Objective Laser-matter interaction research using nanosecond pulse lasers has resulted in industrial applications, such as laser processing, laser marking, and laser cleaning, because of its advantages of moderate cost and high reliability. The repetition rate is an important indicator to describe the number of laser pulses per unit time. The higher the repetition frequency, the more the number of working pulses per unit time. Thus, high-quality processing results can be obtained via high-speed and high-precision processing using nanosecond pulse lasers. Therefore, development efforts in this field focus on achieving a narrow pulse width, high average power, and high repetition rate. With the increase in pump power, traditional lasers such as fiber lasers have strong nonlinear effects, such as stimulated Brillouin scattering and backscattering, resulting in a wider output pulse width; however, the peak power obtained is limited. Because of the radial distribution of the heat generated by the laser rod and the uneven temperature distribution, the radial temperature gradient will cause a serious thermal lens effect, which will affect the high power output and beam quality of the nanosecond laser. The emergence of thin-disk lasers has effectively improved this situation, wherein multipass pump technology is used to compensate for the considerably low single-pass absorption of the gain medium. The ingenious pump method improves its pump absorption efficiency, while the single-sided pumping back cooling method further reduces the thermal lens effect of the crystal, making it widely used in continuous and nanosecond pulse fields.
Methods A cavity-dumped laser based on an independently developed 24-pass thin-disk pump module was investigated with a high repetition rate, narrow pulse duration, and high beam quality. First, the relationship between media absorption and the number of pump passes was calculated according to the Beer-Lambert law. Then, using the quasi-three-level rate equations, the dependence of the threshold pump power and continuous output power on the transmittance of the output coupler was analyzed. Subsequently, to achieve the balance among absorption efficiency, thermal lens effect, and processing difficulty, the thickness of the thin crystal of the multipass pump module was selected to be 200 μm and the multipass number N was 24. Finally, to obtain the continuous and pulsed output of the linearly polarized laser, a Z-shaped cavity in which the thin film polarizer acts as both a polarizer and a high reflectivity mirror in the cavity, was designed.
Results and Discussions The continuous output performance of the Yb∶YAG quasi-three-level thin-disk laser is shown in Fig. 11, and the spot of the output laser and the measured beam quality M2 are shown in Fig. 12. Figure 11 shows that when the pump power reaches 30 W, the laser output starts. The pump power is close to the pump threshold power calculated theoretically (Fig. 5). Simultaneously, the output power increases linearly with the increase in pump power, the slope efficiency is 62.1%, the maximum output power is 94.1 W, and the maximum light-to-light conversion efficiency is 52.84%. Figure 12 shows that the output laser intensity distribution is Gaussian and the beam quality M2 is close to the diffraction limit. In addition, the cavity-dumped nanosecond pulse performance is shown in Fig. 13. Figure 13 shows that the pulse output power increases with an increase in the pump power and the slope efficiency is 45.3%. It can be seen from the right axis of Fig. 13 that the optical-optical conversion efficiency also increases with an increase in the pump power and gradually stabilizes. When the pump power reaches 180 W, the average pulse output power is 65.4 W, the peak power is 88.02 kW, and the optical-optical conversion efficiency is as high as 36.33%. The Yb∶YAG cavity-dumped thin-disk laser pulse sequence and waveform are shown in Fig. 14. Figure 14(a) shows that at a repetition frequency of 100 kHz, the pulse output sequence is stable, further, only some pulse amplitudes are lower than the average pulse amplitude. Air ionization can be observed in the experiment; therefore, it is speculated that the reason for the irregular pulse sequence may be that the entire resonant cavity is placed in an air environment. The excessive peak power causes the air to ionize, and finally, air disturbance affects the stability of the pulse output. It can be seen from Fig. 14(b) that the leading edge of the output pulse is very narrow and the trailing edge is wide, which is considerably consistent with the cavity-dumped waveform obtained from the theoretical calculation in Fig. 6, which proves the accuracy of the theoretical calculation results of the previous cavity dumping.
Conclusions Based on the Beer-Lambert-law and the quasi-three-level rate equations, the performance of the continuous and pulse outputs of the Yb∶YAG thin-disk laser is theoretically calculated and the effect of the multipass number, crystal thickness, and transmittance of the output coupling mirror on the laser output is also analyzed. Based on the 24-pass pump module independently designed by the laboratory and optimized cavity design, the maximum continuous output power can reach 94.1 W and the optical-optical conversion efficiency is 52.8% when the pump power is 180 W. At a repetition rate of 100 kHz, the maximum average output power of the pulse output is 65.4 W, the pulse output sequence is stable, and the pulse width is as narrow as 7.425 ns. The theoretical predictions are verified experimentally, and the experimental results are significantly higher than those reported in the previous literature, which has important reference significance for related research work. Because of the high peak power density in the cavity, air ionization occurs in the cavity. Therefore, to obtain high optical-optical conversion efficiency, better beam quality, and pulse stability, the next step is to place the cavity in a vacuum environment.
