# Compact 15 mJ Fiber-Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper
## 1 Introduction
Furthermore, multistage fiber amplifiers can achieve high average power up to hundreds of watts [16; 17]. However, due to high peak power during the picosecond pulses amplification process, unwanted nonlinear effects and optical damage will be generated. Therefore, it is rather difficult for fiber amplifiers to realize high energy. To obtain a stable picosecond laser source with high pulse energy, the method of combining a fiber seed source with a multistage solid-state laser amplifier can be adopted, and this is a very effective method. For laser amplifiers, according to different shapes of gain media, they can be divided into thin-disc amplifiers, slab amplifiers, and rod or bulk amplifiers. The thin-disc amplifier has obvious advantages under high-energy pumping due to its excellent thermal management mechanism, which can guarantee both high beam quality and high-energy laser output [18; 19]. However, the lower single-pass gain and complex multi-pass structure significantly increase the technical complexity and operation maintenance cost of the amplification system. By contrast, the slab amplifier is quite advantageous in terms of technical complexity [20; 21]. Nevertheless, the slab amplifier has a special pumping structure, which requires a large number of coupling and shaping systems. Moreover, it requires high-precision assembly to control the beam quality of the output; thus, the cost is relatively high. Additionally, it is difficult to grow large-sized slab crystal; hence, it is much less powerful than rod or bulk laser amplifiers in terms of output energy. The rod or bulk laser amplifier still has good development prospects because of its advantages of simple structure, mature pump module technology, and absence of complex shaping system [22; 23; 24]. The Nd:YAG crystal is the most common gain medium in picosecond rod or bulk amplifiers. The upper level lifetime of Nd:YAG gain medium is 230 us, which has high gain and extraction efficiency. Currently, Nd:YAG regenerative amplifiers and Nd:YAG multi-pass amplifiers are promising solutions to further amplify the energy of the seed source for subsequent applications. In 2007, [PERSON] et al. demonstrated a 130 mJfiber-solid hybrid amplifier with a repetition rate of 300 Hz, and its output pulse width was adjustable from 350 ps to 600 ps [25]. In 2014, [PERSON] et al. built a 112 mJ fiber-solid hybrid amplifier, which provided pulse durations from 500 ps to 2 ns at a repetition rate of 100 Hz [26]. In 2021, [PERSON] et al. demonstrated a sub-nanosecond fiber-solid hybrid Nd:YAG laser system, which generated a single-pulse energy of 120 mJ and a pulse width of 500 ps at a repetition rate of 100 Hz [27]. In these reports, the single longitudinal mode continuous-wave (CW) fiber lasers were employed as the master oscillators. The modulation of pulse duration and temporal shape of the master oscillators required both acousto-optic modulation (AOM) and electro-optic modulation (EOM) technology. Therefore, this scheme needs additional modulators, resulting in expensive cost and complicated configuration. Another method is to adopt the fiber master oscillator based on SESAM mode-locked technology. In 2016, [PERSON] et al. developed a 62 mJ, 1 kHz fiber-solid hybrid amplifier system, which was seeded by a SESAM mode-locked fiber seed source with a repetition rate of 60.3 MHz and a pulse width of 520 ps [28]. However, the SESAM was placed on a mirror mount. It is necessary to insert free-space coupling elements to realize the oscillation of the signal light in the cavity. Moreover, in order to achieve a stable mode-locked pulse output, it is necessary to increase the length of the resonant cavity to obtain more longitudinal modes in the cavity. These are not conducive to the miniaturization, integration, and all-fiberized structure. Moreover, the difficulty of system adjustment and alignment is increased, which is unfavorable for achieving stable mode-locked pulses output. In 2014, [PERSON] et al.