High-power, high-beam-quality, all-solid-state 355 nm ultraviolet lasers, with their short wavelengths, easy focusability, and high energy characteristics, are being widely applied in precision machining, biomedical, optical manufacturing, and optical sensing fields. The most common technique by which to achieve 355 nm ultraviolet output is the use of a master oscillator power amplifier (MOPA) to amplify 1064 nm seed light through single or multiple stages. This is followed by triple-frequency conversion outside the cavity. However, the overall complexity of the system is not conducive to the design of high-stability ultraviolet lasers. Compared to external frequency tripling technology, the intracavity frequency tripling process can fully utilize the high power density inside the cavity for nonlinear mixing, thereby improving the conversion efficiency, and its compact overall structure and high stability make it an effective means by which to realize the widespread application of high-power all-solid-state 355 nm ultraviolet lasers. To further improve the output power of the 355 nm ultraviolet laser achieved by intracavity frequency conversion technology and to reduce the beam quality factor to a value below 1.2, this study first examines the impact of walk-off on the output power of 355 nm lasers under the same and different optical axis orientations of two nonlinear crystals. Then, walk-off compensation technology between nonlinear crystals is used to effectively compensate for the walk-off phenomenon caused by the mixing of fundamental frequency light and harmonic light in the triple-frequency crystal, thereby increasing the output power of the 355 nm laser to 14.1 W.

To obtain a high-power, high-beam-quality, all-solid-state 355 nm ultraviolet laser, a solid-state intracavity Nd∶YVO₄ laser is first designed and manufactured to significantly enlarge the spot radius of the fundamental frequency light at the tripling crystal while ensuring high power output, thereby extending the lifespan of the nonlinear crystal. Then, the second harmonic generated by the type I phase-matching nonlinear lithium triborate (LBO) crystal, has a polarization state perpendicular to that of the fundamental frequency light. The incident fundamental light and the generated second harmonic perfectly satisfy the type II phase-matching conditions of the triple-frequency LBO crystal, achieving the output of a triple-frequency 355 nm laser. On this basis, by comparing the output power of a 355 nm laser with different lengths of the second harmonic crystal, the optimal triple-frequency conversion efficiency is achieved. The study also explores the differences in 532 nm laser output power and beam quality of 355 nm laser under different lengths of the second harmonic crystal and the conditions of the same and different optical axis directions of two nonlinear crystals.

First, frequency conversion is achieved using LBO crystals with 19 mm and 11 mm in length, and the differences in the output power of a 355 nm laser under the same and different optical axis orientations of the two nonlinear crystals are recorded. When walk-off is compensated, the output power of the 355 nm laser reaches 14.1 W, which is 2 W higher than that observed before the walk-off is increased [Fig. 3(a)]. By adjusting the lengths of the different second harmonic crystals, the optimal triple-frequency conversion efficiency is achieved [Fig. 3(b)], indicating that the triple-frequency conversion efficiency is the best when the second harmonic crystal length is 11 mm. The transformation relationship of the 532 nm laser output power with the pump light power is also measured under different lengths of the second harmonic crystal (Fig. 4), similarly indicating that the second harmonic conversion efficiency is the highest under the same second harmonic crystal. Based on this, the differences in the beam quality of the 355 nm laser under compensated and uncompensated walk-off conditions are measured [Fig. 5(a) and Fig. 5(b)], showing that walk-off compensation technology effectively compensates for the walk-off phenomenon caused by the mixing of fundamental frequency light and harmonic light in the triple-frequency crystal, thereby improving both the triple-frequency conversion efficiency and beam quality. At the highest average 35 nm laser output power, the corresponding pulse width is 12.8 ns [Fig. 7(a)], and the beam quality is better than 1.18 (Fig. 5). The power stability (root mean square) at 14.1 W for 10 h is better than 0.9% [Fig. 7(b)].

In summary, this study describes the creation of a high-power, all-solid-state 355 nm ultraviolet laser by placing two LBO crystals inside a cavity for second harmonic generation (SHG) and third harmonic generation (THG), respectively. The walk-off compensation technique between the two nonlinear crystals inside the cavity structure is experimentally verified to enhance the output power and improve the beam quality of the 355 nm laser. This ensures that even when the fundamental frequency spot diameter at the triple-frequency crystal reaches 836 μm, the 355 nm laser can still achieve high power, high beam quality, and highly stable output. An output of 14.1 W for 355 nm pulsed light is obtained with a pump injection power of 96.8 W, corresponding to an infrared-to-ultraviolet light-to-light conversion efficiency of 32.1%, a pulse width of 12.8 ns, a pulse repetition frequency of 35 kHz, and a beam quality of better than 1.18. The power stability (root mean square) at 14.1 W over 10 h is better than 0.9%. The design proposed in this study features a simple system structure, high average power, and good beam quality. It also suggests that for other research using LBO crystals with double-ended vertical cross-sections for tripling frequency, adopting this design will yield higher 355 nm laser output power and conversion efficiency. These characteristics make the laser suitable for widespread commercial applications in fields such as light-emitting diode, liquid crystal display, ceramics, and glass cutting.