High-power femtosecond lasers are widely utilized in various research fields such as terahertz (THz) generation, frequency comb, and high harmonic generation because of their short pulsed width, high peak frequency, and good beam quality. However, owing to strong thermal lens effect, the output power of lasers based on traditional bulk-shaped gain medium is limited to the 20 W level. Further, fiber lasers are restricted by nonlinear effects such as stimulated Brillouin scattering at high output power. The emergence of thin disk lasers (TDL) has facilitated the simultaneous solving of these two problems. A thickness of only a few hundreds of micrometers combined with effective back-side cooling technique can significantly reduce the influence of thermal lens effect and nonlinear effects. In addition, multiple pass pump module guarantees high absorption efficiency under the condition that the gain medium is extremely thin. Currently, femtosecond lasers based on thin disk medium are mainly achieved using the semiconductor saturable absorber mirror (SESAM) mode-locking and Kerr Lens mode-locking (KLM). The highest output power obtained using the former (350 W) is higher than that of KLM (270 W); however, KLM lasers exhibit better performance in the aspect of pulse width, peak power, cost, and stability of key components. Thus, KLM TDL is a promising candidate for light sources in high field science and nonlinear optical researches.
We investigate KLM thin disk lasers by using our home-made 72-pass pump module. First, we analyze the principles of Kerr lens mode-locking in thin disk lasers to ensure that the mode radius at the pinhole decreases at a proper ration following the insertion of the Kerr medium, or the virtual Kerr Lens at the beam waist. Subsequently, we demonstrate the cavity design method based on ABCD matrix and soliton mode-locking theory. On the basis of this method, we design the KLM TDL cavity by employing a focusing mirror pair with the radius of curvature (RoC) of 150 mm. In addition, we simulate the mode radius in the cavity under continuous wave (CW) and KLM operation with an iteration algorithm. Finally, a KLM TDL experiment is conducted based on a 72-pass pump module. Moreover, the cavity is optimized based on the former experiment, and the higher output power is achieved.
The experiment conducted with the first cavity structure yields a femtosecond output of 11.78 W at fundamental transverse mode when pumped at 72 W. The pulse train with a frequency of 81.45 MHz measured via an oscilloscope shows good stability in the time scale of 2 μs (Fig. 8). The pulse width measured using an autocorrelator to be 243 fs, assuming sech2 pulse shape, is shown in Fig. 9. Further, the spectral width is measured to be 4.6 mm (Fig. 10). The multiple peaks in the spectrum may be caused by the uneven group delay dispersion (GDD) curve of the home-made dispersive mirrors in the wavelength range of 1025-1035 nm. The corresponding time-bandwidth product is 0.317, which is slightly larger than the theoretic minimum value. On further increasing the pump power to 81 W, stable double pulse output can be observed (Fig. 11). This phenomenon is attributed to the strong spectral broadening effect under high power single pulse operation. If the output mode transforms into double pulse operation, the narrowed spectrum fits better into the emission spectrum of Yb∶YAG.
Following the optimization of the cavity structure by increasing the RoC of the focusing lens pair to 200 mm, the output power is increased to 22.33 W at the pump power of 94 W with an optical-to-optical efficiency of 24%. However, the repetition frequency remains almost unchanged at 79.36 MHz and the pulse energy is 0.28 μJ. The corresponding pulse width is measured to be 393 fs (Fig. 13).
We analyze the principles of Kerr lens mode-locked thin disk lasers based on a 72-pass pump module and the design of the KLM cavity. The design principles are examined according to the ABCD matrix method and soliton mode-locking theory. Finally, the KLM laser oscillator based on thin disk medium is constructed. The stable femtosecond pulse output at 11.78 W with the pulse width of 245 fs and repetition rate of 81.45 MHz is obtained when pumped at 72 W. The output result is nearly identical to the designed target. After changing the RoC of the focusing mirror pair to 200 mm, the output power is increased to 22.33 W at the pump power of 94 W with the corresponding pulse width of 393 fs. In future studies, to increase the output power to a higher level, the RoC of focusing mirrors will be increased, and the optimized dispersion compensation will be provided. In addition, the cavity will be placed in a vacuum environment to reduce the influence of air disturbance and air dispersion.
