## 3 Characteristic of Picosecond Laser Measurements for Debris Targets
### Pulse-Bursts Picosecond Laser System with High Power  
For the picosecond laser with a PRF of 1 kHz, it have high peak power, but the average power and single-pulse energy cannot be increased easily. Therefore, a pulse-burst picosecond laser was developed by SHAO for the DLR to achieve high power and large single-pulse energy. A Michelson interferometer was used to achieve narrowly spaced pulse bursts, as shown in Figure 18 (left). BS1, BS2, and BS3 denote beam splitters with transmission rates of 50% whereas M1, M2, M3, and M4 denote zero-degree total reflectors coated at a wavelength of 1064 nm. Each zero-degree total reflector corresponds to a pulse generator in a burst. Pulses in bursts with equal spacing \(\Delta\)t were obtained by increasing the lengths of M1 to BS1, M2 to BS1, M3 to BS1, M4 to BS1 to \(\Delta\)L [28; 29]. Here, we took the length \(\Delta L=150\) mm and a pulse spacing of \(\Delta t=2\times\Delta L/c=1\) ns (where \(c\) is the speed of light). Pulses of a mode-locked picosecond laser with a PRF of 80 MHz are divided into four pulses in Figure 18 (right).  
To realize high-power lasers with picosecond pulse bursts, a burst-mode regenerative amplifier (RA) and a two-stage traveling-wave amplification have been adopted for the amplifier system [30; 31], as shown in Figure 19. The Nd:YAG RA, which acts as a pre-amplifier, reduces the PRF of the seed laser from 80 MHz to 1 kHz through electro-optic modulation. The seed pulse burst laser is injected into the RA through the optical isolation system from P1 and then comes out from P1 after the amplification. The Faraday rotator (FR) and half-wave plate (HWP\({}_{1}\)) act as an optical isolation system that avoids the effect of the output amplified beam returning to the seed oscillation cavity. The quarter-wave plate (QWP) and Pockels cell (PC) create the pulse.  
Figure 19: Schematic diagram of the amplifier and second harmonic generation construction of a pulse-burst picosecond laser system.  
Figure 18: Schematic diagram of a Michelson interferometer for pulse bursts (**left**) and the waveform of the pulse bursts (**right**).  
To maintain the beam quality during the two stages single pass double module amplification, two 4f imaging systems are used, and all subsequent imaging systems contain vacuum tubes to prevent air breakdown. A half-wave plate and 90-degree quartz rotator help to compensate for the difference in optical power between the tangential and radial polarization components of the thermal lens. Two 45-degree polarizers are used to improve the degree of polarization. A frequency conversion module, which is used to adjust the beam size and divergence, is placed behind the 4f imaging system to maximize the conversion efficiency. In further improving the conversion efficiency, a 1064 nm thin-film polarizer is used to purify the polarization direction of the amplified fundamental laser. After the Nd:YAG RA and two-stage traveling-wave amplification, the seed pulse-burst is amplified to 40 W at a wavelength of 1064 nm and a PRF of 1 kHz.  
The second harmonic waves are generated by the lithium triborate (LBO) crystal, which was chosen for its high damage threshold, relatively high nonlinearity, large acceptance angle, and no spatial walk-off for type-I noncritical phase matching at 1064 nm. The crystal is combined with a half-wave plate to realize the best phase matching condition. Owing to the configuration and technologies addressed above, the pulse-bursts picosecond laser with high power was achieved (i.e., an average power of 20 W at 532 nm and frequency doubling efficiency of 50%).