Solid-state lasers are extensively employed in industrial processing, medical treatment, scientific research, etc. Recently, with the maturity of semiconductor laser devices, solid-state lasers pumped by semiconductor lasers are developing towards higher power and higher beam quality, which is applied to more scenes. However, when the pump power increases, more heat accumulates in the gain medium, resulting in more serious thermal impacts, which will affect the further enhancement of beam quality and laser power. To reduce the thermal effect, different designs, including cryogenically cooled gain medium, direct pumping approaches, and thin disks, are employed. Compared with traditional gain medium configurations including rods and slabs, the thin-disk gain medium has a minimal temperature gradient in the axial direction, which can obtain a high-power laser output with high beam quality. The finite element numerical simulation and experimental tests are carried out to further understand the beam quality degradation and output power limitation caused by thermal effect in the laser diode array side-pumped Nd∶YAG disk. The axial and radial distributions of pump light absorption flux and temperature in the polygon disks with 90° and 45° cutting angles are examined and compared.

The side-pumped pentagon thin disk gain medium discussed in this study is an Nd∶YAG crystal with a doping concentration(atomic fraction) of 0.3%, and a thickness of 1.5 mm. Five laser diode arrays are symmetrically positioned around the disk, and the pump light is vertically incident on the crystal side through the pump coupling structure. In the pump coupling structure, the fast-axis collimator is employed to control the transmission direction of the fast-axis beam to be nearly parallel, and the coupling structure consisted of the cylindrical lens and reflectors is used to compress the large-area pump light to match the thin disk size. Experiments reveal that the pump light coupling efficiency is 97%. The pump light is transmitted along a zig-zag path through total internal reflection inside the medium, and the pump light is incident from numerous directions overlapping into an approximately circular area in the crystal. The absorption flux distribution of the pump light in the gain medium is nearly flat-topped in the radial direction and approximately Gaussian in the axial direction when the cutting angle of thin disk is 90°. The absorption flux of the pump light is primarily concentrated in the middle of the disk along the thickness direction, resulting in uneven temperature gain of the medium. When the cutting angle of thin disk is 45°, the absorption flux distribution of the pump light is nearly flat-topped in the radial and axial directions, and the heat generation inside the crystal is uniform. A three-dimensional finite element analysis model of a polygonal thin-disk gain medium is created using thermal analysis software. When the average pump power is 165 W, compared to the gain medium with a cutting angle of 90°, the overall radial temperature of the gain medium with a cutting angle of 45° decreases by approximately 5 ℃, and the temperature difference decreases by about 2 ℃. The temperature difference increases with an increase in pumping power. Under various cutting angles, the axial temperature difference is larger. When the cutting angle is 90°, the temperature difference between the front-end and back-end surfaces of the thin crystal is about 12 ℃. When the cutting angle is 45°, the temperature difference between the two ends of the thin disk crystal is 9 ℃.

Experimental findings reveal that the fluorescence distribution (Fig. 9), temperature distribution (Fig. 10) and wavefront aberration (Fig. 11) in the gain medium are consistent with theoretical analysis results. The root mean square (RMS) of uniformity of fluorescence distribution in the range with 16 mm diameter is 2.52%. The gain medium's over-temperature distribution is relatively uniform, and the temperature in the inscribed circle of the center is 46-52 ℃. When the pumping power is 165 W, the thermal-induced wavefront distortion of the Nd∶YAG thin disk gain medium is about 0.14λ. Here λ is the test wavelength. Under 220 W pump power, 85 W output power is obtained from the polygonal Nd∶YAG thin-disk laser, with a slope efficiency of 40.1% and beam quality β≈10 (Fig.12).

In this study, numerical simulations and experiments are conducted on a polygonal Nd∶YAG thin-disk gain medium pumped by diode lasers. Experimental findings show that a side-zigzag-pumped polygonal thin disk with 45° cutting angle can achieve a flat-top distribution of energy storage and further reduce the gain medium thermal effect. The thermal effect's reduction in the gain medium is conducive to enhancing the output power of the solid-state laser while maintaining better beam quality.