400 W average power Q-switched Yb:YAG thin-disk-laser
Saeid Radmard, Ahmad Moshaii & Kaveh Pasandideh
Scientific Reports volume 12, Article number: 16918 (2022) Cite this article
Abstract
We report on producing up to 403 W average power directly from an acousto-optically Q-switched Yb:YAG thin-disk laser (TDL). To achieve this power, it has theoretically and experimentally been shown that the laser stability border could be shifted toward higher repetition rates by engineering of the output coupler transmittance. This allows for stable operation of the laser at higher frequencies and a further increase in the power extraction from the active medium. Using an output coupler with 93% reflectivity, a maximum average power of 403 W at the repetition rate of 12.0 kHz has been recorded under the pump power of 1220 W. Furthermore, the maximum pulse energy of 57 mJ was produced at the repetition rate of 1.00 kHz and the pump power of 520 W. The characteristics of the laser at various Q-switching rates and the pump powers have been investigated. In addition, a numerical study for supporting the experimental results has been proposed here. To the best of our knowledge, the achieved average power and the pulse energy are the highest values reported to date from a Q-switched Yb:YAG TDL. The results pave the way to further power scaling of solid-state Q-switched oscillators.
Introduction
Thin-disk-lasers (TDLs) are a class of relatively low-cost and high-average power laser sources1. The unique specifications of these lasers in power and beam quality made them very attractive for producing both CW and pulsed laser systems2. Achieving a promising optical efficiency of 80% makes them more favorable for industrial applications3. The high average power TDLs with µs to ns pulse duration4,5, ultrafast pulses6,7 and high average power green TDLs8 have been reported. TDL devices with average powers of more than 10 kW in CW mode and several hundred watts in pulsed operation have been commercialized9.
In the high power operation, Q-switching, cavity dumping and oscillator-amplifier setup are the three main pulse generation methods in the µs or ns region10. Despite initial efforts to utilize these methods11,12,13, in the TDLs, cavity-dumping has been commonly used for pulse generation in this region, with average powers up to several hundreds of Watts5,14,15,16. However, it has some important drawbacks including high voltage driving, relatively high cost of the dumping elements, and the broad-spectrum17. On the other hand, Q-switching is a common way for generating pulsed solid-state lasers by using Acousto-Optical (AO) or Electro-Optical (EO) modulators. Compared to EO Q-switching and cavity-dumping, the AO Q-switching is attractive because it doesn’t need any high voltage and polarizing elements in the resonator, so it is less complicated and more economical10.
The average power of pulsed lasers is very important in industrial applications since it directly determines the processing speed18,19. In rod oscillators, the maximum average power is restricted by the fracture limit of the active medium20,21. Meanwhile, the thermal effects destroy the laser beam quality, so achieving higher average powers requires various amplification stages22,23. Alternatively, nonlinear effects and fiber damage are the main factors that challenge the power scaling of pulsed fiber lasers24,25. However, due to the active medium geometry, TDLs are less influenced by the above-mentioned restricting factors, and scaling-up of the average power at constant pump power density is realized1.
Noticeably, in Q-switched TDLs, two main factors restrict the scaling up of the output average power26. Both of these factors originate from the low gain coefficient of the active medium. The output coupler (OC) reflectivity typically is near one, so the cavity-internal-energy is high enough to damage the disk even for output pulses in the order of hundred mJ. Therefore, the laser repetition rate should be increased to enhance the average power. However, this could lead to strong output pulse energy fluctuation and the appearance of pulse instabilities13,27. This instability originates from the dynamic of Q-switched lasers and is still the subject of experimental and theoretical studies, also in other kinds of lasers26,28,29. Although pulse instability is an intrinsic property of the Q-switched lasers, it is expected to appear more pronouncedly in TDLs because of their low gain factor28. Active feedback controlling technologies could be implemented to stabilize the laser output in this region but add further complexities to the laser and limit their flexibility16,30.
