Objective In recent years, thin- disk lasers have been applied in various fields such as basic scientific research, industrial production, biomedicine, and defense. Owing to their significant advantages in terms of power scalability, thermal performance, and nonlinear effects, thin- disk lasers show promise for combining high average power and high peak power with excellent beam quality. Regenerative amplification is the technology that best suits thin disk lasers. The output power of the amplifier is increased by increasing gains and reducing the losses. Currently, regenerative amplifiers typically increase the gain by either enlarging the pump area or employing multiple thin- disk modules. However, in the former method, the amplified spontaneous emission (ASE) also increases simultaneously, whereas the latter method involves a more complex and uneconomical optical path. Double- pass regeneration is a promising technology in which a beam passes through a thin disk four times in a round trip, ultimately reducing the total loss by decreasing the number of round trips. In this study, we report a compact Yb:YAG thin- disk double- pass regenerative amplifier. The amplifier has a maximum output power of 130 W and an optical- to- optical efficiency of 26 degrees o. Methods The thermal focal length of a thin- disk medium determines the mode distribution in the resonator cavity and should be measured before designing the cavity. Using a wavefront sensor based on the principle of four- wave lateral shearing interferometry, the thermal focal length is measured at various pump power levels. By applying the ABCD matrix theory, the optical resonator of the thin- disk regenerative amplifier is designed and optimized to ensure operation in the fundamental mode and insensitivity to cavity misalignment. The pulses are passed through the thin- disk medium twice at intervals longer than the pulse width. Additionally, the beam diameters in the medium are similar. The optical layout of the thin- disk double- pass regenerative amplifier is shown in Fig. 1. It includes a seed laser with a narrow spectral width, optical isolator, Faraday rotator, Pockels cell, thin-film polarizers, resonator cavity, and Yb:YAG thin- disk module with a 24- pass pumping system. The thin- disk module consists of a doped Yb:YAG thin- disk crystal with a 9 mm aperture and a thickness of 215 mu m. The pump laser can deliver power up to 500 W at a wavelength of 969 nm. The multipass pump spot on the Yb:YAG thin- disk crystal has a circular shape with a super- Gaussian distribution, and its diameter is approximately 3.9 mm. In addition, the particle rate equation is used to calculate the saturated output pulse energy values of the single- pass and double- pass regenerative amplifiers at a continuous pump power of 500 W. The results indicate a significant improvement in the output of the optimized cavity. Results and Discussions When a single longitudinal- mode seed laser with a pulse width of 3.4 ns, repetition rate of 10 kHz, and energy of approximately 1 nJ is injected for amplification, the regenerative amplifier delivers an average power of 130 W at a pump power of 500 W. This results in an optical- to- optical efficiency of 26 degrees o. The amplifier outputs a pulse close to the diffraction limit with beam quality factors of 1.20 and 1.15 in the horizontal and vertical directions, respectively. The near- and far- field patterns of the amplified beam are measured, and are shown in the insets of Figs. 5 and 8, respectively. Another advantage of double- pass regenerative amplification is the reduced impact of the cavity offset on stability. This is owing to the reduction in the number of round trips. The peak- to- valley values (PVs) and root mean square (RMS) of output power stability for the double- pass regenerative amplifier within 3.5 h are 5.77 degrees o and 0.77 degrees o, respectively. When amplifying the pulses with a repetition rate of 1 kHz, the amplifier delivers an average power of 67 W at a pump power of 500 W. The corresponding optical- to- optical efficiency is 13.3 degrees o. In addition, we measure the output powers of the pulses with multiple repetition rates at a pump power of 500 W, as shown in Fig. 9. The waveforms of the pulses with repetition rates of 1 kHz and 10 kHz at the maximum output power are measured, with some waveform distortion occurring in the former. Conclusions In this study, we present the results of our study on a double- pass regenerative amplifier that utilizes a single Yb:YAG thin- disk module. When the pump power is 500 W, the amplifier delivers output powers of 67 W and 130 W at repetition rates of 1 kHz and 10 kHz, respectively. The corresponding optical- to- optical efficiencies are 13.4 degrees o and 26 degrees o. The output beam is close to the diffraction limit. The cavity type of the double- pass regenerative amplifier shows good stability, with PVs and RMS of output power stability measuring 5.77 degrees o and 0.77 degrees o, respectively, within 3.5 h. Thin- disk double- pass regenerative amplifiers demonstrate excellent performance. In the future, we will continue to increase the pump power or the mode diameter to achieve improved laser output. In addition, we will amplify the broadened nanosecond chirp pulse to obtain a high- repetition picosecond laser output.
