Laser diodes (LDs), leveraging their prominent advantages in high efficiency, long lifespan, and high repetition rates, have gradually supplanted flashlamps as the core pump source for solid-state lasers. LD side-pumped lasers introduce pump light via the crystal sidewalls. They boast an extended effective pump region that enables high-power pumping, thus emerging as an efficient solution for medium-to-high energy laser output. These lasers find extensive application in industrial processing, laser fusion, laser guidance, and other fields.

However, thermal effects induced by the non-uniform gain distribution within the crystal, such as thermal lensing and thermally induced birefringence, severely constrain the performance of high-power laser systems. This limitation restricts large-aperture modules to operating below hundreds of Hz and impedes their utilization in high-repetition-rate scenarios.

(1) To probe into the factors that influence the gain field uniformity in a Nd∶YAG crystal with a diameter of 15 mm and a doping concentration (atomic fraction) of 0.6%; (2) To optimize the system structure to enhance the homogeneity of the gain distribution; (3) To alleviate thermal effects, thereby attaining high-power and high-beam-quality laser output.

A theoretical model was established using ray-tracing methods. This model was employed to simulate and analyze the impacts of different LD arrangements (7-dimensional, 9-dimensional, and 11-dimensional) and the distances between LDs and the crystal on the gain field distribution (Fig. 2, Fig. 3).

Based on the optimization results, an integrated LD side-pumped module was devised (Fig. 6). The module adopts an 11-dimensional annular array, with the distance from the LD to the crystal center being 21 mm. In each dimension, two LD bars are packaged using AuSn hard solder to form a ring-shaped unit structure (Fig. 4). The module is composed of eight serially connected ring structures and integrates a total of 176 LD bars. Additionally, it is equipped with a dual-coolant system for the independent thermal management of LDs and the crystal (Fig. 5).

The experimental characterization encompassed wavelength consistency (Fig. 7), gain distribution uniformity (Fig. 9), thermal lens focal length (Fig. 10), and single-pass gain factor (Fig. 12).

The designed 11-dimensional pump structure exhibited stable operation at a current of 140 A, a repetition rate of 500 Hz, and a pulse width of 200 μs. Under these conditions, the module achieved a peak pump power of 25 kW. Moreover, a 90% gain uniformity was realized across 90% of the crystal volume (Fig. 9), representing a notable improvement compared to the 7-dimensional and 9-dimensional configurations (Fig. 2). Additionally, a single-pass small-signal gain of 5.3 was attained (Fig. 12).

Thermal lensing tests revealed that the focal length was 435 mm under high-power pumping (Fig. 10). Spectral analysis indicated that the wavelength deviations among all eight ring units were within -0.5 to +0.5 nm (Fig. 7). Meanwhile, the LD junction temperature remained at only 41.7 ℃ during operation at a 10% duty cycle, ensuring long-term reliability.

Through theoretical modeling and experimental validation, this study successfully developed an LD side-pumped Nd∶YAG module with a 15 mm diameter crystal, featuring high gain uniformity. The module caters to the requirements of high-repetition-rate operation at 500 Hz and delivers an integrated peak pump power of 25 kW.

The optimized 11-dimensional pump configuration and the 21 mm pump distance design achieved a 90% gain uniformity. Concurrently, an extended thermal lens focal length (435 mm) and a high single-pass gain (5.3) were accomplished. This work furnishes a reliable pump solution for high-power solid-tate lasers in industrial and scientific applications.