High-power ultrafast solid-state lasers play an important role in various industrial and scientific applications, including high-precision micromachining, high-resolution microscopic imaging, and medical diagnosis. However, the thermal effects in solid-state media become a limiting factor at high pumping powers, restricting the scalability of the output power in the ultrafast pulse regime. In high-power ultrafast lasers, a gain-medium geometry with high aspect ratio is critical for efficient heat dissipation. Common solid-state laser geometries, such as fibers and thin disks (TDs), are widely used for the generation and amplification of ultrafast laser pulses. Although fiber lasers offer advantages, high nonlinear effects remain a significant challenge in achieving high peak power. In contrast, TD, with favorable characteristics, is the medium of choice for directly generating high-power ultrafast femtosecond laser pulses. TD has a disk-shaped gain medium, typically with a thickness of 100?300 μm, and a much larger diameter of up to several mm. The back side of the disk is thermally coupled to a water-cooled heat sink, effectively mitigating the thermal lensing effects. The TD-geometry facilitates the direct generation of ultrafast laser pulses in mode-locked oscillators because of its good thermal management combined with small optical nonlinearities and has successfully generated femtosecond pulses using Kerr lens mode-locking (KLM) and a semiconductor saturable absorber mirror (SESAM). Although SESAM-based oscillators are not limited by average power scaling, achieving shorter pulses beyond the bandwidth limit remains challenging because of intrinsic SESAM characteristics. In contrast, the TD concept, integrated with a mode-locked (ML) regime, is the method of choice for generating pulses beyond the gain bandwidth limit, owing to its large modulation depth and ultrafast relaxation time. Thus, the study of KLM thin-disk laser (TDL) is of significant importance for generating high-power ultrafast pulses. However, because of the technical constraints related to thin-disk laser heads, most disk heads used for TDLs are sourced from the German companies Trumpf and D&G GmbH, with few reports on KLM TDLs based on domestic disk heads. The potential of domestic disk heads in high-average-power ultrafast oscillators in the femtosecond regime is yet to be fully developed and applied.

In this study, we investigated a KLM TD oscillator using a homemade thin-disk laser head that used doped Yb∶YAG crystal as the gain medium with a thickness of 150 μm and a diameter of 8.8 mm. The back end of the crystal was coated with highly reflective (HR) films at 1030 nm and 969 nm (reflectivity R>99.9%), and the front end was coated with anti-reflection (AR) films (R<0.1%). The Yb∶YAG disk head employed a 969 nm zero-phonon-line (ZPL) pumping source and a 48-pass pumping system to achieve high absorption of the pump laser (absorptivity of >95%). The crystal pump diameter was 2.3 mm. For the Kerr mode-locked-operation oscillator, a simple Z-shaped folded cavity was adopted, and the TD was placed between the HR end-mirror and output coupler (OC). For a high gain per pass, the laser beam was reflected twice through the TD using a pair of 45° mirrors, enabling an 8-pass configuration in one round trip. The mode diameters of the thin disk for the two reflections were 2 mm and 1.65 mm. To compensate for the high gain, the transmittance of the output coupling mirror was set to 15%. The total length of the cavity was about 4.8 m, and the repetition rate was 31.35 MHz. Considering the small thickness of TD and large mode size, a separate Kerr medium (KM) of 2 mm thick CaF₂ plate was placed at the Brewster angle at the focal point of concave mirrors CM1 and CM2. The radius of curvature of both curved mirrors was 500 mm. A Kerr medium with a lower nonlinear refractive index and higher band gap is beneficial for obtaining high peak-power pulses and preserving the single-pulsed regime. Along with a hard aperture of 3.8 mm near the OC, anomalous group delay dispersion was introduced through highly reflective mirrors HD1 and HD2, with a total negative dispersion of -24000 fs² per round trip.

To start off KLM, a strong sensitivity to the resonator mode was obtained by operating the laser at the edges of the stability zones. This corresponds to an increase in the separation between the curved mirrors. At this stage, mode-locked operation was initiated by perturbing the end mirror. A stable mode-locked operation with an output power of 50.4 W at 206 W pumping was realized using 15% OC. In the laboratory environment, we recorded the pulse train using an oscilloscope, and no Q-switched mode locking was observed. The output spectrum width at full width half maximum (FWHM) was 3.01 nm, and the pulse duration was 392 fs. This corresponds to a time-bandwidth product of 0.333, which is slightly larger than the theoretical value of 0.315. The signal-to-noise ratio (SNR) of the 31.35 MHz fundamental signal in the radio frequency (RF) spectrum was 67 dB. The laser beam quality factors were measured to be Mx2=1.05 and My2=1.1, which are close to the diffraction limit. The mode-locked operation remained stable for over 30 min, with an average root mean square (RMS) power fluctuation of 1.07%.

In this study, a femtosecond thin-disk oscillator was demonstrated with high average power and short pulse duration using the Kerr lens mode-locking technique based on our custom-designed thin-disk head. The laser beam underwent multiple direct passes (8-passes) through the thin disk to enhance optical-to-optical conversion efficiency. The TD-KLM oscillator generated 392 fs pulses with an average power of 50.4 W at a repetition rate of 31.35 MHz, using a 969 nm ZPL pumping source with a power of 206 W. The corresponding pulse energy was 1.6 μJ. Future improvements in average power, pulse energy, and optical efficiency are anticipated by increasing the mode size of both the Kerr medium and thin disk, using larger radius of curvature (ROC) focusing mirrors, thereby increasing the transmittance of the output coupler and the number of passes through the TD per round trip. Additionally, the oscillator can be placed inside a water-cooled housing in clean, dust-free environment to decrease the influence of air and temperature fluctuations.