Hardening laser surface offers new possibilities for the enhancement of the mechanical resistance of superficial layers. Essential for the laser treatment processes is the efficiency at which the incident laser power is coupled into the work piece. The aim of the work was to study the effect of some factors such as spot shape, beam angle of incidence and surface coating on the steel surface absorptivity. Samples made of carbon steel were laser processed using a CW CO2 - 1,2 kW laser unit. Different process parameters (beam power, spot dimension and traverse speed), were used. The microstructure of laser hardened layers which have been investigated by optical microscopy. The mechanical characterization of the layers has been done by hardness measurements. Correlation has been established between the structure of the laser-processed layers and the process parameters.

A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity profile (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser efficiency will be increased. This resonator structure was applied for efficiency power extraction (as a low order TEM modes laser beam) from a DC excited transverse flow CO2 laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity profile of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near field as well as in the far field; the gain asymmetry along the flow direction was compensated by the gas circulation fluidodynamical circuit with two counterflowing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry.For the practical construction of these two types of optical resonators we have developed two new types of 90degrees deflection elements: the first one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90degrees angle. Both elements exhibit good focusability if they are equipped with two concave mirrors. (C) 2002 Elsevier Science Ltd. All rights reserved.

An effective method for surface heat treatment with 10.6 mum linear polarized laser beam at oblique incidence is reported. A circular focused laser spot on the workpiece surface, simultaneously with 2.2-4 times increasing of the absorption are obtained in the 70-80degrees range of the incidence angle. The main element of the experimental setup is the astigmatic focusing head which focalize the laser beam into an elliptical spot of ellipticity epsilon > 3 at normal incidence. At a proper incidence angle (obtained by the focusing head tilting) the focused laser spot on the work piece surface gets a circular form and p-state of polarization is achieved.We performed laser heat treatment (transformation hardening, surface remelting) of the uncoated surface, as well as the alloying and cladding processes by powder injection. An enhancement of the processing efficiency was obtained; in this way the investment and operation costs for surface treatment with CO2 laser can be significantly reduced. Several technical advantages concerning the pollution of the focusing optical components, powder jet flowing and reflected radiation by the work piece surface are obtained. (C) 2002 Published by Elsevier Science Ltd.

The amount of the heat transferred to the workpiece is of high importance on the laser surface treatment. A possibility to increase the energy coupling efficiency by using oblique incidence of the laser beam is proposed. The drawback of oblique incidence technique consists in the deformation of the laser intensity profile on the irradiated surface at large incidence angles (70-80 degrees). This paper is dealing with a method to overcome this disadvantage: focusing the laser beam in an elliptical shape at normal incidence; using oblique incidence at large angles (70-80 degrees) a circular focused p-polarized spot is obtained. The absorption is (2.5-4) times enhanced and the focused spot on the surface has a circular shape. The use of oblique incidence of the laser beam allows another important advantage: the avoidance of the contamination of the focusing optics. The cladding experiments at 75-degree incidence angle have been carried out at only 650 W laser power level. An improved cladding efficiency was obtained.

We developed and experimented a method for obtaining a transverse flow CO 2 laser-TFL- with a matching coefficient (the cross section of the laser beam/the cross section of the electrical discharge) -(eta) m greater than 0.8 for a Fresnel number of the optical resonator -N F less than 2, in which both types of the active medium asymmetries are compensated. This corresponds to a TFL with high efficiency and TEM 00/01 good optical quality laser beam. The main elements for this laser are: gas circulation fluido-dynamical circuit in cylindrical geometry with two opposite electrical discharges; U-type laser resonator with a 90 degree deflection element (pentaprism alike), instead of one corner mirror, which reverse the wavefront of the laser beam in one of the two discharge channels. Also, it was developed a new very stable 90 degree-beam deflection optical element with properties alike pentaprism. This element might be used in optical resonator construction, transport and focusing systems of the laser beam.

`Special mirror' concept is presented as a `spare module' for a GaAs extracting mirror technological processed with GaAs thin films, mounted within two kinds of c.w. CO2 laser resonators: U-type resonator (with optical pathlength Lr equals 4 m) and U- type folded resonator with V-discharge on each branch, reversal mirror provided (with the optical pathlength Lr equals 8 m), experimentally used to obtain good quality laser beam. The extracting mirror presents variable reflectivity, acting as a super-Gaussian mirror and working as an output coupler in the resonator, providing total compensation of the refractive index and of the small gain asymmetries.

A new optical resonator configuration for transverse flow CO2 laser (TFL) was developed. A high value of the matching coefficient (the cross section of the laser beam/the cross section of the electrical discharge) - eta m, is obtained for laser beams with TEM00/01 intensity distribution (that corresponds to the low values of the Fresnel number - NF Of the optical resonator). Simultaneously, both types of the gain and refractive index asymmetries of the active medium (parallel and perpendicular on the flow direction) are compensated. A 900 deflection device is the main constructive element (this is built by two mirrors at 45 degrees relative angle and offers the pentaprism properties). This deflection device reverses the wavefront in one of the two discharge channel and in addition improved the resonator stability. The main results are eta m congruent to 0.83 for N-F congruent to 1.9 and the beam pattern is independent of the power level in the range of 300 W to 1800 W.

The variable reflectivity mirror whose behavior was followed in dynamic regime while used in C.W. CO2 technological laser had the structure: AIR (ZnSe, BaF2)/GaAs(ZnS)AIR acting as a super-Gaussian mirror. It worked as an output coupler in an U type resonator mounted in a C.W. CO2 transverse flow laser, the output power characterizing the emitted beam belongs to the interval 800 - 1000 W in the fundamental mode. Following 300 hours of operations, in technological regime and 600 hours of overall operation, the laser beam quality exhibited unmodified characteristics from the point of view of industrial processing of materials.

The design of a variable reflectivity mirror (VRM) is reported, according to the requirements imposed on the output coupler of an U-type resonator for an industrial, CW, transverse flow CO2 laser. The reported method consists of building a variable thickness layer of BaF2 on GaAs substrate material using RF magnetron sputtering coating. The obtained VRM final structure is air-(ZnS)/GaAs/(BaF2, ZnSe)-air exhibiting a maximum reflectivity of the central zone, at 10.6 mu m, between 66% and 69%. The component worked, mounted on the laser resonator, for more than 600 h allowing a single mode beam structure at power levels between 800 W and 1 kW CW.

The influence of a nonhomogeneous small signal gain on the cw CO2 laser output is investigated. Simplified Gaussin variations of the small signal gain are considered. At certain resonator parameters, a maximum output power in the fundamental transverse mode can be obtained for laser operation providing a sharp gain variation with a pronounced maximum on the resonator axis.

The aim of this work is to report a new method to achieve a light weight, compact, high power CO2 laser which can be used in industrial applications. It is shown that a metallic cylinder with a relative small diameter (0.6 m) can assure the same flow velocity and cooling of the gas mixture like in previous systems using aerodynamic wind tunnels and special heat exchangers. The construction and operation of a gas transport CO2 laser with cylindrical geometry and two parallel electrical discharges are presented.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text I. Ursu, I. N. Mihailescu, I. Gutu, A. Hening, T. Julea, L. C. Nistor, M. Popescu, V. S. Teodorescu, A. M. Prokhorov, V. I. Konov, and V. G. Ralchenko, "Surface nitridation of zirconium and hafnium by powerful cw CO2 laser irradiation in air," Appl. Opt. 25, 2720-2724 (1986) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

The aim of this work is to report the achievement of a light weight, compact, high power CO2 laser which can be used in industry. The construction and operation of a CO2 gas transport laser with cylindrical geometry and two parallel electrical discharges are presented. Using a 600 mm diameter, 1400 mm length, 250 kg laser head, we have obtained more than 1.6 kW laser power with 2% stability and 2 mrad full angle divergence. For comparison the results are presented for one electrical discharge laser.

The construction and operation of a CO/sub 2/ gas transport laser with cylindrical geometry is presented. The aim of this work is the achievement of small-size and small-weight gas transport lasers at high output level. We have accomplished this by using a single metallic cylinder for the electric discharge, recirculation, and the cooling of the gas mixture. More than 1-kW of laser power was obtained from a 1.45 m long, 0.59 m diameter laser weighing about 180 kg. Typical parameters were: 40 Torr pressure, CO/sub 2/: N/sub 2/: He = 1.: 8:11, discharge current 8 A, 12% efficiency, 28 x 22 mm/sup 2/ laser spot.