# Research Progress in the Flight Characteristics of Flyers Driven by Different Technologies
## 3 Research Progress in Flyer Velocity
### Investigation of Flyer Velocity
#### 3.1.1 PDV  
Figure 7 shows the conventional PDV configuration. The initial laser beam is divided into two equal beams. One beam is delivered to the surface of a moving target and then reflected back to the detector, while the other beam is directly sent to the detector. A beat frequency is generated through the detector by combining the initial and reflected beams, and the accompanying signal is displayed with an oscilloscope. The target's moving velocity is equal to half of the laser wavelength times the beat frequency [16].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Flyer Launch** & **Advantage** & **Disadvantage** & **Flyer Velocity** \\ \hline Explosive-driven flyer & Simple structure, small size, easy synchronization, large testing range, multiple sample layout & Limited ability to drive flyers & 3.5 km/s [2] \\ \hline Light gas gun & precise adjustment of projectile velocity, smooth flying of the flyer, good data repeatability, high measurement accuracy & High cost, complex debugging & 15 km/s [6] \\ \hline Electrical exploding foil accelerator & Compact structure, easy operation, available high strain rate (usually 10\({}^{4}\)-10\({}^{6}\) s\({}^{-1}\)) & Dependent on pulsed high-voltage equipment & 14 km/s [8] \\ \hline Magnetically accelerated flyer & Loading pressure approaching TPa level, available isentropic line, launches multiple samples simultaneously & High cost, the free surface of the flyer is easily ablated & 45 km/s [10] \\ \hline Laser-driven flyer & Compact structure, small size flyer, imparts shocks in excess of TPa level with sub-nanosecond durations to materials & High requirements for laser power density control & 23 km/s [12] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of different flyer-driving technologies.  
The most recent version of PDV has a larger detector bandwidth, as well as a higher sampling rate and bandwidth of the oscilloscope; it uses the latest fiber optic technology, making PDV useful for acquiring precise velocity data with high temporal and spatial resolutions. [PERSON] et al. investigated the flight of titanium flyers driven by small detonators. The rising, holding, and decreasing of flyer velocity was clearly observed in the velocity history collected by the PDV system [17]. [PERSON] et al. developed a device that combines ultra-fast stroboscopic optical microscopy with PDV to investigate impact events of laser-launched flyers and glass targets [18].  
The traditional PDV equipment is compact, robust, and has strong speed measuring ability. PDV probes of various forms can be conveniently positioned in a variety of experimental configurations. Nevertheless, single-point measurements do not have the ability to quantify velocity distributions on the target line or in the plane. Time division multiplexing [19], frequency division multiplexing [20], and the spectroscopic approach [21] are all methods for achieving multi-point measurement. [PERSON] et al. built a 16-channel PDV system based on optical heterodyning and multiplexing technologies and employed it to measure the speed of an explosive-driven 3.8 \(\upmu\)m thick Parylene flyer [22]. [PERSON] et al. developed a multi-point PDV system by incorporating an optical communication component that can divide the laser into several parts to deliver the light source to different lines. This device was used to measure the particle velocity of the back surface of Al targets of varying thicknesses under laser shock [21].