Diffusion-cooled CO2 Laser Research Articles

Surface modification of AISI H13 tool steel by laser cladding with NiTi powder

This paper presents laser cladding of NiTi powder on AISI H13 tool steel surface for surface properties enhancement. The cladding process was conducted using Rofin DC-015 diffusion-cooled CO2 laser system with wavelength of 10.6 µm. NiTi powder was pre-placed on H13 tool steel surface. The laser beam was focused with a spot size of 90 µm on the sample surface. Laser parameters were set to 1515 and 1138 W peak power, 18 and 24 % duty cycle and 2300–3500 Hz laser pulse repetition frequency. Hardness properties of the modified layer were characterized by Wilson Hardness tester. Metallographic study and chemical composition were conducted using field emission scanning electron microscope and energy-dispersive X-ray spectrometer (EDXS) analysis. Results showed that hardness of NiTi clad layer increased three times that of the substrate material. The EDXS analysis detected NiTi phase presence in the modified layer up to 9.8 wt%. The metallographic study shows high metallurgical bonding between substrate and modified layer. These findings are significant to both increased hardness and erosion resistance of high-wear-resistant components and elongating their lifetime.

Analysis of Deformation of Reflector for RF-Excited CO<sub>2</sub> Laser

The paper aims to study the deformation and compensation of the reflector for RF-excited diffusion-cooled CO2 laser. Based on ANSYS, the thermal deformation and compensation of the reflector have been calculated, It has been proved that the effect of temperature compensation is obvious, and can correct the thermal deformation of the reflector to the tolerance of the parameter of optical cavity.

Polarisation properties of radiation of high-power industrial multitubular CO2 lasers

The polarisation of radiation of high-power industrial multitubular, diffusion-cooled CO2 lasers is studied. The conditions of the appearance of linear and elliptic polarisation and of polarisation instability are determined. The relation between the laser parameters and its polarisation characteristics is established.

High frequency pulser-sustainer for preionization in dc excited continuous wave CO2 lasers

Slow flow diffusion cooled CO2 lasers mostly use dc electrical discharge for excitation. Preionization in the discharge tubes of such a laser helps to initiate and maintain the dc discharge at voltages below breakdown voltage. A high frequency pulser using an insulated gate bipolar transistor as switching device is developed for preionization of the 20 discharge tubes of the 1 kW multibeam CO2 laser that we have developed. This pulser produces positive pulses of 10 kV peak voltages; frequency variable from 1 to 5 kHz and its pulse width is variable from 1 to 10 μs. The rise time of the pulses is between 600 and 700 ns. Preionization allows dc discharge excitation simultaneously in all discharge tubes of the multibeam CO2 laser even at high operating pressures. Other advantages of the preionization technique, such as the discharge stability at lower discharge currents with lower ballast resistance and the shorter inactive zone in the discharge tubes, are discussed. This pulser can also be used with dc excited fast axial flow CO2 lasers and other discharge devices.

Lasers for materials processing: specifications and trends

An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO2 and the Nd: YAG laser. The domain of CO2 lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO2 laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO2 lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO2 lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO2 and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.

Mode properties of unstable off-axis resonators with large Fresnel numbers

Off-axis unstable resonators play an important role for high-power diffusion-cooled CO 2-lasers with slab geometry. Mode properties of these unstable resonators are investigated by means of direct iterative diffraction calculations. The method of resonance excitation that has formerly been applied to stable resonators is used to obtain a set of low order modes. Far-field properties and beam quality of these modes are discussed and compared with experimental results.

A versatile microwave plasma source and its application for a CO2 laser

A single ridged waveguide with nonconstant ridge height, operated with a traveling wave has been proven to be a versatile microwave plasma applicator. Easy dimensioning, inexpensive production, and compatibility to standard rectangular waveguide components are the main advantages compared to other designs. The described plasma source is especially suitable for any applications where an exceptionally uniform plasma is required. Design equations are given for easy adaption of the applicator to individual requirements. The applicator has been investigated for microwave excitation of a diffusion-cooled CO2 laser. With a nonoptimized system, laser output power of 9 W could be achieved in a quasi-cw mode. A traveling discharge operating mode is described which is useful for plasma diagnostics and for pulsed laser applications.

Microwave excitation of a diffusion-cooled CO2 laser

A microwave-excited (2.45 GHz), diffusion-cooled CO2 laser, capable of operating in a quasi-cw mode, has been developed. By separating the discharge structure from the feeding RF waveguide and using distributed coupling, previous problems with local contracting discharges could be surmounted. For effective diffusion cooling a planar 1.9 mm high discharge channel was used, a structure which easily allows area scaling of laser output power. Two discharge channel designs (metal/glass and all ceramic) were investigated, differing mainly in thermal conductivity and waveguide losses for the 10.6 mu m laser radiation. A conventional magnetron for microwave ovens was used as microwave source. System design and extensive measuring results are presented. With a first prototype non-optimized in gas mixture, optical resonator and waveguide surface quality laser output power above 20 W was obtained with an efficiency of about 10%.

CO2 Laser Discharge Influenced by a Rotating Magnetic Field

This paper deals with experimental investigations of the effect of a rotating magnetic field on a diffusion cooled CO2 laser discharge. Some properties relevant to laser activity are discussed, e.g. neutral gas temperature, magnetically driven gas rotation, pumping rate, and distribution of small signal gain within the discharge tube. By means of an intracavity method the radial distribution and the time behavior of the laser output power were measured.

Diffusion-cooled CO2 laser with coaxial high frequency excitation and internal axicon

A report on the design and performance of a coaxial laser system with a stable resonator with internal axicon is given. Efficient diffusion cooling is obtained by using an annular transverse high-frequency excited discharge. Experimental results are compared with the results of a loaded resonator model that takes tilted mirrors into account. The agreement between theory and experiment for extraction efficiency, alignment sensitivity, polarization and mode structure is very good.