Masao Hishii

Present and future of lasers for fine cutting of metal plate

Abstract This paper describes the present state and future prospect of the art of industrial lasers used or to be used for fine cutting of metal plates. The cw-CO 2 lasers are widely used for the fine cutting of the steel plate in the automotive, electric, and electronic industries. The CO 2 laser for the fine cutting has the characteristics of high power output, high beam quality and highly controlled power-mode, which well meet the users demands. On the other hand, a 250 W Nd:YAG laser with as good focusability as the CO 2 lasers has been developed. It is now gradually replacing the CO 2 laser of that power level in the cutting and welding fields. The CO 2 laser will be further replaced by the Nd:YAG laser as the output power of the Nd:YAG laser increases in the future. Innovative technologies, such as the novel laser resonator for the CO 2 laser and the LD (Laser Diode) pumping for the Nd:YAG laser, will make the improvement of both CO 2 lasers and Nd:YAG lasers on the output power and the beam quality, which widen the application field of respective lasers in industry.

Gain characteristics of an atmospheric sealed CW CO 2 laser

Experimental and analytical investigations have been made on unsaturated gain g 0 of a CO 2 electric-discharge convection laser, in which discharge current flow, gas flow, and the optical axis are mutually perpendicular. Stable glow discharges in sealed gas mixtures of CO 2 , CO, N 2 , and He were maintained at pressures up to 780 torr with an input power density of about 90 W/cm3. The ratio of electric field to neutral particle density E/N was 1.7 \times 10^{-16} V . cm2and was independent of the total gas pressure P . The electron density in a positive column of the glow discharge was about 4 \times 10^{10} cm-3. Detailed spatial distributions of g 0 at a wavelength near 10.6 μm were measured in the pressure range from 100 to 780 torr. Measurements were also made on the current dependence of g 0 and on the change in g o with discharge time. The g 0 distributions along the gas flow direction were found to agree with those calculated from the electron density distribution and the relaxation rate constant of the upper laser level on the basis of continuity equations for a two-level model. The integrated value of g 0 along the flow direction was proportional to P-0.8when E/N , electron density, and gas temperature were held constant. A maximum value of the g 0 distribution, which was proportional to P-0.3, was 0.14 percent/cm at 780 torr.

High-pressure sealed CW CO 2 laser with high efficiency

Discharge stabilization, long-term operation, output power characteristics, and efficiency of the high-pressure CW CO 2 laser have been investigated under sealed conditions. A comparison is made with low-pressure CW CO 2 lasers. Two types of electrode structures suitable for operations in the pressure range 100-760 torr are presented. Effects of O 2 and CO on the discharge stability and unsaturated gain are described. By using molecular sieve 3A as an adsorbent of water vapor, which was the most detrimental impurity, sealed operation with no decrease in output power was achieved at 0.5-1.5 kW for more than 150 h elapsed time including about 30 h of discharge time. It has been demonstrated that high efficiency can be obtained in spite of high-pressure and sealed operation. The efficiency was improved by reducing the cavity loss due to the absorption of intracavity radiation by CO 2 molecules in the unexcited region, and by finding the optimum of gas mixture. A maximum efficiency of 19 percent was obtained at a 1 kW power level for a 100 torr gas mixture of either CO 2 -CO-N 2 -He = 2-1-19-19 or CO 2 -CO-N 2 -He-Ar = 2-1-18-10-10. The effects of Ar and N 2 proportion on the unsaturated gain and saturation parameter are discussed.

CW 20-kW SAGE CO/sub 2/ laser for industrial use

A CW CO/sub 2/ laser with power output of more than 20 kW is described. The laser consists of a discharge excitation system named SAGE (Silent-discharge Assisted Glow discharge Excitation) and a ZnSe or KCl transmissive window for extracting high-quality beams with axisymmetric intensity profile from a confocal positive-branch unstable resonator. SAGE is effective in forming a uniformly distributed and stable discharge in a large-volume, high-pressure gas excitation medium. Together with appropriate high-pressure gas conditions and a properly selected zeolite to adsorb water vapor in the laser chamber, the transmissive windows enable gas-sealed operations. The structure of the 20-kW SAGE laser and its performance characteristics are described. The maximum CW power of 26.5 kW with an efficiency of 16.5% is attained with the ZnSe window, and a 20.3-kW power output is extracted through the KCl window. Long-term gas-sealed operation at a CW power level of 20 kW has been demonstrated for a period of 65 h. >

Influence of self‐absorption on output power characteristics of a high‐pressure cw CO2 laser

Absorption of intracavity radiation by CO2 molecules in an unexcited region has detrimental effects on output power characteristics, especially above a certain level of output power, i.e., the threshold level. These absorption effects have been investigated experimentally in detail. The threshold level depends both on CO2 concentration of the laser gas and on the mirror transmission of the optical cavity. It is found from these dependences that the absorption effect becomes serious above ∼0.2 W/cm3, which is the density of optical power absorbed by the unexcited CO2 molecules. The influence of self‐absorption on the laser efficiency and the oscillating line of 10.4‐μm band can be explained well by a usual theory for laser oscillation. Furthermore, a novel method for monitoring the concentration of CO2 molecules is demonstrated.

Output power fluctuations in a high‐pressure cw CO2 laser

Large fluctuations of output power have been observed above a certain level of the radiation power accumulated inside the laser cavity. These fluctuations can be effectively suppressed by circulating the gas mixture through an unexcited region between the active medium and one of the cavity mirrors, or by removing CO2 molecules from the gas mixture in the unexcited region. Experimental evidence indicates that the fluctuations are due to the absorption of intracavity radiation by the CO2 molecules contained in that region.

The application of an unstable resonator to a high-pressure CW CO2 laser

Abstract Unstable resonators provide high quality beams of high output power needed for material processing. We have developed a CO2 laser of over 5 kW for material processing with an unstable resonator of positive confocal configuration under high-pressure, sealed condition. High quality beams with high extraction efficiency of the power were obtained and deep pentration was attained in welding tests.