# Compact 15 mJ Fiber-Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper
## 1 Introduction  
Picosecond pulsed laser sources have been widely used for a variety of industrial manufacture and basic scientific applications, such as material processing [1, 2], laser-induced damage [3], medical diagnosis [4], nonlinear optics [5, 6], and laser ranging [7, 8], all of which benefit from the narrow pulse width, large single-pulse energy, and sufficiently high peak power. In particular, the millijoule-level pulsed laser source with hundreds of picoseconds has great application potential in the field of material processing and laser-induced damage. Nanosecond laser sources can achieve high processing efficiency by virtue of their high-energy characteristic. However, the ablation mechanism of the nanosecond laser sources is mainly the photothermal effect, which will generate a large amount of heat. As a result, cracked areas and molten areas tend to be generated near the processing area, resulting in worse processing quality. For ultrashort pulsed laser sources (<10 ps), the ablation mechanism is mainly photoionization. Ultrashort pulsed laser sources have narrow pulse width and high peak power density, which can cause a sharp rise in the temperature of the processed material in a short period of time. Therefore, there is no slow accumulation of lattice heat, enabling high-precision processing quality. However, when a narrow pulse is amplified, it is extremely easy to cause damage to optical components because of its high peak power. It can be seen that it is still difficult to obtain high energyand high peak power simultaneously for ultrashort pulsed laser sources. For this reason, the lower energy limits their processing efficiency. In contrast, the hundred-picosecond pulsed laser source has a pulse width in the nanosecond to femtosecond range. It not only has the characteristics of narrow pulse width, small thermal effect range, and high accuracy, but can also ensure high-energy and high peak power output, thereby reducing the damage threshold of the material and accelerating the damage process. Therefore, the hundred-picosecond pulsed laser system is the most promising application light source to realize the above characteristics. In the traditional industrial field, product quality and production efficiency can be improved by using a millijoule-level hundred-picosecond laser source [9]. Therefore, it has certain theoretical significance and practical value to provide a reliable and stable millijoule-level hundred-picosecond pulsed laser source for research on material processing and laser-induced damage, which can enhance the damage efficiency of materials. The high-energy picosecond laser pulses are usually obtained by a master oscillator power amplifier structure with high gain medium [10; 11; 12; 13]. In general, this structure is based on a fiber or solid-state mode-locked laser oscillator as the front-end seed source, followed by further energy enhancement through regenerative amplification or multi-pass amplification technology. It can be seen that a stable front-end seed source is the foundation of a laser amplifier. In the current reports, most of them used solid-state Nd:YVO\({}_{4}\) mode-locked laser oscillators based on a semiconductor saturable absorber mirror (SESAM), which had a repetition rate of 80 MHz [14; 15]. However, the realization of stable mode-locking by the solid-state mode-locked laser oscillator is limited by the length of the resonator, which is harmful to the miniaturization of the system front-end. In addition, a large number of spatial separation elements are very sensitive to the environment, which leads to the instability of the mode-locked pulses. Furthermore, the etalon was used to broaden the pulse width and thus achieve efficient energy amplification. However, the broadening was limited to tens of picoseconds. Compared with a traditional all-solid-state seed source, a fiber seed source has unique advantages, including good beam quality, high conversion efficiency, compact structure, good thermal management, and strong anti-interference ability.