Research on All-solid-state Continuous-wave 228 nm Deep Ultraviolet Laser

Deep UV laser has very important applications in sterilization, Raman spectroscopy and material processing. In this study, the 914 nm fundamental frequency optical cascade nonlinear optical frequency conversion was realized by the frequency doubling in the V-shaped cavity and Lens focusing method,extra-cavity quadruple frequency structure, 228 nm CW laser was obtained. In general, higher average power and better beam quality were required for the continuous operation laser to achieve the extra-cavity frequency doubling compared with the pulse operation. In order to obtain a higher output performance of 457 nm CW laser, this paper firstly analyzes the influence of the length change of the V-shaped cavity arm on the spot size at different positions in the cavity by theoretical calculation. The effects of different arm lengths of V-cavity on the output performance of 457 nm laser generated by LD end pump Nd: YVO4/ LBO was investigated experimentally. Finally,at pump power of 26 W, 457 nm CW laser output of 2.2 W, TEM00, has been achieved. On the basis of frequency doubling, extra-cavity frequency doubling using Type-I phase matching BBO crystal, was performed, and 228 nm CW DUV laser with 6 mW power has been achieved, the laser spot was elliptic and the power stability was 1.8% within one hour. The pump source is an 808 nm fiber-coupled LD with a core diameter of 400 mu m and a numerical aperture of 0.22, with maximum CW power of 110 W. The pump light coupling system consists of two plano-convex mirrors with a focal length of 10 mm and a 45 degrees polarizer, and its imaging magnification is close to 1:1. The parameters of Nd: YVO4 crystal are: Nd3+ atomic doping concentration is 0.1%; 4x4x5 mm(3) in size; the left facet was antire-flection coated at 808 nm and 1 064 nm and high reflection coated at 914 nm; the right facet was antireflection coated at 914 nm, 1 064 nm and 1 342 nm wavelengths. The laser crystal was wrapped in a layer of indium foil on the side and secured on a copper heat sink, which is capable of controlling the temperature through circulating water cooling. The output mirror M is a flat concave mirror with a curvature radius of 100 mm. It is coated with 914 nm high reflection film, 457 nm, 1 064 nm and 1 342 nm antireflection film. The high reflection mirror M2 was a flat concave mirror with 200 mm in radius of curvature which was high reflection coated at 457 nm and 914 nm. The V-shape cavity was formed by the left facet of Nd: YVO4 crystal M1 and M and M2, where the angle between the two arms is 5 degrees. The size of LBO frequency doubling crystal is 4x4x15 mm(3), both facets of which were antire-flection coated at 457 nm, 914 nm and 1 064 nm. M3 is a 457 nm focusing lens with a focal length of 150 mm and coated with 457 nm antireflective film. The size of BBO frequency doubling crystal is 4x4x8 mm(3),and the Type-I phase matching was adopted,the cutting angle is theta=61.4 degrees. The 457 nm laser is output from mirror M and focused through M3 focusing lens,BBO crystal is placed at the focus, and 228 nm CW deep UV laser was obtained by frequency doubling of BBO crystal. The splitting prism M4 was used to separate 457 nm and 228 nm lasers. Considering the thermal focal length generated by Nd: YVO4 crystal,ABCD matrix and stable cavity conditions were used to calculate by Matlab program,and the influence of arm length L1 and L2 on the size of light spot in the cavity was obtained. As can be seen spot size at different positions of the cavity is insensitive to L1 length change, but sensitive to L2 length change. The L2 length of the V-cavity was optimized experimentally to obtain a more appropriate mode matching between the pump light and the fundamental frequency light. The maximum output power of 457 nm CW laser was 2.2 W and the light spot was TEM00 mode when the pump power was 26 W. Finally, 457 nm CW laser with output power of 2.2 W passes through the BBO crystal to produce 228 nm deep UV laser with 6 mW power. The relationship between the output power of 228 nm laser and the injected power of 457 nm laser is shown, and the stability is 1.8%.