Abstract

The high power deep ultraviolet (DUV) laser output with a good beam quality and high conversion efficiency produced using CLBO crystals. The 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.

The CLBO crystal boasts several advantages, including low walk-off effect, the ability to grow into large blocks, and a relatively high effective nonlinear coefficient in the deep ultraviolet region. These characteristics make it suitable for generating high-power, high-beam-quality solid-state deep ultraviolet lasers. However, CLBO crystals are prone to deliquescence, which limits the output of deep ultraviolet lasers.

CLBO crystal for 266 nm lasers

The system described utilizes a solid-state 532 nm laser as the fundamental frequency pump source, with a pulse width of 2.7 ns and a repetition rate of 100 kHz, delivering a maximum output power of approximately 35 W, and an M² beam quality factor of ~1.5. Control over the pump power is achieved through a combination of a half-wave plate (HWP) and a polarizing beam splitter (PBS).

Adjustment of the pump beam diameter is facilitated by using a plano-convex lens (L1) with a focal length of f=200 mm and a plano-concave lens (L2) with a focal length of f=-100 mm.

The CLBO crystal, sized 5 mm × 5 mm × 20 mm, with a cut angle (θ) of 61.7°, has both of its optical faces polished but remains uncoated. Due to its susceptibility to deliquescence, the crystal is heated to above 150°C and placed in an inert gas environment. Placing the CLBO crystal on a rotating translation stage enables adjustment of the fundamental frequency incident angle within the horizontal plane to achieve phase matching.

To prevent damage to the beam splitter components, an uncoated calcium fluoride (CaF₂) prism is used to spatially separate the generated 266 nm laser from the 532 nm pump light.