310 W picosecond laser based on Nd:YVO4 and Nd:YAG rod amplifiers

https://doi.org/10.1016/j.optlastec.2021.107668Get rights and content

Highlights

  • A high-power stable picosecond laser based on Nd:YVO4 and Nd:YAG rod amplifiers was proposed.

  • A maximum average output power of 310 W at 4.27 MHz was obtained.

  • A maximum peak power of 86.6 MW at 300 kHz was generated with a pulse energy of 700 μJ and a pulse duration of 8.08 ps.

Abstract

Herein, a high-power picosecond laser system based on master oscillator power amplifier structure is demonstrated. The seed source is a SESAM mode-locked fiber oscillator; the pulse repetition frequency is selected by an acoustic optical modulator with a tunable range from 100 kHz to 4.27 MHz. The amplification system includes one fiber amplifier stage, four stages of end-pumped Nd:YVO4 amplifiers, and two stages of side-pumped Nd:YAG amplifiers. Finally, the average output power of >300 W is achieved. The maximum peak power is 86.6 MW at 300 kHz with a pulse energy of ∼700 μJ and a pulse duration of ∼8.08 ps.

Introduction

For material micromachining applications, including precise drilling, surface ablation, and cutting, a high average output power laser with short pulse width and low pulse repetition frequency (PRF) is instrumental [1], [2], [3]. Furthermore, the rapid development of ultrafast high-power lasers is essential for a wide range of scientific applications such as nonlinear frequency conversion [4], [5], [6].

To obtain high average output power, there are two common methods: one method involves using a high-power oscillator, which has been confirmed to produce average output powers of several hundreds of watts with picosecond or femtosecond pulse duration without requiring any extra amplification stages. The gain medium could be conventional Yb:YAG thin disk, and the pulses are generated using semiconductor saturable absorber mirror (SESAM) mode locker. However, the power would be limited by the damage threshold of the SESAM [7], [8]. Another approach for achieving high-power output involves using master oscillator power amplifier (MOPA) system in which a low power seed laser is amplified by cascaded amplifiers to increase the output power. Furthermore, the MOPA structure has certain intrinsic advantages. There are two popular amplifier structures for achieving picosecond lasers with output powers of several hundred watts: fiber amplifiers and solid-state amplifiers.

Fiber amplifiers have the advantages of good beam quality, good heat dissipation characteristics, high efficiency, compact structure, and high reliability. Furthermore, a picosecond fiber amplifier has been reported to deliver kilowatt output power levels [9]. However, in ultrafast pulse operation, the fiber introduces strong nonlinear effects such as Raman scattering and self-phase-modulation. Consequently, the PRF of a fiber amplifier is always tens of MHz or even GHz to decrease the pulse peak power [9], [10], [11], [12]. Therefore, fiber lasers cannot achieve ultra-high peak power and ultra-high single pulse energy.

To achieve high peak power ultrafast pulses, all-solid-state amplifiers would be a better choice. In recent years, the all-solid-state laser technology has recorded a series of new developments, providing new solutions for building ultrafast high-power lasers. Nd:YAG and Nd:YVO4 crystals have confirmed to be suitable gain media for high-power pulsed laser amplification [13], [14], [15], [16], [17], [18]. Usually, the reduction of thermal distortions in high average power lasers is performed by increasing the surface/volume ratio of the gain medium such as slab lasers and thin disk lasers. Moreover, the beam sizes can be designed to be sufficiently large to avoid laser induced damage. The Yb:YAG slab has been used to achieve average power of up to 1.1 kW while producing 55 μJ pulses with a duration of 600 fs, and 300 W while producing 3 mJ pulses with a duration of 900 fs [19]. However, for the slab laser, the optical setup is complex and the beam quality is relatively poor. For thin disk laser system, the average output power for a femtosecond and picosecond pulses have exceeded 1 kW [20], [21]. However, the propagation length available for the absorption of the pump is extremely small, in addition to the gain per pass of the gain medium. Moreover, the production process of thin-disk gain medium is extremely complicated. Therefore, to achieve a higher average output power with good beam quality, simple structure and low cost, the traditional rod gain medium would be a better alternative [18], [22], [23].

In this study, we demonstrate a high average power MOPA laser system, which comprises fiber seed laser, fiber amplifier and all solid-stated amplifiers. The seed laser is a mode-locked picosecond fiber laser, and is amplified to 50 W using a one-stage fiber amplifier and two-stage end-pumped Nd:YVO4 rod amplifiers with a beam quality factor (M2) of 1.26. Subsequently, to avoid laser-induced damages, the laser is amplified by another two stages of end-pumped Nd:YVO4 rod amplifiers and two stages of side-pumped Nd:YAG rod amplifiers. The maximum average output power reaches 310 W when the PRF is 4.27 MHz and the pulse duration is 8.08 ps. The maximum peak power is 86.6 MW @300 kHz with average output power and single pulse energy of 210 W and 700 μJ, respectively. Moreover, the seed and amplifiers are designed into modules, which can be easily assembled for commercial and scientific applications.

Section snippets

Experimental setup

Fig. 1 shows the experimental setup of the high-power picosecond MOPA system. The seed is an all-fiber laser mode-locked by SESAM with a pulse duration of 7.6 ps and a PRF of 20 MHz. A pulse picker, [an acoustic optical modulator (AOM)], reduces the PRF to enhance the peak power and pulse energy. To increase the output power, the milliwatt pulses are amplified first by a fiber amplifier and then by four stages of end-pumped Nd:YVO4 amplifiers. The amplified pulses from the second end-pumped

Results and discussion

To reduce the thermal effect and increase the output power, a laser system with multi-stage amplifiers was introduced. After the pulse picker, the PRF can be tuned from 100 kHz to 4.27 MHz. The laser central wavelength is 1064.8 nm with a spectral linewidth of 0.74 nm. The beam quality (M2) was measured as 1.26.

To achieve a higher average output power, the PRF of the seed laser was first set to 4.27 MHz, and the temperature of the water cooling system was set to 20 °C. The average output power

Conclusion

In summary, we demonstrated a picosecond MOPA system based on end-pumped Nd:YVO4 rod amplifiers and side-pumped Nd:YAG rod amplifiers. Using the multi-stage amplifiers configuration, we achieved a maximum output power of 310 W at 4.27 MHz with a pulse duration of 8.08 ps. The maximum peak power was 86.6 MW at 300 kHz with a pulse energy of ∼700 μJ. Unfortunately, the beam quality was deteriorated by the thermal lensing effect and thermal-induced birefringence. However, this issue can be

CRediT authorship contribution statement

Deqin Ouyang: Conceptualization, Investigation, Data curation, Validation, Writing – original draft. Yewang Chen: Formal analysis, Methodology. Minqiu Liu: Software. Xu Wu: Methodology, Data curation, Validation. Qiguo Yang: Methodology, Investigation. Fanghua Xu: Methodology, Investigation. Murong Zhong: Methodology. Qitao Lue: Methodology, Project administration, Supervision. Shuangchen Ruan: Resources, Project administration, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant No. 61905146, Free Exploration Project of Shenzhen Basic Research under Grant No. JCYJ20180301171044707, Shenzhen key Project for Technology Development under Grand No. JSGG20190819175801678 & JSGG20191129105838333 and Guangdong Provincial Department of Education Youth Innovation Talent Project under Grant No. 2018KQNCX400.

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