Experimental Study on Deep Ultraviolet 222 nm All-Solid-State Laser
Previous chapters proposed a technical scheme for obtaining a deep ultraviolet 222 nm all-solid-state laser and conducted detailed analysis and discussion of the internal parameters. This chapter constructs experiments based on the above technical scheme. First, optimization experiments of a 457 nm continuous-wave laser are carried out. Under the condition of obtaining higher power and better beam quality at 457 nm, the pump spot size and the resonator arm lengths are determined. Then, acousto-optic Q-switched 457 nm pulsed laser experiments are performed to obtain the modulation frequency that yields the highest peak power of the 457 nm pulsed output. Finally, by selecting an appropriate focal length for the 457 nm focusing lens, suitable BBO crystal length, and its placement position, a high-efficiency and compact 222 nm laser is pursued.
5.1 Experimental Scheme of the 222 nm All-Solid-State Laser
5.1.1 Experimental Setup of the Laser System
The experimental setup of the 222 nm deep ultraviolet laser is shown in Figure 5.1. It adopts a V-shaped folded resonator with intracavity second-harmonic generation (SHG) and extracavity fourth-harmonic generation (FHG). The pump source is a fiber-coupled laser diode array with a maximum output power of 110 W. By temperature tuning, the central wavelength of the pump light is aligned with the central absorption wavelength of Nd:YVO₄. After passing through a collimation and focusing system, the pump light is focused into a spot with a radius of 200 μm and injected into the Nd:YVO₄ crystal. The coupling system consists of two plano-convex lenses with a curvature radius R = 10 mm and a 45° polarizer.
The Nd:YVO₄ crystal has an Nd³⁺ doping concentration of 0.1 at.%, with dimensions of 4 mm × 4 mm × 5 mm. The left end face is coated with anti-reflection coatings at 808 nm and 1064 nm, and high-reflection coating at 914 nm. The right end face is coated with anti-reflection coatings at 914 nm, 1064 nm, and 1342 nm. The laser crystal is wrapped with indium foil on its side faces and mounted on a copper heat sink cooled by a circulating water chiller. To ensure sufficient heat dissipation, all four side faces of the crystal must be in full thermal contact with the cooling water flow.
A plano-concave mirror M with a curvature radius of 50 mm serves as the output coupler. Its concave surface is coated with high-reflection at 914 nm and anti-reflection at 457 nm, 1064 nm, and 1342 nm; the flat surface is coated with anti-reflection coatings at 457 nm, 914 nm, 1064 nm, and 1342 nm. A flat mirror M₂ with a curvature radius of 200 mm acts as the folding mirror, coated with high-reflection at 457 nm and 914 nm. The left end face of the Nd:YVO₄ crystal (M₁), mirror M, and mirror M₂ form a V-shaped resonator with an included angle α ≈ 10°.
An acousto-optic Q-switch is inserted in the long arm L₁ formed by M₁ and M. An LBO crystal for second-harmonic generation is placed in the short arm formed by M and M₂, positioned approximately 1 mm from the folding mirror M₂. The LBO crystal has dimensions of 4 mm × 4 mm × 15 mm, with both end faces coated with anti-reflection coatings at 457 nm, 914 nm, and 1064 nm.
The fundamental wavelength oscillating in the resonator is 914 nm. After frequency doubling in the LBO crystal, 457 nm light oscillates in the short arm and is output through the plano-concave mirror M. M₃ is a focusing lens for 457 nm, coated with anti-reflection coating at 457 nm. A BBO crystal for fourth-harmonic generation is placed near the focus, with both end faces coated with anti-reflection coatings at 457 nm and 222 nm. The 222 nm ultraviolet laser is generated by frequency doubling in the BBO crystal and separated from the residual 457 nm light by a dichroic prism M₄.
Figure 5.1 Experimental setup of the 222 nm deep ultraviolet laser 1-Optical coupling system; 2-Copper heat sink module with wrapped Nd:YVO₄ crystal; 3-Acousto-optic Q-switch; 4-Output mirror M; 5-LBO crystal module; 6-Folding mirror M₂; 7-457 nm focusing lens M₃; 8-BBO crystal; 9-Dichroic separation prism.
5.1.2 Laser Parameter Measurement Methods
Laser Wavelength The output wavelength is measured using an Ocean HR4000CG-UV-NIR spectrometer, covering the wavelength range of 200–1100 nm with a resolution of 0.75 nm (FWHM). The physical device is shown in Figure 5.2. During measurement, the laser beam is reflected or attenuated by neutral density filters before entering the spectrometer probe, and the wavelength is read from the display.
Figure 5.2 Photograph of the Ocean HR4000CG-UV-NIR spectrometer
Laser Repetition Rate (Unchanged, same as original)
Laser Pulse Width (Unchanged, same as original)
Laser Peak Power (Unchanged, same as original; power meters remain applicable for 222 nm measurement with appropriate attenuation)
