Research On Al₂O₃Waveguide Dielectric Film Of Electrode Surface For RF Slab CO₂Laser

High power RF slab CO2laser is one of the main light sources in medium thickness or thick plate laser cutting and welding, which electrode plate has the functions of RF discharge,Diffusion cooling,and optical waveguide etc, ultra precision machining and surface film feature of the electrode are the key technologies which restrict Laser development. In this thesis, heat conduction、optical waveguide loss and the anomalous dispersion of the Al2O3film on the surface of electrode have been analyzed and the film structure has been designed also; based on hot filament chemical vapor deposition (HFCVD) and magnetron sputtering technology, the physical and chemical mechanism, process parameters, micro-structure of film,surface waveguide loss and optical characters etc. were deeply researched; the actual performance of Al2O3waveguide dielectric film on the surface of the finished electrodes have been tested. The main contents are classified as follows:Based on the slab waveguide eigenmodes of RF slab CO2laser, the waveguide transmission mode loss coefficient for CO2laser, γTE and γTM, in the copper surface and Al2O3dielectric film were calculated, combining with unstable-waveguide hybrid resonator structure, where in copper surface γTE=1.74e-06,γTM=9.45e-05, and in Al2O3dielectric film yTE=1.68e-06, γTM=2.95e-06. Waveguide transmission anomalous dispersion effect and high temperature resistance, anti-oxidation, anti-laser ablation characteristics of the Al2O3dielectric film were deeply studied. The results show that, Al2O3dielectric film is an ideal waveguide ceramic film for CO2laser and is the best choice for slab electrode film.Thermal transmission property of Al2O3film on electrode surface has been analyzed and calculated. According to built-waterway diffusion cooling mechanism of2kW RF slab CO2laser electrode, heat conduction process in four medium films was derived.The result shows the working surface highest temperature of coated electrode is300.4℃when the thickness of Al2O3film is5um and the highest temperature of exposed Cu electrode surface is299℃,and the temperature difference between the coating layer is less than2℃, which theoretically confirms Al2O3film on the surface of electrode doesn’t influence the diffusion cooling; an Al+Al2O3gradient film structure coated on a copper substrate was designed to effectively solve some issues, such as the bond strength and self-healing of the film, heat dissipation of the electrode, etc..The technology and optical characteristics of the Al2O3film obtained by high temperature chemical vapor deposition on the surface of copper has been studied. Under800℃, with AlC13、H2、CO2as reactant, H2S as catalyzer, Al2O3coatings were deposited on the copper surface. The test result of spectrograph shows the film’s reflection on entry light of10.6um is as high as85%. Spectrometer testing result shows that, when the incidence angle is between10°and80°, the average coatings reflectivity of the10.6μm incident light is85%, and the anomalous dispersion effect of Al2O3on CO2laser is verified. However high-temperature depositing process is likely to cause electrode welding spots off and thermal deformation, so it is not very suitable for coating slat electrode.The performance of Al2O3film which has been got by low temperature Magnetron sputtering on the surface of Cu has been especially studied. With Al as target material, Ar as working air, and oxygen as reaction gas to deposit Al2O3film on Cu surface. The macro analysis shows that the ratio of Oxygen and argon is the key factor in the film color. When ratio of Oxygen and argon is between1:10and1:4, the color is changed from black to gray, white and transparent. The micro Analysis shows the film is composite of Al and O, the atomic ratio is over2/3, and the micro-structure is Al+Al2O3which meets the design of electrode surface film structure; the reflectance under different incident angle was tested, when incident angle is between10°to80°, the highest reflectance of Al2O3film surface to CO2with a wavelength of10.6um is85%.The discharge stability and waveguide effect of different films in2kW RF slab CO2laser were studied and the influence of discharge time on electrode surface features was tested also. Long time discharge test shows the optical efficiency of coated electrode laser increases from10.4%to11.2%. However, that of naked Cu electrode laser drops from6.5%to4.4%. The waveguide loss of coated electrode drops from1.51dB/m to1.12dB/m, and that of naked Cu electrode increases from1.69dB/m to3.52dB/m.The analysis shows discharge process helps Al in coating layer oxidized to Al2O3and implement film self-healing or Increase and no influence of discharge stability.