High power, high energy ultrafast thin-disk laser oscillators have many important applications in the fields of scientific research, industry, biomedicine, and defense. Kerr-lens mode locking (KLM) is one of the most commonly used methods to generate ultrashort pulses directly from the thin disk oscillators. In this work, we review and discuss about the development of the KLM thin disk oscillators. The principle of KLM is introduced firstly, following by a presentation of the state-of-the-art of the ultrafast thin disk oscillators with respect to high average power, high repetition rate, short pulse duration, and new wavelengths. The applications and development prospect of the KLM thin disk oscillators are finally discussed.
Semiconductor saturable absorber mirror (SESAM) and KLM are two main mechanisms to realize ultrashort pulses from the thin disk oscillators. Compared with SESAM, KLM shows more advantages in modulation depth, damage threshold, and so on, and thus it enables higher power pulsed laser generation with a shorter pulse duration in the ambient air. In terms of high average power, Poetzlberger et al. introduced the active multi-pass cell (AMC) into the KLM thin-disk oscillator, increasing the laser gain within one roundtrip by the multiple passes of the laser pulse through the thin disk medium. The increased gain enables a higher output coupling rate and results in a higher output power. Under an output coupling rate of 50%, 150-W pulses with a pulse duration of 290 fs can be achieved (Fig. 2). For the shorter pulse duration, limited by the narrow emission bandwidth of the gain medium, it is difficult to achieve laser pulses with durations below 100 fs directly from a Yb∶YAG thin-disk oscillator. To solve this problem, on the one hand, one can utilize new materials with wider emission bandwidths as thin-disk gain media, such as Yb∶Lu2O3 and Yb∶CALGO. On the other hand, a broader spectrum can be generated by improving the mode locking technique with a stronger modulation depth. In 2021, Zhang et al. invented a distributed Kerr-lens mode-locking (DKLM) technique comprising of multiple Kerr-lenses in a Yb∶YAG thin-disk oscillator. It significantly increases the self-amplitude modulation (SAM) coefficient for KLM. The resulting spectral width exceeds the emission bandwidth of the Yb∶YAG gain medium by a factor of approximately four, leading to 47-fs pulse generation directly from the Yb∶YAG thin-disk oscillator (Figs. 3 and 4). Very importantly, the new concept is also applicable to other types of gain media, which may lead to new records in the generation of ultrashort pulses. With the same concept, Drs et al. realized 27-fs pulse generation from a Kerr-lens mode locked Yb∶YAG thin-disk oscillator by increasing the modulation depth, which is the shortest pulse duration ever obtained from a Yb∶YAG thin-disk oscillator. For a higher repetition rate, a strongly asymmetric configuration comprising of two concave mirrors with different radii of curvature is used to ensure a large beam size on the thin disk in a short oscillator cavity, and the 75-W, 260-fs pulse with a repetition rate of up to 260 MHz is obtained in this scheme, which is the highest repetition rate ever realized in a thin-disk oscillator (Fig. 5). Besides, for the generation of a new wavelength directly from the thin disk oscillator, Zhang et al. successfully realized the first femtosecond Ho∶YAG KLM thin-disk oscillator with an average power of up to 25 W, providing a solid foundation for the further development of 2 μm ultrafast thin-disk lasers. Compared with 1 μm laser, 2 μm femtosecond laser shows more advantages in driving the nonlinear frequency conversion for the generation of a mid-infrared laser due to the low multiphoton absorption and high conversion efficiency. Besides, more crystals can be used for the nonlinear conversion process with the 2 μm driving source, which is not possible with 1 μm driving source. With the 2 μm femtosecond thin-disk oscillator, Zhang et al. generated a broadband mid-infrared laser via intra-pulse difference-frequency generation (IDFG). The 2 μm femtosecond pulses delivered from the oscillator are first coupled into a photonic crystal fiber (PCF) for a further spectral broadening and temporal compression, and then focused onto the nonlinear crystal for the IDFG process. Nonlinear crystals such as GaSe and ZnS/ZnSe are used, and mid-infrared laser pulses with spectra ranging from ~3 μm to ~20 μm are generated finally.
Great progress has been made in 1 μm KLM thin-disk oscillators in terms of average power, pulse energy, pulse duration, and repetition rate during the last decades, which is beneficial for a lot of applications in the fields of fundamental research, industry, and medical science. At the same time, the realization of 2 μm KLM thin disk oscillators also makes itself more attractive for mid-infrared laser generation. In the future, more and more new materials will be explored as the thin-disk gain media, and pulses with shorter pulse duration, kW-level average power, and mJ-level energy can be expected from thin-disk oscillators.
