continuous data records of explosive detonations driving a copper flyer. The velocimeter allows recording the speed data of five channels in a single experiment. The optical fiber transmission system allows independent positioning of each channel to achieve spatial flexibility. Furthermore, each data channel has a variable recording length [44].  
Figure 14: A schematic of five-beam Fabry–Perot velocimeter.  
Figure 13: The main parts of FPI system.  
The traditional FPI converts the wavelength change into the ring diameter change through the F-P etalon. However, the thick etalon for obtaining a clear image will lead to a slow response time, which is not enough to study the high-acceleration motion process. The F-P optical resonator can change the continuous wide spectrum of the incident light into the quasi-discrete spectrum of the transmitted light, so that it plays a role in frequency selection. Therefore, the absolute amount of Doppler frequency shift can be measured by using the F-P optical resonator. Compared with other laser interferometers, this method has the characteristics of fast response, low performance requirements for recording equipment, easy operation, compact structure, and low cost.  
As shown in Figure 15, the laser beam irradiates the flyer surface through the F-P probe and returns from the surface to port 3 with a Doppler frequency shift. The reflected beam is divided into two parts by the beam splitter. One part passes through the F-P optical resonator, and the transmitted light intensity \(I_{1}\) generated by coherence is measured by photo-detector 1; the other part is directly monitored by photo-detector 2, thus producing light intensity \(I_{2}\). The relative transmittance \(T(f)=I_{1}/I_{2}\) is calculated for inversely solving the frequency, and then the speed-time curve can be obtained. [PERSON] et al. developed a type of velocity interferometer on the basis of the F-P optical resonator. The polyimide flyer driven by an electric exploding foil is monitored by this system [45].