High-power ultrashort pulse lasers operating in the picosecond and femtosecond time domains have important applications in strong-field physics, biomedical imaging, optical frequency conversion, and nuclear laser fusion. One approach for obtaining such high-power ultrashort pulses is the use of the amplification systems, such as chirped pulse amplification (CPA) or a master oscillator power amplifier (MOPA). However, amplifier chains produce amplified spontaneous emission (ASE), and the complex structure of the amplifier reduces the stability of the system. Another method is to use the direct output from the mode-locked oscillator, represented by a semiconductor saturable-absorber mirror (SESAM) mode-locked disk laser, which was demonstrated in 2000. The disk gain medium has a high area-to-volume ratio, which leads to excellent heat dissipation and therefore stabilizes the operation at high power. However, its low single-pass gain requires a complex multi-pass pump for improving the absorption efficiency of the pumped light, which can reduce the system stability. In 2022, our group reported a large core-size crystal waveguide mode-locked laser, providing a new solution for realizing high-power mode-locked lasers. Crystal waveguides used as gain media have good mode limiting capability and high heat dissipation capability, and they can achieve a stable high-power output. We believe that this new mode-locked oscillator has potential for power scaling. As studies related to dispersion compensation for crystal waveguide mode-locked lasers have been scarce, investigation of dispersion compensation to find ways to increase power is essential.
The semiconductor saturable-absorber mirror used for mode-locking introduces the tendency of the laser towards Q-switching instabilities; thus, the Q-switching mode-locking (QML) operation of the laser can be realized with a repetition frequency of the order of kHz. This leads to the generation of high-energy pulses that will cause irreversible damage to the SESAM. The cavity design follows the conditions proposed by H?nninger et al. with regard to achieving continuous-wave mode locking (CWML). We calculate the threshold powers for different spot radii on the SESAM and different cavity lengths (Fig. 1) and then select suitable cavity parameters. The spot radius on the SESAM is set as 200 μm, the cavity length is 4.5 m, and the theoretical value of power for CWML is 3.4 W. To achieve a stable CWML, a commercial SESAM with a small modulation depth is selected, an output coupler with a lower transmittance is used, multiple 4f systems are employed to increase the cavity length, and the focal length ratio of the last set of mirrors is changed to scale the spot radius on the SESAM. Furthermore, we analyze the main sources of dispersion in the cavity and experimentally implement dispersion compensation by inserting five Gires-Tournois-Interferometer (GTI) mirrors in the cavity (Fig. 3) and the -23500 fs2 group velocity dispersion (GVD) per round trip is achieved. The experimental results show a significant improvement compared to the configuration without dispersion compensation, and no obvious saturation is observed in the output power curve [Fig. 5(a)], indicating further power expansion.
At a pump power of 160 W, the waveguide core absorbs a pump power of 84 W, the pump absorption efficiency is 52.5%, the output power is 21 W, and the optical-to-optical efficiency is 25%. The beam quality values in the x and y directions of the output beam are 1.17 and 1.05, respectively, which indicates impressive beam quality. The wide-span radio frequency (RF) spectrum and autocorrelation curve indicate single-pulse operation. Figure 6(a) shows a single-pulse shape, illustrating an autocorrelation trace with an full width at half-maximum (FWHM) of ～2 ps, corresponding to a time-bandwidth product of 0.4. The time-bandwidth product is closer to the Fourier limit compared with that of the configuration without dispersion compensation. We analyze the reasons for the low optical-to-optical efficiency and suggest ways for further power expansion: 1) No significant saturation is observed in the experiment and a higher output can be obtained when a high-power pump laser is used. 2) Using a crystal waveguide with a large core size as a double-cladding structure, optimizing the length of the crystal waveguide, or connecting multiple crystal waveguides in series can help to improve the pumping efficiency and increase the output power. 3) The focal length ratio of the last set of mirrors can be adjusted so that the spot area on the SESAM will also increase by a corresponding multiple; when the power in the cavity increases by a certain multiple, then stable mode locking can be achieved. 4) With an increase in the output power, the use of an output coupler with a higher transmission ratio can improve the slope efficiency of the laser.
An all-solid-state passively mode-locked laser based on a Yb∶YAG large-core-diameter crystal rectangular waveguide is reported. Using a GTI mirror to compensate the dispersion in the cavity as well as using a laser with average power of 16 W, a pulse width of 2 ps, time-bandwidth product of 0.4, and repetition rate of 31.7 MHz can be obtained. The mode-locked output characteristics of the large-core-diameter crystal rectangular waveguide mode-locked laser are experimentally studied. The main sources of intracavity dispersion are analyzed, and methods to expand the output power are proposed.
