Yb:YAG Single Thin-Disk Regenerative Amplifier with High Stability and High Optical-to-Optical Efficiency

Objective In recent years, thin -disk lasers have been applied in many fields such as basic scientific research, industrial production, biomedicine, and defense. Owing to the significant advantages, such as power scalability, thermal performance, and nonlinear effects, thin-disk lasers hold great promising for high average and peak power laser while maintaining excellent beam quality. Scaling of both the average and peak powers of thin-disk lasers is possible by increasing the beam cross sections, while all internal intensities and the brightness of the pump laser are kept constant. However, the width of the dynamic stability zones of resonator cavities becomes narrower, and the output performance becomes more sensitive to cavity misalignment when the mode beam cross-section in resonators increases. These issues limit the further increase of output power of the thin-disk laser. This study reports a large-mode Yb:YAG thin-disk regenerative amplifier with active compensation for cavity misalignment. Methods The thermal focal length of a thin-disk module determines the mode distribution in the resonator cavity and should be measured before designing the cavity. The thermal focal length is measured at different pump powers using a wavefront sensor based on the principle of four -wave lateral shearing interferometry. By applying the ABCD matrix theory, the optical resonator of the thin -disk regenerative amplifier is designed and optimized, to ensure the operation of the fundamental mode and to enhance resistance to cavity misalignment. The optical layout of the thin -disk regenerative amplifier is shown in Fig. 1. The regenerative amplifier contains a seed laser with a narrow spectral width, an optical isolator, a Faraday rotator, a Pockels cell, thin-film polarizers, a resonator cavity, and a Yb:YAG thin-disk module with a 24 -pass pumping system. The thin disk module contains a Yb:YAG thin-disk crystal with free aperture and thickness of 9 mm and 215 p.m, respectively. The pump laser can deliver up to 500 W at a wavelength of 969 nm. The multipass pump spot on the Yb:YAG thin-disk crystal is circular with a super-Gaussian distribution and diameter of similar to 3.9 mm. To improve the output stability, a feedback system is applied in the regenerative amplifier for the active compensation of the cavity misalignment. The numerical results show that the cavity misalignment caused by the mirrors in the branch with a small mode size results in smaller displacement of mode beam on the thin -disk crystal compared to that caused by the mirrors in the branch with a large mode size. In addition, the cavity misalignment caused by curved end mirror M8 results in a minimal displacement of the mode beam on the thin-disk crystal, implying that the active compensation for the cavity misalignment by the mirror M8 leads to the highest adjustment precision. Results and Discussions When a seed laser with an energy of less than 1 nJ and a pulse width of 3.4 ns is injected into the thin -disk regenerative amplifier, and the pump laser operates continuously at 400 W power, the regenerative amplifier delivers average power values of 40.9 W and 53.3 W at repetition rates of 1 kHz and 10 kHz, respectively. The optical-to-optical efficiencies are 10.2% and 13.3%, respectively, and the single -pass small-signal net gain values are 1.147 and 1.129, respectively. The near- and far -field patterns of the amplified beam are measured and are shown in the insets in Figs. 4 and 5, respectively. The spatial quality factors Mx2 and My2 of the amplified beam at 1 kHz repetition rate are 1.12 and 1.10, respectively. Moreover, the M-x(2) and M-y(2) of the amplified beam at 10 kHz repetition rate are 1.07 and 1.06, respectively. The amplified beam exhibits an excellent power stability. The power stability is measured to be 6.42% (PV) and 0.56% (RMS) over a continuous period of 2 h, owing to the active compensation for cavity misalignment. By contrast, without active compensation for cavity misalignment, the average power of amplified beam decreases by 20% after more than 1 h of operation. In experiments of pulsed pump, when the pump pulse width and pump peak power are 600 p.s and 400 W, respectively, the amplifier delivers an average power of 38.7 W at a repetition rate of 1 kHz, with a high optical -to -optical efficiency of 16.1%. When the pump pulse width is 900 p.s, the amplifier delivers an average power of 42.0 W at a repetition rate of 1 kHz, with an optical -to -optical efficiency of 11.7%. Conclusions This study presents a regenerative amplifier with a Yb:YAG thin-disk module. When the pump power is 400 W, the amplifier delivers average powers of 40.9 W and 53.3 W at repetition rates of 1 kHz and 10 kHz, respectively. The amplified output exhibits a nearly diffraction -limited beam. Based on the active compensation for cavity misalignment, the Yb:YAG regenerative amplifier exhibits excellent output power stability, with a stability of 6.42% (PV) and 0.56% (RMS) over 2 h. In the pulsed -pump experiments, the optical -to -optical efficiency is as high as 16.1% when the pump pulse width is 600 p.s. In future work, the resonator cavity will be optimized, and the pump laser will be replaced by a laser with higher power.