00-kHz Level High Energy Double-Pass Nd:YVO4 Picosecond Amplifier with Optic-Optic Efficiency of 51%  
[PERSON]  
1 Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (H.H.); 33 [EMAIL_ADDRESS] (Y.S.); [EMAIL_ADDRESS] (C.W.); [EMAIL_ADDRESS] (Z.W.)  
2 Beijing Engineering Research Center of Laser Technology, Beijing 100124, China  
3 Key Laboratory of Trans-scale Laser Manufacturing Technology, China Ministry of Education, Beijing 100124, China  
4 The 53 rd Research Institute of the China Electronics Technology Group Corporation, Tianjin 300000, China  
5 Correspondence: [EMAIL_ADDRESS]; Tel.: +86-138-1043-543  
with pump power of 55 W at an optical-optical efficiency of 23%, achieving pulse peak power of 1.2 MW and pulse energy of 130 \(\upmu\)J. The beam quality factor was \(M_{\pi}^{2}=1.30\), \(M_{\pi}^{2}=1.28\).  
In 2016, [PERSON] [12] had demonstrated a high pulse energy, high peak power, and high beam quality picosecond (ps) master oscillator power amplifier system of 1064 nm at a pulse repetition rate of 5 kHz. This system consisted of a ps mode-locked oscillator, a regeneration amplifier, and three-stage Nd:YAG slab amplifiers, which enabled us to manage good thermal issues and provide an average power of 41 W. Up to 8.2 mJ of energy per pulse was achieved at the total absorbed pump power of 460 W with an average extraction efficiency of 8.5%. The pulse duration was 25.3 ps, which corresponds to a peak power of 324 MW. The beam quality factors of \(M^{2}\) were measured to be \(M_{\pi}^{2}=2.8\) along the x-axis direction and \(M_{\pi}^{2}=2.2\). along the y-axis direction, respectively.  
In 2017, [PERSON] [13] reported a non-pulse-leakage optical fiber pumped 100 kHz level high beam quality Nd:YVO\({}_{4}\) picosecond amplifier. An 80 MHz, 11.5 ps mode-locked picosecond laser was used as the seed with single pulse energy of 1 nJ, a maximum output power of 24.5 W was generated corresponding to the injected regenerative amplified power of 9.73 W at 500 kHz. The output pulse duration is 16.9 ps, and the beam quality factor \(M^{2}\) is 1.25, the optic-optic efficiency of amplifier is 39.7%.  
In 2018, [PERSON] [14] reported a high peak power and high beam quality fiber--a solid hybrid amplification laser system. A fiber laser with a repetition frequency of 50 kHz, pulse width of 3.9 ps, and average power of 10.9 mW was used as the seed source. The average output power of 27.65 W was obtained in the double-passing amplifier of two stages, with the pulse peak power of 65 MW. The first amplifier stage is end-pumped Nd:YVO\({}_{4}\) solid amplifier and the second is side-pumped Nd:YAG solid amplifier. By optimizing the beam filling factor in each solid amplifier, the \(M^{2}\) factor of the output beam reaches 1.30.  
In this study, we demonstrate a highly stable and high beam quality industrial grade double-pass picosecond Nd:YVO4 amplifier with repetition rate of 500 kHz. The average output power of 16.19 W was obtained at 500 kHz with the pulse width of 17.6 ps and a pump power of 31.7 W, corresponding to the output pulse energy of 32.38 \(\upmu\)J. The beam quality factors of \(M^{2}\) were measured to be \(M_{\pi}^{2}=1.28\) along the x axis direction and \(M_{\pi}^{2}=1.17\) along the y axis direction, respectively. Compared with the results of existing amplifiers [11; 12; 13], the optic-optic efficiency of 51.07% is higher with good beam quality and high frequency. Our measurements show that this amplifier structure enables the laser to be fully amplified and operates stably. The amplifier can be used in the fine processing of hard and brittle materials, such as laser drilling, laser scribing, laser cutting for glass, ceramics, silicon wafers, etc.