High Power Industrial CO2 Lasers Research Articles

Generation of polarisation-nonuniform modes in a high-power CO2 laser

A method describing radially and azimuthally polarised laser modes is presented, which is devoid of intrinsic contradictions and unjustified restrictions. The solutions of the wave equation found in the paper satisfy Maxwell's equation ▽E = 0. This allowed the calculation of all the components of the fields of such modes, including the longitudinal one. Expressions for tight focusing of radially and azimuthally polarised modes are obtained in the Debye approximation. The methods of intracavity generation of such modes in a high-power industrial CO2 laser are experimentally realised and methods of mutual transformation of these modes outside the cavity are developed. The intracavity generation of azimuthally polarised modes was achieved by using as a rear highly reflecting mirror either V-shaped axicons or metal diffraction mirrors with the relief period comparable to the wavelength. Radially polarised radiation was generated by using diffraction mirrors with the relief period of the order of two wavelengths. The polarisation-nonuniform TEM01* and TEM11* modes of power up to 1.8 kW are generated for the first time. In this case, the degree of polarisation was close to 100% within the entire radiation power range both in the near- and far-field zones. By using two half-wave phase shifters, the azimuthally polarised radiation is transformed to radially polarised radiation and vice versa, the energy efficiency of this transformation being 92%. These results open up the possibility for industrial applications of such radiation.

Phase correction of radiation emitted by a powerful industrial laser with higher mode selection

Special features of operation of a high-power industrial CO2 laser with selection and correction of the phases of higher transverse modes are investigated theoretically and experimentally. An effective conversion of the higher-mode oscillation field into a narrow beam with inphase optical oscillations with an output power up to 1 kW with the help of a phase corrector is reported for the first time. The angular divergence is reduced to one-third of the value typical of a multimode beam. The physical origin of intracavity aberrations, which deteriorate the beam quality, is analysed. It is shown that the most significant effect is produced by an optical wedge-type aberration emerging due to transverse circulation of the active medium and transverse discharge. The prospects of application of this method for obtaining more powerful narrow laser beams are discussed.

Optical inhomogeneities of the active media of high-power industrial CO2 lasers with fast axial flow

Theoretical and experimental investigations showed that a diverging thermal gas lens, which forms in high-power industrial lasers with fast axial flow of the active medium, can influence the diffraction losses in the laser oscillator and the beam parameters. The nonlinear optical properties of the lens, demonstrated by the dependence of its optical power on the intensity of the output radiation, are described.

The Influence of laser cavity gaseous impurities on the performance of an industrial CO2 laser

Contamination in the cavities of high power industrial CO2 lasers is a cause of laser performance degradation. Such degradation can result in laser downtime and significant loss of productivity. The laser cavity gas supply can contribute to this contamination, but there is confusion about the level of gas impurity concentration which can cause a degradation process. The overall objective of this investigation was to study the effects of cavity gas contaminants on the performance of an industrial fast‐axial‐flow CO2 laser. A 3 kW fast‐axial‐flow CO2 laser operated for 2100 h prior to this investigation was employed in a series of gas contamination experiments. The operating power was selected as 1500 W. After cleaning the laser cavity, the absorptivities of output coupler and rear mirror were measured after laser operation using both ultra‐high‐purity laser cavity gases (99.999%) and premixed contaminated helium (100 ppm, 1000 ppm and 5000 ppm for C3H8, C2H4, CH4 and O2). The output powers were also measured. The results show that increased absorptivities in the output coupler were observed with both hydrocarbon and oxygen impurities compared to those with ultra‐high‐purity gases (99.999%). The increased absorptivities in the output coupler and rear mirror seem to correlate with the total carbon content in the laser cavity due to the gaseous hydrocarbon impurities. Deposition on the surfaces of corner mirrors were identified as carbon using energy dispersive electron spectroscopy analysis. Several flakes of deposited carbons were also identified on the surface of the NaCl window inside the resonator after operating the laser using 5000 ppm propane in helium. 1000 ppm oxygen in the laser gas mixture resulted in an increase of 1.4 % in the output power (4.8 % for 5000 ppm oxygen). 1000 ppm of propane resulted in a decrease of 17 % in the output power. A 44.8 % of power drop was observed when using 5000 ppm propane. The negative effect of contamination (especially hydrocarbon) on high power laser performance was demonstrated.

Low hydrogen content diamond-like carbon coatings of KCl optics for high power industrial CO2 lasers

Abstract Magnetron sputtered low hydrogen content diamond-like carbon (DLC) was studied for possible application as antihygrosity protective coatings of KCl IR optics for high power industrial CO2 lasers. The absorption coefficient of sputtered DLC was shown to vary with sputtering parameters and hydrogen content in the deposition environment. With proper control of the parameters, DLC with an absorption coefficient as low as 80 cm−1 at 10.6 μm is obtainable. Laser damage due to pulsed and continuous CO2 lasers was studied. The damage threshold due to the pulsed CO2 laser was 4–7 GW cm−2, which was relatively insensitive to sputtering power. The damage threshold due to a continuous wave CO2 laser of DLC prepared under favourable conditions was shown to be 7.2–7.4 kW cm−2, which was very sensitive to sputtering parameters, cleanness of the film surface, and defects in the film and the KCl substrate. The damage behaviour of DLC was studied in detail. A DLC protected KCl focusing lens (φ 50 mm, f = 200 mm) was successfully used with an industrial CO2 laser processing machine ar 1.5 kW power for months in a humid atmosphere in Shanghai, China.

Effects of exposure time and pulse parameters on CO2 laser osteotomies.

Two sets of experimental linear osteotomies were performed on frozen cortical bone by means of a high-power industrial CO2 laser in the rapid superpulsed mode, varying the velocity of the operating table, the mean power of the laser beam, the pulsewidth, the pulse repetition rate, and the peak power. Both the depth and the width of the osteotomies were inversely related to the velocity and directly related to the mean power and to the pulsewidth, but they were not affected by changes in the repetition rate. The depth/width ratio, which describes the profile of the osteotomies, was affected by the mean power only. These data permit greater accuracy in the planning of laser osteotomies in cortical bone when using the rapid super-pulsed mode.