High-power femtosecond lasers pumped by laser diodes (LDs) play a significant role in industrial processing and scientific research. Femtosecond lasers are generated directly using mode-locked lasers. The components of a mode-locked laser include a gain medium, pump source, mode-locked device, and dispersion compensation device. The gain medium is the core of the laser; it provides population inversion and generates excited radiation. With the emergence of high-brightness and high-power semiconductor lasers, ytterbium ion (Yb³⁺)-doped solid-state laser materials have rapidly developed and become one of the most important gain media in the field of high-power and femtosecond lasers. High-power Yb-femtosecond lasers are mainly based on semiconductor saturable absorber mirror (SESAM) mode-locking technology and Kerr lens mode-locking (KLM) technology. Generally, both passively mode-locked and Kerr lens mode-locked femtosecond lasers need to introduce a certain amount of negative group delay dispersion (GDD) to balance the self-phase modulation in the cavity and produce stable femtosecond solitons. Particularly, as the average power increases, the intracavity self-phase modulation becomes stronger and more negative GDD is needed. It is well known that dispersion compensating devices include prism pairs, chirped mirrors, and Gires-Tournois interferometer (GTI) mirrors. Prism pairs lead to complex oscillator structures, while chirped mirrors and GTI mirrors are more expensive. Therefore, high-power Kerr lens mode-locked lasers without dispersion compensation devices are of great research significance for reducing the cost of femtosecond lasers. Based on the above background, this study evaluates the Kerr lens mode-locking technique using non-GDD-optimized broadband highly reflective mirrors for dispersion compensation. Because broadband highly reflective mirrors tend to be negatively dispersive in the band at wavelengths longer than their center wavelengths, we propose the use of broadband highly reflective mirrors instead of GTI mirrors to realize high-power Kerr lens-locked operation based on Yb∶CYA crystals.

The experimental setup is a dual-confocal cavity, and the pump source is a 50-W fiber coupled output LD at a wavelength of 976 nm with a beam quality factor (M2) of approximately 25. The pump is imaged into the crystal with a 104 μm-diameter spot by an imaging system. The laser spot size in the designed resonant cavity is simulated in the simulation software, and the beam waist radius size of the laser in the gain medium is calculated to be 70 μm. The spot diameter of laser mode is slightly larger than that of the pump light mode, which is conducive for the formation of a soft-aperture diaphragm. The cross-section area of 6-mm long Yb∶CYA crystal used in the oscillator is 3 mm×3 mm. The absorption slope of the pump is 93%. For the thermal load dissipation, the crystal is wrapped in indium foil and mounted tightly on a water-cooled copper heat sink maintained at a constant temperature. The nonlinear Kerr medium is a thin slice of CaF₂ with a thickness of 2 mm. The Kerr medium and the incident light are placed at the Brewster angle to compensate for the astigmatism introduced by the folding angle of the concave mirrors and maintain the linear polarization of the laser inside the cavity. Moreover, we use four broadband highly reflective mirrors covering 750-1100 nm in the cavity for dispersion management, because these highly reflective mirrors have negative dispersion in the bands at wavelengths larger than the central wavelength of 950 nm and positive dispersion in the bands at wavelengths smaller than the central wavelength. We measure the amount of negative dispersion introduced by the broadband high-reflective mirrors near the wavelength of 1030 nm, and each of them can provide GDD of approximately -550 fs2 for a single bounce; thus, the net dispersion in the cavity is -1520 fs2 using four high-reflective mirrors. The resonant cavity has a single cavity length of 1.85 m, corresponding to a repetition rate of approximately 81.1 MHz.

A stable mode-locked operation with an output power of 3.6 W, spectral full width at half-maximum of 15.2 nm, and pulse width of 92 fs is achieved at 18 W pumping using an output coupler with transmittance of 15%. The root mean square (RMS) of the average power fluctuation during the mode-locking operation is 0.46% over 100 min, and the beam quality factors of the mode-locked laser in x and y directions are 1.24 and 1.22, respectively.

Using broadband highly reflective mirrors instead of expensive GTI mirrors, a stable mode-locking operation with a high average power and short pulse duration is achieved, significantly decreasing the laser cost. Moreover, it is believed that such low-cost all-solid-state femtosecond lasers, which can directly produce high power, narrow pulse widths, good stability, and high beam quality, will become popular in frontier scientific research and industrial processing.