High Efficiency Yb:YAG Thin Disk Laser Based on Zero Phonon Line Pumping

There is a growing demand for high-power lasers in industries such as laser cutting, welding, heat treatment, and marking. High efficiency oscillators with scalable power have the potential to streamline the system complexity and cost per Watt compared to amplification stages. The main challenge for power scaling is the heat load on the gain medium. Yb:YAG crystals are attractive gain mediums because they offer high thermal conductivity, wide pump bands, low quantum defects, relatively long fluorescent lifetime, and high-quality crystal growth. Their energy-level scheme eliminates the problem of excited-state absorption, which makes the quantum defect become the primary heat source in the crystal. Complementing these advantages with suitable architectures with high surface-to-volume ratios such as fiber and Thin-disk Lasers(TDLs) enables excellent heat extraction. However, fiber lasers still face the challenge of nonlinear effects such as self-phase modulation and stimulated Raman scattering at high power. Alternatively, thin-disk lasers have improved the situation. We demonstrate a Yb:YAG thin-disk laser that uses zero phonon line pumping. This type of laser system offers excellent power stability, high optical-optical conversion efficiency(> 73%), and remarkable thermal management ability. Our design provides theoretical support for future research on the generation of kilowatt-level continuous and ultrafast pulses laser. We hope that our basic strategy and findings can be helpful in the design of higher power, more efficient thin-disk lasers and make them widely available in the industry. The Yb:YAG crystal is simple in its energy level configuration, with only two energy states of the Yb3+ ion. The absorption peaks of the crystal are at 940 nm and 969 nm, with broadband pumping achieved near 940 nm resulting in a quantum defect of 8.7%. By opting for 969 nm wavelength pumping, the quantum defect decreases to 5.9%, reducing the thermal load by over 30% and improving the beam quality by diminishing thermal lensing and distortion effects. The pump module of the experimental device consists of a pumping source, a parabolic mirror, and two sets of folding prisms. A thin-disk crystal is placed at the focal point of the parabolic mirror and undergoes 12 reflections, equating to 24 round-trip passes, as the pumping light passes through the folding prisms and mirror. The final step involves redirecting the pumping light path using a planar reflector, ultimately achieving 48-pulse absorption. The frontal side of the thin-disk crystal has an antireflection layer, and the back side has a high reflection coating. The crystal is bonded onto a diamond substrate to facilitate superior thermal contact, low thermal resistance, and high mechanical strength, and the jet impingement technology is employed to cool the diamond heat sink. This setup and efficient cooling design improves absorption efficiency and reduce crystal thermal load, significantly enhancing laser output performance. We calculated the correlation between the absorption efficiency of the crystal and the number of pump cycles. As the number of cycles increases, so does the absorption efficiency. However, the efficiency eventually reaches a saturation point after a specific number of cycles. In this case, at cycle number 48, the crystal absorbs 95.3% of the pump light, indicating that it is almost fully saturated at this point. In the experiment, the impact of two pumping wavelengths, 969 nm and 940 nm, on the conversion efficiency of a Yb:YAG thin-disk laser was analyzed. The results clearly demonstrate that the selection of the pumping wavelength can improve the quantum conversion efficiency and reduce the waste heat in the crystal. Besides, the design of the multiple pass pumping system can also significantly improve the resonator efficiency and crystal absorption efficiency. Utilizing a 969 nm zero phonon line pumping results in a reduced threshold pump power and a significant increase in optical-optical conversion efficiency. With the gradual increase in pump power, the optical-optical conversion efficiency also enhances. An output coupling experiment is performed using an output coupler with a transmission rate of 2%, and the overall structure of the laser system is also shown. A continuous wave Yb:YAG thin disk laser system has been successfully developed, which exhibits exceptional capabilities. The system utilizes a VBG locked-wavelength 969 nm pump source and a 48-pass design. The cooling system employs jet impingement technology on a diamond heat sink. The experiment successfully achieved a maximum output power of 373 W and a peak optical-optical conversion efficiency of 73.37%. The Root Mean Square (RMS) power stability over a 2-hour period is less than 0.2%. The research shows the exceptional thermal management capabilities of this thin-disk laser system. It provides a foundation for future studies involving multi-kilowatt continuous thin-disk lasers, kilowatt-level green lasers, and ultrafast thin-disk lasers. We believe that the thin disk lasers with many advantages can be the ideal solution for the next generation of high-power, high-energy, peak-power lasers.