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Under the laser processing, the material is effected in a focused beam, and the governing parameter of the laser processing is the intensity in a focal spot. The most rigid requirements to radiation characteristics are imposed by the laser cutting: the radiation-spot diameter on the material is up to 0.1 mm, the radiation intensity in the spot - up to o iO - 108 W/cm2. In the case of CO2 laser, such parameters can be reached if the radiation divergence approaches the diffraction limit, i.e. the laser beam is of high quality. Quantitatively the beam quality is expressed by dimensionless parameter K, K = 4k/mOD, where 0, D, are correspondingly the beam divergence and diameter measured by the level of 0.86 of the power,, -the length of a radiation wave. For a Caussian beam which divergence is considered as a limit diffraction one, parameter K is equal to one by defmition. Parameter K dictates diameter d of the focal spot, and intensity I in the spot, with the nonaberrational focusing these values can be written as follows: d=------F 1= ,rWK2 KD 4,%2(F/D)2 here F is the focal distance of the lens, W -radiation power. The high quality of the cut can be achieved when a stable resonator is used in the laser, and the generation occurs on TEM00 mode (K = 0.6 - 0.9), or TEM01 mode (K = 0.55) [1]. The beam of the TEM00 mode allows to obtain a narrower cut and higher cutting speed due to the smaller size of the focal spot. The higher beam quality permits to reach the power density, needed for the cutting, at smaller angles of focusing. It is especially important for the cutting of thick sheets when the sheet thickness is much larger than a focal region length. The smaller focusing angles allow to reduce the width of a cutting channel on a sheet surface, to make the channel walls more straight and to raise the cutting speed. At the moment the power of the industrial CO2 lasers during the generation of the TEM mode does not exceed 3.5 kW. The low laser power restrains the cutting speed which is one of the basic processing parameters. As the radiation power is of 8 -12 kW, the quality parameter is of 0.26 [1]. The present paper presents the results of investigations and engineering works aimed to the increasing radiation quality of high-power CO2 lasers. In the lasers with a crossflow, a self-filtering unstable resonator (SFUR) is applied [2, 3], which has permitted to increase the radiation power of the lasers from the unit of the active-medium length, as compared to the stable resonator, on the level of the TEM00 mode, the beam quality being retained. As has been shown in steel-cutting experiments, the self-filtering resonator allows to achieve approximately the same quality indices for the cut and cutting speed as the stable resonator during the generation on the lowest modes.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only. |
