Solid-state lasers represent a class of compact and efficient high-power laser sources, which are attractive for a broad range of medical, commercial, scientific, and military applications. However, due to the risk of serious thermal optical aberration and fracture of the gain medium, thermal effects become the primary limiting factors in further increasing the output power and beam quality of the laser. To meet the requirements of various practical applications, a compact high-power system with efficient thermal management needs to be developed. Direct-liquid-cooled configuration lasers (DLCLs) have become highly attractive in the high-power laser field due to their excellent heat dissipation capabilities. Multiple disk pieces, arranged as an array, are integrated into a single gain module (GM), leading to a low heat density in each gain disk by dispersing the heat across the entire gain disk array. The circulating liquid flows over the largest surface of the disk, efficiently carrying away the heat. Thanks to easier laser output, DLCL resonators have been extensively studied. However, due to the repetitive superposition of complex wavefront aberrations in DLCL resonators—caused by the coolant flow and gain medium—the laser beam quality is compromised. To overcome the challenge of achieving both high power and good beam quality, new DLCL configurations must be explored. This study demonstrates a new DLCL scheme with high performance (high power, high beam quality, and high efficiency) referred to as the thermal-dispersed reflectivity-type Nd∶YAG disk array MOPA (master oscillator power-amplifier) laser.

This study employs a research method that combines configuration proposal, parameter design, and experimental integration verification. A novel high-power direct-liquid-cooled distributed-reflective-type laser is designed, featuring a distributed gain system composed of tens of Nd∶YAG disks densely stacked. A specialized laser cooling liquid flows through planar micro-channels between the gain media disks. Additionally, a Zig-Zag-like laser path is designed within the gain system to achieve high power output. This laser configuration merges the advantages of direct-liquid cooling and the Zig-Zag path. The laser’s configuration is optimized. The key factors of the gain media disks, the laser gain of the laser system, temperature distribution, and wavefront aberration are simulated theoretically. Furthermore, an experimental verification platform based on the direct-liquid-cooled distributed-reflective-type MOPA laser was constructed. The laser characteristics, including output power, optical-optical (O-O) efficiency, and far-field distribution, have been obtained.

In the MOPA system, a QCW Nd∶YAG rod oscillator was used as the seed, providing an average output power of 0.5 kW with a repetition frequency of 500 Hz and a pulse width of 220 μs. The temporal profile of the output laser is shown in Fig. 16. As depicted in Fig. 15, a maximum average output power of 21.2 kW was obtained from the entire amplifier chain, corresponding to a peak power of 192.7 kW and a single-pulse energy of 42.4 J, achieved under an average pump power of 56 kW. An O-O conversion efficiency of 36.9% was achieved with an output of 21.2 kW. Attention is drawn to the extracted power and efficiency of the direct-liquid-cooled laser GMs, as shown in Fig. 17. Notably, the extracted power and O-O efficiency of GM1# were lower than those of GM2#. Specifically, GM1# achieved an extracted power of 9.3 kW with an O-O efficiency of 33.5%, while GM2# achieved an extracted power of 11.2 kW with an O-O efficiency of 40%. Two identical gain modules with opposite flow directions were placed in the MOPA to self-compensate for tilt aberration. Figure 18 shows the wavefront and far-field distribution of the amplified output beam. The beam quality, denoted by the diffraction limit multiplier, was measured using a beam analyzer. The analyzer images the beam into the far-field distribution, which is then compared to the ideal far-field distribution to determine the beam quality parameter. The peak-to-valley (PV) and root-mean-square (RMS) values of the output beam were 1.1 μm and 0.23 μm, respectively. After defocus and tilt aberration compensation, the wavefront consisted of high-order aberrations. The corresponding beam quality was measured at 4.8 times the diffraction limit.

A 20 kW-class direct-liquid-cooled MOPA for a direct-liquid-cooled distributed-reflective-type Nd∶YAG disk array laser is designed, representing a new scheme with the potential for high laser performance. An average output power of 21.2 kW with an O-O efficiency of 36.9% is realized in the amplifier chain, with corresponding beam quality measured at 4.6 times the diffraction limit. Due to the high injected peak power density, the extraction efficiency of GM2# reached 40%. The experimental results demonstrate the validity and feasibility of this novel configuration for high-power operation, particularly in terms of distributed gain and distributed cooling. To our knowledge, the output power demonstrated in this study is the highest reported for a YAG direct-liquid-cooled multi-disk MOPA laser. Furthermore, the direct-liquid-cooled distributed-reflective-type laser shows potential for achieving high beam quality, high efficiency, and high power output in compact solid-state lasers.