High Power CO2 Research Articles

Performance comparison of CO2 and diode lasers for deep-section concrete cutting

Layer-by-layer laser machining with mechanical removal of vitrified dross between passes is a new technique with a demonstrated capability for deep-section cutting, not only of concrete, but of ceramic and refractory materials in general. For this application fairly low power densities are required. A comparison of experimental results using high-power CO2 and diode lasers under roughly equivalent experimental conditions, cutting to depths of >100 mm, is presented. A marked improvement in cutting depth per pass is observed for the case of the diode laser. The increased cutting rate is rationalized in terms of the combined effects of coupling efficiency and beam shape.

High-power self-controlled volume-discharge-based molecular lasers

An investigation is made of the characteristics of the formation of a self-controlled volume discharge for pumping CO2 lasers, i.e., self-sustained volume discharge (SSVD), which involves the preliminary filling of a discharge gap by an electron flux from an auxiliary discharge plasma. It is determined that this method is suitable for large interelectrode gaps, that distortion of the electric field in the gap by the space charge of the electron flux plays an important role in the formation of the discharge, that the electrodes could be profiled dynamically during propagation of an electron flux through the discharge gap, and a SSVD could form in systems with a strongly inhomogeneous field. High-power SSVD-based CO 2 laser systems are created and investigated. Another type of self-controlled volume discharge without preionization, i.e., a self-initiated volume discharge (SIVD), in nonchain HF lasers with SF 6 –C 2 H 6 mixtures is also investigated. It is established that, after the primary local electrical breakdown of the discharge gap, the SIVD spreads along the gap in directions perpendicular to that of the electric field by means of the successive formation of overlapping diffuse channels under a discharge voltage close to its quasi-steady-state value. As new channels appear, the current that flows through the channels formed earlier decreases. The volume occupied by the SIVD increases when the energy deposited in the plasma increases, and, when the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously when the current increases. The possible mechanisms that explain the observed phenomena, namely the dissociation and electron attachment of SF 6 molecules, are examined. A simple analytical model is developed that makes it possible to describe these mechanisms at a qualitative level. High-power SIVD-based HF(DF) lasers are developed and tested.

Fabrication of Multilevel Copper Computer Generated Hologram for High Power CO2 Laser Beam Shaping

A multilevel computer generated hologram (CGH) was fabricated for shaping a laser beam profile of a high-power CO2 laser. The surface relief has eight height levels with 0.94μm in step heights. A copper surface relief was formed on a flat copper substrate by electroplating with photo-resist masks. Standard deviation of the surface step height was less than 22% of the designed height. The CGH changes a CO2 laser beam of 14mm in diameter to a rectangular beam of 1mm×4mm and its diffraction efficiency was 69.2%. A 200W CO2 laser beam of 14mm in diameter did not damage the CGH surface.

大出力CO<sub>2</sub>レーザによる厚板貫通溶接時の凝固割れ発生機構とその抑制

大出力CO<sub>2</sub>レーザによる厚板貫通溶接時の凝固割れ発生機構とその抑制

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.

Transmission of high-power CO2 laser pulses through a plasma channel

A 5 J, 180 ps CO2 laser pulse is channeled by a 17 mm long capillary discharge. Plasma dynamic simulations confirm occurrence of optical guiding conditions along a plasma column of a quasiparabolic radial profile with the minimum axial free-electron density ∼1017 cm−3.

Laser beam welding hard metals to steel

Laser beam weldability of hard metals to steel was examined with high power (cw) CO 2 laser, (cw) Nd:YAG laser and (pw) Nd:YAG laser. Steel/hard metal joints, with 2.5 mm thickness, welded by the three laser systems, for comparison, will be presented. Two different hard metals compositions (K10 and K40) were examined. Power ( P), speed ( s) and vertical focal point position (f.p.p.) were investigated. To reduce the problem of porosity and crack formation in the hard metal, the parameter, t w, horizontal distance from the beam to the joint was also investigated. Weld bead size, microstructure, bending tests and hardness were evaluated.

High-power multibeam CO 2 laser for industrial applications

Multibeam CO 2 lasers consist of a large number of closely packed parallel glass discharge tubes, all sharing a common plane parallel resonator. This paper describes construction and operation of a CW multibeam CO 2 laser consisting 20 discharge tubes and cooled length of 1500 mm , delivering 1 kW power. A high-frequency pulser is used for producing preionization in all discharge sections for initiating the main DC discharge simultaneously in all discharge tubes. Plane parallel resonator consists of a plane ZnSe mirror of 90 mm diameter having 60% reflectivity and a gold-coated copper mirror of same diameter. This laser operates in waveguide regime and laser power is not critically sensitive to mirror misalignment.

Generation of intense secondary pulsed molecular beams of high kinetic energy controllable by a powerful IR laser

A method for generation of intense secondary pulsed molecular beams and beams of radicals of high kinetic energy controllable by a powerful IR laser is described. A pressure shock (shock wave) is used as a source of secondary beams. The pressure shock is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beams were studied. Their intensities and the degree of gas cooling in them were shown to be comparable with the corresponding characteristics of the unperturbed primary beam. The acceleration of molecules in the secondary beam is achieved due to vibrational excitation of them by high-power IR laser pulse in the pressure shock and subsequent vibrational to translational (V–T) relaxation, which occurs when a gas expands through the orifice into a vacuum. Intense [⩾1020 molecules/(sr s)] beams of SF6 and CF3I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases. The SF6 molecular beams with kinetic energies approximately from 2.5 to 2.7 eV with carrier gases H2, He and CH4 (SF6/carriergas=1/10) were obtained. The possibility of generation of intense beams of cold radicals by this method is demonstrated. The intense beams of cold and accelerated CF3 radicals were generated when the CF3I molecules in the shock were dissociated by high-power CO2 laser radiation. The spectral and energetic characteristics of acceleration of SF6 and CF3I molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules.

Factors affecting the optical performance of CVD diamond infrared optics

Diamond is a unique optical material having the combination of high strength and transparency in the visible, near and far infrared regions. As a far infrared window it has already found applications in demanding environments, e.g. high-power CO2 laser exit windows. In this paper, the far infrared optical properties of diamond are described together with the near infrared properties and the potential for using CVD diamond at shorter wavelengths is discussed. How such optical properties as absorption, scatter and image quality are related to the characteristics of CVD diamond are described. The far infrared absorption properties of diamond at elevated temperatures are presented and the mechanisms that cause the residual absorption and its temperature dependence in this region are discussed. An empirical formula for predicting the temperature-dependent absorption coefficient is presented.

High power large bore CO2 laser small signal gain coefficient and saturation intensity measurements

Two methods of measuring the gain medium parameters of a laser are presented (Hodgson N and Weber H 1997 Optical Resonators–Fundamentals, Advanced Concepts and Applications (Berlin: Springer) pp 583–91). The first method used measured only the output power of the CO2 laser (10.6 μm is the wavelength used) using three separate output couplers, each having different couplings. Once the three different output powers were measured, the results were used in the Rigrod equation to solve for two separate Rigrod equations knowing all the variables except for two unknowns; the small signal gain coefficient and the saturation intensity. The second method was a single-pass measurement where a very low power CO2 laser was injected into the high power CO2 laser in question. Power readings were made before and after the injection to determine the gain. Both methods agree with each other well, within 5%, for the small signal gain calculation, which provides confidence that the calculation for saturation intensity in the first method is correct as well.

Dielectric-barrier Discharges: Their History, Discharge Physics, and Industrial Applications

Dielectric-barrier discharges (silent discharges) are used on a large industrial scale. They combine the advantages of non-equilibrium plasma properties with the ease of atmospheric-pressure operation. A prominent feature is the simple scalability from small laboratory reactors to large industrial installations with megawatt input powers. Efficient and cost-effective all-solid-state power supplies are available. The preferred frequency range lies between 1 kHz and 10 MHz, the preferred pressure range between 10 kPa and 500 kPa. Industrial applications include ozone generation, pollution control, surface treatment, high power CO2 lasers, ultraviolet excimer lamps, excimer based mercury-free fluorescent lamps, and flat large-area plasma displays. Depending on the application and the operating conditions the discharge can have pronounced filamentary structure or fairly diffuse appearance. History, discharge physics, and plasma chemistry of dielectric-barrier discharges and their applications are discussed in detail.

Operational characteristics and power scaling of a transverse flow transversely excited CW CO2 laser

Transverse flow transversely excited (TFTE) CO2 lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO2 laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO2 laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.

Laser welding phenomena of mild steel with oxide film. Influence of oxide film on high power CO2 laser welding of mild steel (Report 1)

Influence of oxide film, such as laser cutting edge or mill-scale, on CO2 laser welding phenomena and weld characteristics of mild steel was investigated. Laser welding of butt joints, which were different in location and thickness of oxide film, were performed. Examinations were made on the weld characteristics; which were bead geometry, defect and microstructure. Light emitted from laser induced plasma was also analyzed to understand welding phenomena. The results of these experiments lead to understanding that oxide film especially on joint face causes substantial spatters, under-filled surface, disappearance of nail-head, porosity formation and reduction of hardness. It is considered that these are attributed to oxygen which reduces surface tension of molten metal and react with carbon, manganese and silicon. These results suggest that reduction of oxide film thickness or killing oxygen activity by supplementary deoxidizing elements may produce acceptable weld.

Reduction mechanism of porosity in tandem twin-spot laser welding of stainless steel

Porosity is easily formed in a keyhole-type of deeply penetrated laser weld beads. In this study, therefore, high power CO2 laser welding utilizing a single spot or a tandem twin-spot beam was performed on thick plates of Type 304 steel with the objectives of investigating feasibility of porosity prevention and establishing the optimum conditions in tandem twin-spot beam welding. Porosity formation tendency was first investigated in a partial penetration weld made with a single-spot CO2 laser beam at low speed. Consequently, it was confirmed that many pores were present near the bottom parts of the weld beads produced without and with assist gas at a power exceeding 10 and 15 kW, respectively. On the other hand, it was revealed that the number of pores was drastically reduced by twin-spot laser beam welding. Especially, some weld beads produced at 30 kW indicated the best grade of the inspection test result according to JIS Z3106. Subsequently, the reason for the reduced porosity was investigated by using the microfocused X-ray transmission in-situ observation system with a high speed video camera. During high-power laser welding of Type 304 with the single-spot beam, it was confirmed that the keyhole fluctuated up and down and consequently a lot of bubbles were generated from the keyhole tip near the bottom part of the molten pool, leading to the formation of porosity. On the other hand, when an enlarged keyhole was produced by a twin-spot beam, few bubbles were generated from the keyhole tip. Moreover, even if the bubbles were formed, most of them did not move away from the formation location and were soon absorbed in the keyhole. This is interpreted by considering that no strong melt flow was induced near the bottom part of the molten pool just behind the tip of the widened keyhole. Consequently, the difference of porosity formation between single-spot and twin-spot laser beam welding is attributed to the differences of their bubble formation mechanisms, liquid flow near the bottom part of the molten pool, and the bubble disappearance via the keyhole.

大出力ＣＯ２レーザ・ＭＩＧハイブリッド溶接特性と開先裕度拡大効果―ハイブリッド溶接による厚鋼板の突合せ溶接特性（第１報）―

There are few applications of laser welding to thick plates joining. One reason is attributed to narrow gap tolerance due to a small laser beam diameter leading to unacceptable seam defects such as underfill and burn through. Another is easy formation of weld imperfection such as porosity and cracking in a keyhole—type of deeply penetrated laser weld beads. Hybrid welding with a laser beam and an arc has been receiving considerable attention as one of the methods to suppress such defects and imperfection. This paper reports fundamental investigation results of hybrid welding with combined high power CO2 laser and Metal Inert Gas (MIG) arc, and especially hybrid welding results of thick plates with such a wide gap that the majority of a laser beam can pass through. The behavior of laser-induced and arc plasmas near the molten pool during the hybrid welding was observed with a black-and-white CCD camera. Penetration depth were confirmed to depend upon the distance between laser beam irradiation location and MIG wire target. The different depths were interpreted in terms of the interaction between laser-induced plasma and molten metal, melt flows, and so on. Moreover, under the formation conditions of the deepest weld beads, the wide gap groove was fully filled with the molten metal. The observation suggests that the recoil pressure due to the laser-induced evaporation should force the molten metal fed with a MIG wire to fill the leading gap and a fully penetrated keyhole was formed during hybrid welding. Consequently, a sound hybrid weld bead could be produced in joining the plates of 22 mm thickness with 4 mm gap, and moreover some weld beads indicated the best grade of the inspection test result according to JIS Z3104.

Laser welding phenomena of mild steel with oxide film. Influence of oxide film on high power CO2 laser welding of mild steel (Report 1)

Laser welding phenomena of mild steel with oxide film. Influence of oxide film on high power CO2 laser welding of mild steel (Report 1)

Laser surface alloying of medium carbon steel with SiC (P)

Laser surface alloying (LSA) is increasingly recognised as a powerful surface modification tool to enhance the wear and corrosion resistance of engineering components. The present work deals with laser alloying of medium carbon steel with silicon carbide using a high-power CO 2 laser. A processing regime, identifying an appropriate laser power-scan speed combination for achieving defect-free alloyed layers, has been established during the study. The influence of repetitive scans on the alloyed layer properties was also subsequently investigated in a comprehensive manner. Repetitive scanning was found to affect substantially the laser-alloyed zone (LAZ) and heat-affected zone dimensions, as well as the phase constitution of the laser-alloyed layers. The microstructure in the LSA layers was also observed to vary significantly with the number of scans, besides changing quite distinctly with depth in the LAZ. Tribological tests revealed considerable improvement in abrasive wear performance by laser surface alloying, and the results also emphasise the need to control the number of repetitive scans to achieve optimum performance.

Optical resonator for high-power transverse flow CO 2 lasers

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 CO 2 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 90° 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 90° angle. Both elements exhibit good focusability if they are equipped with two concave mirrors.

Rapid prototyping of diffractive optical elements for high-power lasers using laser ablation lithography fabrication and coherence probe microscopy analysis

An alternative technique for rapid prototyping of diffractive optical elements (DOEs) for use with mid-infrared high-power lasers is proposed. The solution is based on the photoablation of an intermediate resist layer on various substrates using an excimer laser in a micromachining station, followed by classical etching of the substrate to form the DOE. Being able to achieve rapid prototyping also depends on having available a rapid, precise, nondestructive profilometry technique in order to optimize each stage of the manufacturing process. After comparing several techniques for structural characterization, we found that coherence probe microscopy was the best one suited for the rapid and precise measurement of the squareness, depth, and surface roughness of the transparent resist mask and the substrate features. Several results of the characterization of transparent resist layers and etched substrates are given. The technique of rapid prototyping by the means proposed is illustrated with the fabrication of a DOE for direct marking of an industrial logo using a high-power CO 2 laser.