# Research Progress in the Flight Characteristics of Flyers Driven by Different Technologies
## 3 Research Progress in Flyer Velocity
### Investigation of Flyer Velocity
#### 3.1.3 Visar  
Discrete VISAR  
As shown in Figure 9, the laser beam reflected from a mirrored specimen surface is split, and part of it is delayed before being recombined with the undelayed portion. Thus, the light frequency involving the Doppler shift induced by the moving specimen surface constantly interferes with the light frequency which existed at delay time \(\tau\). The constant \(\lambda/2\tau\), where \(\lambda\) is the laser light wavelength, quantifies interferometer sensitivity. In order to eliminate the luminous interference and improve the flyer surface reflectivity, it is generally necessary to conduct surface coating treatment on the flyer. However, due to the limitation of the photo-multiplier tube and oscilloscope, the time resolution of the VISAR in its typical configuration is inadequate to observe the events such as the study of detonation fronts. [PERSON] et al. measured the highest velocities of aluminum samples on the GEPI with a VISAR interferometer. The stabilized velocity on the sample of 0.9 mm thickness was 7.65 km/s, and the thin one of 0.4 mm thickness reached 10.24 km/s before the extinction of the VISAR signal [27]. When the observed signal frequency exceeds the recording system bandwidth due to a considerable acceleration of the object or a small fringe constant, high-frequency information from VISAR measurements is ignored. With the use of known target plate displacement or theoretical prediction, the lost fringe constant may be determined [28]. Furthermore, the loss fringes may be identified by measuring the flyer velocity using two fringe constants with a non-integral multiple relation [29]. Moreover, combining observations with the same delay but different dispersions could be beneficial [30]. The typical VISAR has no spatial resolution, making multi-point or multi-directional measurements on the same target problematic. Installing multiple optical fiber probes to allow multichannel optical signals with velocity information to enter the interference cavities is a simple and cost-effective method. [PERSON] et al. used multi-point dual-sensitivity VISAR to measure the velocity histories of the metal hemisphere at three different angles and obtained the velocity-time curve of the flyer motion [31]. All-Fiber VISAR (AFVISAR)  
Discrete VISAR has many disadvantages such as complex optical path adjustment, insufficient spatial coherence, and low light energy utilization. With the advancement of optical fiber technology, AFVISAR was developed using the laser Doppler approach to tackle the concerns listed above. Figure 10 diagrammatically depicts a typical AFVISAR system. The light from the fiber laser is directed to the 2 \(\times\) 2 coupler I, where it is split into two beams. The two beams  
Figure 9: Schematic diagram of VISAR. are routed to the 2 \(\times\) 2 coupler II through the fiber direct line (L1) and the fiber delay line (L2), then subsequently to the moving object via the collimator. The reflected beams carrying moving information are returned to coupler II and subsequently to coupler I via the L1 or L2. In the coupler I, only beams with appropriate coherent paths might interfere. [PERSON] et al. derived the relationship between the velocity of a moving object and the frequency of the interfered signal [32]. AFVISAR is distinguished by its compact construction, short coherence length, easily adjustable light route, and any fringe constant. [PERSON] et al. developed a two-point all-fiber VISAR system. [PERSON] et al. established a two-point all-fiber VISAR and applied it to measure velocities on the same specimen impacted by the split Hopkinson pressure bar [33]. Optically Recording Velocity Interferometer System (ORVIS)  
With the exception of the streak camera recording interference fringe motion, the ORVIS system is similar to a VISAR system. The observed picture of a moving fringe pattern over time directly represents the velocity history of the monitored surface [34].