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The experimental setup is shown in Fig. 1. Single-mode fiber-coupled LDs with center wavelengths of 976 nm and 979.5 nm are selected as the pump sources. To improve the mode matching, the pump light is collimated and focused through a coupling system with an imaging ratio of 1∶3, and the radius of the focused pump spot is 11 μm. A 2-mm long Yb∶KGW crystal coated with an anti-reflection film @ 980-1100 nm on both transparent surfaces is used as the gain medium. During the experiment, the crystal is wrapped with indium foil and mounted on a water-cooled copper heat sink block to ensure effective heat dissipation. The resonant cavity is a four-mirror ring cavity with a total cavity length of 274.5 mm, corresponding to a repetition rate of 1.093 GHz. M1 and M2 are concave mirrors with a radius of curvature (ROC) of 50 mm, and M2 and M3 are interferometer (GTI) mirrors providing -1600 fs2 (1035-1055 nm) group delay dispersion (GDD) in total per round trip. An OC with a transmittance of 0.4% is selected to ensure a sufficiently high power in the cavity.
Optical frequency combs based on femtosecond lasers are powerful tools for the development of precision metrology. Femtosecond lasers with high repetition rates are particularly important when optical-frequency combs are applied to spectroscopy. However, it is difficult to realize high-repetition-rate Kerr-lens mode-locking because of the weak Kerr nonlinear effect. Due to the mode matching limitation of soft aperture Kerr-lens mode-locking, a single-mode fiber-coupled laser diode (LD) is usually used as the pump source, and an output coupling (OC) mirror with low transmittance is chosen to improve the intracavity peak power, resulting in a low output power. Output power is an important parameter when a high-repetition-rate laser is used for optical frequency comb generation. Therefore, it is essential to improve the output power of GHz-mode-locked lasers.
The Yb∶KGW crystal is an excellent candidate for Kerr-lens mode-locking because of its high nonlinear refractive index of 2×10-15 cm2/W. It has a large absorption cross section (1.22×10-19 cm2) at 980 nm, which additionally supports its high pumping efficiency. Moreover, the emission cross section at 1023 nm is as high as 2.8×10-20 cm2, meaning it has a high gain, which is expected to achieve a high power output. Based on the above properties, we demonstrate a high-power Kerr-lens mode-locked Yb∶KGW femtosecond laser with 1 GHz repetition rate.
First, using the 976 nm single-mode LD as the pump source, mode-locking with an average power output of 90 mW at a center wavelength of 1045 nm and a full width at half maximum (FWHM) of 4.5 nm is achieved [Fig. 2(a)], and the corresponding pulse duration is 265 fs [Fig. 2(b)]. To increase the output power of mode-locking, the pump source is replaced with a 979.5 nm single-mode LD, whose wavelength is closer to the main absorption peak wavelength of the crystal. In addition, the polarization of the pump light is adjusted using a half-wave plate to further improve the absorption efficiency of the crystal. Consequently, the average power of unidirectional mode-locking is increased to 151 mW. The FWHM of the optical spectrum is approximately 4.2 nm at a center wavelength of 1045 nm [Fig. 3(a)], and the corresponding pulse duration is 249 fs [Fig. 3(b)]. In the laboratory environment, mode-locking can be self-started only by increasing the pump power, and the root mean square (RMS) value of power fluctuations within 24 h is 0.76% [Fig. 4(b)]. The power jitter in the vicinity of 3 h and 6 h is because of environmental vibration and other factors that cause the laser to move out of the stable mode-locked state; however, it can restart in a short time. The signal-to-noise ratio of the 1.029 GHz fundamental frequency signal in radio frequency spectrum is 62 dB [Fig. 4(a)].
We report a 1 GHz Kerr-lens mode-locked Yb∶KGW laser pumped by a laser diode. A 976 nm single-mode fiber-coupled laser diode is used as the pump, and a stable mode-locked operation at a repetition rate of 1 GHz with an output power of 90 mW is obtained in a bow-tie ring cavity with a pulse duration of 265 fs. By using a 979.5 nm single-mode laser diode as the pump source and adjusting the polarization of the pump beam, the output power is increased to 151 mW at a center wavelength of 1045 nm, and the corresponding pulse duration is 249 fs.
