## 3 Experimental Results and Discussions  
We firstly characterized the CW properties and wavelength tunability of the Yb:GYSO laser by a simple three-mirror cavity consisting of a plane dichroic mirror, a concave mirror with ROC of 200 mm, and a plane output coupler. Three output couplers with different transmissions (0.8%, 2.5%, and 10%) were used to test the output power. For three OCs the laser wavelengths were all at 1091 nm. The dependence of the CW laser output power on the incident pump power is illustrated in Figure 2. The output power was saturated when the pump power reached 5.6 W. The highest output power of 2.62 W was obtained with a 10% OC. Before saturation, the slope efficiency was about 71%, and it was 58.0% and 47.4% for 2.5% OC and 0.8% OC, respectively. The pump threshold for 0.8% and 2.5% OCs was only 0.61 W. Such low lasing threshold verified the large ground state splitting of the Yb:GYSO crystal (995 cm\({}^{-1}\)), where thermal population of the upper laser level was rare. Continuous wavelength tuning of the Yb:GYSO laser was carried out by inserting a highly dispersive SF6 prism into the three-mirror cavity. A 0.8% transmission output coupler was used to explore the wavelength tunability. At a pump power of 3.8 W, the tunable wavelength range is from 1004 to 1110 nm, and as broad as 106 nm, as shown in Figure 3. This was the broadest tuning range from an Yb:GYSO laser, even broader than an Yb:GSO laser [24]. The highest output power appeared at 1092 nm, which indicated that at longer wavelengths, the reabsorption loss was greatly reduced. The very broad and  
Figure 3: Wavelength tuning curve of the Yb:GYSO laser with a 0.8% transmission output coupler at 3.8 W pump power. Figure 2: CW output power of the Yb:GYSO laser _versus_ the incident pump power for three OCs with different transmissions (0.8%, 2.5%, and 10%). smooth wavelength tuning of the Yb:GYSO crystal were superior for generating ultrashort femtosecond pulses by Kerr-lens mode-locking. As shown in Figure 1, KLM was realized by adjusting the cavity to the edge of the stability region by translating the M2 mirror toward the crystal and finely adjusting the angles of the HR mirror and OC. At this point, the CW output power was 280 mW under 5 W pump power. Stable mode-locking was initiated by fast moving of the HR mirror. When mode-locked, the oscillator delivered a stable pulse train at a repetition rate of 118 MHz acquired by a fast photodiode and monitored by a digital oscilloscope. The mode-locked output power as a function of the pump power is shown in Figure 4. For comparison, the CW output power when mode-locking was terminated is also displayed. Stable mode-locking could be sustained for a pump power ranging from 3.3 W to 6.0 W. The highest output power of 237 mW was obtained at 5 W pump power. The spectrum of the femtosecond pulses was characterized with an optical spectral analyzer (Yokogawa, Tokyo, Japan; AQ6370). It was centered at 1094 nm with a 10.1 nm wavelength bandwidth. As shown in Figure 5a, the mode-locking spectrum reached above 1110 nm. The new wavelength component above 1110 nm was mainly due to self phase modulation in the crystal. The pulse duration was characterized by an intensity autocorrelator (APE PulseCheck USB, Berlin, Germany), as shown in Figure 5b (black curve). If a sech\({}^{2}\)-shape pulse was assumed, the pulse duration was 141 fs. In a 50 ps scanning range, only one pulse was observed, indicating that the mode-locking was in single-pulse  
Figure 4: Mode-locked output power (red dots) _versus_ the pump power. Blue dots indicate the CW power when mode-locking was terminated. Figure 5: (**a**) Spectrum of the femtosecond pulses centered at 1094 nm; (**b**) Autocorrelation trace of the femtosecond pulses (black curve) and the sech\({}^{2}\)-fitting curve (red dashed line). operation. The time-bandwidth product of the pulses was 0.357, close to the Fourier transform limit of sech\({}^{2}\) pulses.