Ytterbium-doped aluminate crystals have attracted increasing attention as gain media for ultrafast solid-state lasers owing to their broad gain bandwidth, favorable thermal properties, and high compatibility with diode pumping. Among them, Yb:(Y,Gd)AlO₃ (Yb:GYAP) is a newly developed multi-ion co-doped crystal that exhibits a higher degree of structural disorder than conventional Yb:YAlO₃. This feature effectively broadens the absorption and emission spectra and enables high-quality, high-concentration ytterbium doping. Therefore, Yb:(Y,Gd)AlO₃ shows strong potential for femtosecond pulse generation. However, experimental studies on its passive mode-locking performance, particularly those focusing on high average output power, remain relatively limited. This work experimentally investigates the passive mode-locking characteristics of Yb:(Y,Gd)AlO₃ and demonstrates a stable, high-power femtosecond laser output based on this gain medium.

The experimental system was built as a diode-pumped, passively mode-locked solid-state laser using Yb:(Y,Gd)AlO₃ crystal as the gain medium. The laser resonator employed a five-mirror folded cavity configuration. The optical layout for continuous-wave operation was shown in Fig.1, whereas the mode-locked cavity configuration was shown in Fig.2. A single-mode fiber-coupled 976 nm laser diode was used as the pump source to ensure high beam quality and stable pumping conditions. The pump beam was focused into the crystal to achieve efficient overlap between the pump mode and laser mode, and good mode matching was achieved by careful optimization of the cavity length. Passive mode locking was achieved by inserting a semiconductor saturable absorber mirror (SESAM) into the resonator. To support femtosecond pulse formation, negative dispersion was provided by Gires-Tournois interferometer (GTI) mirrors to precisely manage intracavity dispersion. The dispersion management strategy was designed to balance nonlinear phase accumulation and dispersion effects inside the cavity, which was critical for stable mode locking. The average output power, beam quality, spectrum, pulse duration, and radio frequency (RF) spectrum were systematically measured to evaluate the performance and stability of the mode-locked operation.

Stable continuous-wave (CW) mode locking was achieved when the absorbed pump power exceeded the mode-locking threshold. At the maximum operating point, the laser delivered an average output power of 223 mW, indicating efficient energy extraction from the Yb:(Y,Gd)AlO₃ crystal and demonstrating strong power capability under passive mode-locking. The spectrum of the mode-locked laser was shown in Fig.5, with a central wavelength of 1047 nm and a full width at half maximum (FWHM) bandwidth of 9.1 nm. The temporal characteristics of the output pulses were measured using an intensity autocorrelator, as shown in Fig.6. Assuming a sech² pulse profile, the fitted pulse duration was 138 fs and the time–bandwidth product was 0.343, which was close to the theoretical value of 0.315 for sech² pulses, indicating effective dispersion compensation in the resonator. The pulse train was shown in Fig.7, and no Q-switched mode locking or multiple-pulse instability was detected during long-term operation, demonstrating robust and reliable mode-locked performance. The RF spectrum was shown in Fig.8 and Fig.9. Within a 1 MHz span, the RF signal-to-noise ratio exceeded 60 dB, confirming a low level of amplitude noise in the pulse train. The pulse repetition rate was measured to be 100.5 MHz, which was consistent with the designed cavity length. Over a 1 GHz span, the RF spectrum remained uniform and well-structured, and no noticeable satellite pulses were observed, indicating stable operation of the mode-locked laser. Such stable RF characteristics were essential for practical applications requiring low-noise and long-term stable femtosecond laser sources. Notably, the achieved average output power of 223 mW represented the highest value reported so far for a SESAM-passively mode-locked Yb:(Y,Gd)AlO₃ femtosecond laser, highlighting the strong power scalability and excellent spectroscopic properties of this crystal.

A high-power, passively mode-locked femtosecond laser based on Yb:(Y,Gd)AlO₃ is successfully demonstrated. Using a five-mirror folded cavity, a SESAM enabling self-starting mode locking, and GTI mirrors for dispersion compensation, stable femtosecond pulse generation is achieved. These results confirm the great potential of Yb:(Y,Gd)AlO₃ for high-performance all-solid-state ultrafast lasers and provide valuable experimental guidance for further performance improvement and application development. Although the reported average output power has reached the highest level so far, the pulse duration still has room for further improvement. Future work will focus on optimizing the cavity configuration and adjusting the SESAM modulation depth to shorten the pulse durations while maintaining high average output power.