In order to obtain a good light-matter interaction effect, it is usually required laser has the characteristics of high power and short pulse duration to obtain a strong peak power and improved time resolution. The ultrawide gain bandwidth and high thermal conductivity of the Ti∶sapphire crystal make it a good gain medium. However, because of the power limitations of the pump source, the thermal lens effect, and the mode-locked pulse stability, it is difficult to increase the average output power of the Ti∶sapphire femtosecond laser. Therefore, Ti∶Sapphire femtosecond lasers with high average power, short pulse width, and high repetition rate have always been a research hotspot in ultrafast lasers and their applications.

The experimental setup is shown in Fig. 1. A continuous-wave green laser with a maximum output power of 16 W at 532 nm is used as a laser pump. The 150 mm plano-convex lens is used to concentrate the pump light onto the Ti∶sapphire crystal. The crystal is mounted on a water-cooled copper crystal frame and wrapped in an indium foil. The water temperature is regulated at 14 ℃±0.1 ℃ using a water cooler with a cooling capacity of 600 W. Concave mirrors C1 and C2 have a curvature radius of 150 mm, a folding angle of 24°, and exhibit strong reflection in the spectral region of 750?850 nm. Flat mirrors M1?M4 exhibit strong spectral reflections in the 750?850 nm range. The output coupling mirrors (OC) have a transparency of 20%. P1 and P2 are a pair of prisms used to compensate for intracavity dispersion, and their separation is fixed at 340 mm. The total length of the resonator is 2.02 m. Unlike commercial laser oscillators in which a slit is placed at the end mirror to suppress high-order transverse modes, we position an adjustable slit in the optical path between the prism pair, which suppresses the high-order transverse mode in the cavity and selectively suppresses the continuous wave (CW) component at a specific spectrum when the high-power laser in the cavity is running. A lens with an extended focal length and a concave mirror with an extended radius of curvature are used to focus the pump light, and the beam waist radius of the spot (23 μm) matches the intracavity laser waist spot radius (26 μm). The laser beam waist is enlarged to prevent damage to the crystal owing to high laser power density, and the laser mode volume is increased to increase the output power. Because of the long gain crystal, the calculated spacing needs to be at least 1700 mm if a pair of fused silica prisms with a low refractive index is selected to compensate for second-order dispersion. Therefore, the prism with a high refractive index is selected, which can provide sufficient negative dispersion within a short distance and reduce the space occupied by the resonant cavity.

The resonator parameters are described using an ABCD matrix [Figs. 2(a)?(c)]. The mode-locking starting area and the astigmatism compensation angle are determined, and the distribution of the beam waist in the resonator is calculated to guide the experiment. The ray-tracing method is used to compute the dispersion generated by the prism pair [Fig. 2(d)], providing a basis for dispersion compensation in the cavity. With an increase in pump power, a slope efficiency of 37% is obtained without the pump saturation effect. Self-phase modulation decreases the pulse width with increasing power. At a pump power of 16 W, a femtosecond pulse output with an average power of 4.1 W is obtained [Fig. 3]. A spectrum with a central wavelength of 795 nm and full width at half-maximum of 17 nm is measured [Fig. 4(a)], and an intensity autocorrelation curve with a pulse duration of 48 fs is obtained [Fig. 4(b)]. Additionally, adjusting the slit width reduces the CW component [Fig. 5]. In the laboratory environment, we record the pulse train using an oscilloscope, and neither Q-switched mode-locking nor multipulse are observed [Fig. (6)]. The root-mean-square (RMS) value of the power fluctuation within 1 h is lower than 0.1% [Fig. 7(c)]. The signal-to-noise ratio of the 74.15 MHz fundamental frequency signal in the radio frequency spectrum is 52 dB [Fig. 7(a)].

Using the Kerr lens mode-locking technique, we demonstrate a Ti∶sapphire femtosecond oscillator with high average power and short pulse duration. The laser generates 48 fs pulses with an average power of 4.1 W at a repetition rate of 74 MHz by employing a 532 nm continuous-wave pump source with a power of 16 W, a high-refractive-index prism pair for dispersion compensation, and a slit to facilitate mode locking. The average output power increases by 2.5%, the pulse duration decreases by 63%, the optical-to-optical efficiency increases by 63%, and the peak output power increases by 2.8 times compared to those of the existing model of the same laser type (pump power of 20 W, average output power of 4 W, pulse duration of 130 fs, and repetition rate of 76 MHz).