Deep-ultraviolet (DUV) lasers have important applications in fields such as laser processing and semiconductor photolithography because of their high photon energy. A DUV laser source must have both a high output power and good beam quality for laser machining. Sum-frequency generation (SFG) in nonlinear optical crystals can be used in a DUV laser to produce shorter and even vacuum ultraviolet wavelengths such as 193 nm. Hence, high-power DUV lasers with a good beam quality have been a hot research topic in recent decades. On the other hand, it is now possible to use a domestic commercially produced CLBO crystal as a key nonlinear optical material for high-power DUV solid-state laser generation. However, the crystal quality for high-power DUV operation has not yet been verified because of the hydroscopic problem. Here, we report a high-power DUV laser output with a good beam quality and high conversion efficiency produced using domestic commercially available CLBO crystals, which demonstrates the potential to achieve a higher power DUV laser output.

In our study, we investigate DUV laser generation using CLBO crystals with different lengths. The research focuses on the characteristics of these high-power solid-state DUV light sources, including the output power, efficiency, power stability, beam quality, spectrum, and pulse width. The experimental setup is shown in Fig.1. The pump power is regulated by a combination of a half-wave plate (HWP) and polarizing beam splitter (PBS). The pump spot diameter is adjusted using a plano-convex lens (L1) with a focal length of f=+200 mm and plano-concave lens (L2) with a focal length of f=-100 mm. The dimensions of CLBO crystals are 5 mm×5 mm×10 mm and 5 mm×5 mm×20 mm, respectively. In both, cutting angle (θ) is 61.7°, and the two end-faces of the crystals are polished but have no coating. The CLBO crystals are heated to a temperature greater than 150 °C and exposed in a noble gas environment to avoid the hydroscopic problem. The 266 nm DUV laser output is spatially separated from the 532 nm pump light using an uncoated CaF₂ prism.

A 20 mm long CLBO crystal pumped by a solid-state 532 nm laser generates a 266 nm DUV laser with an average power of 14 W, a repetition rate of 100 kHz, and a pulse width of 1.8 ns. The pump power is 34.2 W, and the optical conversion efficiency reaches 41%. The results are shown in Fig.2. Comparative experiments are conducted on a 10 mm long CLBO crystal using another light source system with a pump power density similar to that mentioned above. The output power of the 266 nm laser is 1.7 W with a pump power of 8 W, corresponding to an efficiency of 22%. This indicates that the crystal length is an important parameter to achieve a high conversion efficiency. The power stability of the 266 nm laser generated by the 20 mm long CLBO crystal reaches 1.52% within 10 min. There are several factors that influence the power stability, including the stability of the fundamental pump power, inhomogeneous temperature distribution of the crystal, and instability of the mechanics. The measured beam quality of the 266 nm laser at an output power of 7 W is shown in Fig.4. The transverse beam quality factor (Mx2) and longitudinal beam quality factor (My2) are 1.54 and 1.97, respectively. The inset shows the beam profile acquired at a distance of 1.5 m away from the crystal after beam expansion by a concave lens. The circular beam shape and homogenous distribution of the intensity also indicate the high beam quality of the generated 266 nm laser.

A nanosecond 532 nm fundamental laser and a 20 mm long domestic commercially available CLBO crystal are used to generate a high-power DUV solid-state laser at 266 nm, with an average power of 14 W and a conversion efficiency of 41%. The beam quality factors, Mx2 and My2, of the 266 nm laser have values of 1.54 and 1.97 at a power of 7 W, respectively. The root mean square value of the power stability at 10 W reaches 1.52% within 10 min. The temperature distribution and mechanical stress are the main factors influencing the DUV power stability. A 266 nm laser with higher power and better beam quality can be achieved by improving the temperature control system and mechanical design, as well as by increasing the pump power. This can be applied to laser machining, lithography, and vacuum ultra-violet laser generation in the future.