High Power CO2 Laser Research Articles

RF-excited unstable-resonator planar CO2 laser on all-metal electrode—waveguide structure

The radiation characteristics of a planar CO2 laser excited by a diffusion-cooled rf discharge at a frequency of 40 MHz are studied. A single-mode cw lasing power of ≈50 W is achieved with an efficiency of ≈10% for a nearly diffraction-limited radiation divergence of 4—7 mrad. The spatial structure, output power, stability and laser radiation quality are studied as functions of longitudinal and angular alignments of the resonator mirror for two types of hybrid unstable-waveguide resonators of the laser. It is shown that for the resonator corresponding to the negative branch of the stability diagram, a misalignment of 0.02 rad of the mirrors leads to a 50% decrease in the output laser power, while its value for the positive branch resonator is about 100 times smaller. It is found that for the resonator corresponding to the negative branch, the sensitivity to the violation of confocal arrangement of the mirrors upon an increase in the resonator length is an order of magnitude higher. The dependence of the density of input rf power on the working gas pressure is studied experimentally in the interval 50—110 Torr. Power density values of 1—4 W cm-2 are obtained for normal discharge current density. These values are important for optimisation and scaling of the lasing characteristics of high-power planar CO2 lasers.

Stable all-optical formation of Bose–Einstein condensate using pointing-stabilized optical trapping beams

By stabilizing the beam pointing of optical trapping beams, we have succeeded in stable formation of Bose–Einstein condensate (BEC) of 87Rb with all-optical method. The thermal effect of acousto-optic modulator (AOM) is usually one of the most serious problems to induce beam-pointing instability, especially for high power CO2 laser. By passing the beam through AOM twice, we have improved the beam pointing from about 4.8 mrad to less than 0.4 mrad, which has been experimentally confirmed to be small enough to stably form BEC at the crossed region of CO2 lasers as well as to perform experiments using an optical lattice which might have been affected by beam-pointing instability.

Characteristics of high-power CO2 laser welding and porosity suppression mechanism with nitrogen shielding. Study of high-power laser welding phenomena

In its role as a welding heat source providing high energy density in ambient air, the laser can be used to perform high-precision, deep penetration welding under low-strain conditions. In the wake of the substantial high-power and high-beam quality enhancements of laser oscillators, the laser has most recently seen growing applications in a wide variety of industrial fields.1, 2 Available applications, however, are largely focused on high-precision welding of thin sheet metal in the automotive industry.3, 4 Deep penetration welding applications as another viable application avenue have been reported in shipbuilding and nuclear power generation, but remain relatively little documented.5 This is reportedly due to the fact that, with the increasing penetration depth of laser welding, welding defects such as porosity and solidification cracking more readily occur. Particularly in partial penetration welding of carbon steel, development of porosity prevention methods intended to allow laser welding to proceed at a depth of more than 10 mm has gone entirely unstudied. In this context, the authors have mounted an intensive research programme with the goal of developing a laser welding technology to prevent defects in welds at a penetration depth of more than 20 mm. In these circumstances, the oscillators used even now are high-power CO2 lasers suitable for deep penetration applications.

An analysis of the shielding gas flow from a coaxial conical nozzle during high power CO2 laser welding

An experimental and theoretical study on the role of the nitrogen gas stream, exiting from a conventional conical nozzle tip during a laser welding process, has been carried out. A mathematical model has been used, based on the Navier–Stokes equations which express fundamental conservation laws of mass, momentum and energy for a compressible fluid. Numerical simulations of the gas stream colliding onto a plane surface have been performed showing the effects of variations of inlet gas pressure, nozzle exit diameter and standoff distance on the density and Mach number contours, axis pressure of the gas jet and plate pressure produced on the workpiece surface. Laser welding experiments have been performed on carbon and stainless steel specimens, by varying the process parameters in the same range as in the simulations and keeping constant the incident power and the travel speed. Two different gas stream regimes were found, namely sonic and subsonic, which were experimentally verified to produce cutting and welding conditions, respectively.Weld performances have been evaluated in terms of bead width, penetration depth and melted area. Nozzle standoff distance was found to have a negligible influence, while the exit diameter and the flow rate significantly affect the weld results. The numerical predictions allowed an explanation of the experimental results yielding useful suggestions for enhancing the weld quality, acting simply on the shielding gas parameters.

Multilevel Computer Generated Hologram on a Curved Surface Made of Copper for High Power CO2 Laser Beam Shaping

A multilevel computer generated hologram (CGH) was made on a parabolic copper mirror for laser beam shaping. The CGH consisting of eight height-levels with 0.94μm in step height has been fabricated with a laser-beam direct writing lithography and the mask-electroplating method. The CGH was designed for laser surface modification with a high-power CO2 laser. The Gaussian distribution of an incident laser beam was shaped into a Rectangle-Gaussian distribution on a focus plane of the parabolic mirror. The diffraction efficiency was 68%. When the shaped laser beam of a 1.5kW CO2 laser irradiated a steel surface, a uniform melting layer was obtained.

Longitudinal Power Distribution and Corresponding Temperature Distribution in a RF Waveguide CO2 Laser

The output power of a gas laser can not be increased by increasing the input power due to the increase of the gas temperature. As the temperature of the laser gas mixture increases up to 600K, the ampliflcation of the amplifying medium decreases rapidly and so the output power decreases. In this paper, longitudinal power distribution and corresponding temperature distribution along the CO2 waveguide laser are studied theoretically. Efiect of voltage and current distributions along and transverse to the electrode direction respectively are accounted for the studies. It is seen that the temperature decreases exponentially along the length of the electrodes on both sides of the feed point. DOI: 10.2529/PIERS060829221216 Several papers (1) have already described the huge potentiality of large area difiusion cooled RF capacitive discharges for the construction of medium | to high power CO2 lasers for materials processing. Indeed, some industrial sources based on this technology are presently appearing (2). However, there exist some open problems that limit the simple scaling of the above-mentioned technique to higher power segments. One of the major problem is certainly the attainment of a uniform gain medium excitation because of transmission line efiects (3) naturally determined by electrode dimensions comparable with the exciting RF wavelength. Typical counter measures to this di-culty are the adoption of sectioned discharges or the use of reactive elements along the electrodes. The principles for the correct design of systems based on these schemes are well established in the case of narrow channel devices (4{6), in which the equivalent-line characteristic impedance is mainly determined by the electrode structure. In (7) it is demonstrated that the same smoothing principals could be applied to wide channel high power discharges provided that the in∞uence of the discharge loading be taken into account. Indeed, from the analysis carried out in (7), it is possible to conclude that the main role played by the discharge loading is that of determining a new equivalent transmission line whose charac- teristic impedance is closer to that produced by the capacitance of the ion sheaths surrounding the electrodes rather than by the channel capacitance. This efiect determines the voltage and thus power distribution closer to a lossless-line rather than lossy-line models. Moreover (8,9), the typical impedances of sheaths and neutral plasma for the mixtures and pressure of the interests for CO2 laser construction produce distributions hardly distinguishable from uniform transmission line models (7,10,11). In this paper the temperature distribution along the longitudinal direction of the electrodes is studied theoretically by considering the longitudinal power distribution by applying the trans- mission line theory. Voltage distribution is considered along the longitudinal direction while the current in transverse direction of the electrodes. The voltage and current distributions along the electrodes are given by solving the simple transmission line difierential equations (2,7,10{12):

High-power supersonic CO laser on fundamental and overtone transitions

Lasing parameters of a rf discharge-pumped CO laser in a subsonic gas flow with subsequent cooling of the laser medium in a supersonic flow are studied. The output power of the laser achieved 2.1 kW for the efficiency up to 21% in the spectral range 4.9–5.7 μm corresponding to the V+1→V fundamental vibrational transitions in the CO molecule. The laser output power at the V+2→V first vibrational overtone transitions (2.6–2.7 μm) was 50 W. Possible ways of improving the laser design for generating radiation at higher overtone transitions of the CO molecule are discussed.

Two-dimensional-simulation of thermal blooming effects in ring pattern laser beams

Thermal blooming is a major effect in high power laser beams transmitting through gaseous medium as well as the atmosphere. Due to this effect, the beam pattern is deformed through the propagation path. In this work, a ring pattern beam from a high-power CO2 laser with an unstable resonator is evaluated when propagating through a high-pressure CO2 gas, and the results are compared with those of the Gaussian beam. The results show that unlike the initial Gaussian beam from which a ring pattern formed when propagating through an absorbing medium, for the initial ring pattern beam, in the absence of the gas flow, a symmetrical two-fold pattern is formed due to the blooming effect. The results also show that such symmetry can be deformed in the presence of gas flow.

High-power, high-pressure pulsed CO2 lasers and their applications

The paper is devoted to problems associated with the construction of high-power pulsed CO2 lasers and high-pressure amplifiers and to an analysis of the possible ways of their solution. Prospects of the development of such lasers and their applications in technological processes are considered. Original designs of a laser complex for obtaining the carbon-13 isotope and a superatmospheric-pressure CO2 amplifier are presented.

Acoustic Microscopy Applied to Ceramic Pressure Vessels and Associated Components

Alumina ceramic is being used extensively for external pressure vessels in naval applications. The material is also used in valves and other components, where reliability, immunity from corrosion, and high temperatures are required. This report presents a technique for the nondestructive evaluation of alumina ceramic components. The cracks were produced by CO2 laser radiation. Since there is a need for the detection of very fine cracks, scanning acoustic microscopy was found to be superior to optical methods for imaging surface and subsurface cracks. We address the issue of crack initiation with the use of postirradiation analysis. This article reports our results on the evaluation of the surface and interior cracks with optical, scanning laser, scanning electron, and scanning acoustic microscopy. We present images of surface and subsurface microcracks generated at different power levels of a high power CO2 laser system. The spatial variation of the Rayleigh wave velocity is measured by the V(z) curve technique. These preliminary data suggest that, with some improvement, the V(z) technique may detect residual stress with high spatial resolution.

High-power CO2 laser design “from resonator end mirror to cutting lens focal point”

An algorithm is derived that allows calculating all optical parameters relevant to the design of a high-power CO2 laser, taking into account the practical needs of a flying-optics type of laser cutting machine. The novelty of the approach taken is that the resonator and the beam delivery optics are treated simultaneously, which allows for greater flexibility in design. To begin with, the often-used optical configuration of a stable resonator combined with a beam expander for collimation is modeled and optimized. The remaining focal shift over the cutting table, mainly due to the fact that the part of the beam close to the resonator still has appreciable wavefront curvature variation, is calculated. The resulting function is programmed into the machine CNC, which continuously adjusts the lens position depending on the actual beam length, to compensate for this effect. Next, attention is paid to the well-known problem of changing beam parameters with thermal load of the output coupler. It is found necessary to model both mechanical deformation of the ZnSe window and lensing due to the temperature dependent index of refraction. The former is derived from a finite element model, for the latter, the formula of Miyamoto et al. [Proc. SPIE 1276, 112 (1990)] is used. Finally, as an example of novel resonator designs studied using this approach, the characteristics of an intracavity reflective telescope are determined for two different goals: first, to increase the fundamental mode spot size for the given resonator (and hence to improve beam quality without sacrificing power), and second to preserve beam quality when extra resonator layers are added to increase power. The beam is modeled using the concept of the “embedded Gaussian” and the “complex radius of curvature” due to Siegman [Lasers (University Science Books, CA, 1986)].

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.

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

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.

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 cooling of calcium in a ‘golden ratio’ quasi-electrostatic lattice

A 3D lattice based on a high-power CO2laser is considered in the context of laser cooling and trapping ofatomic calcium. We expect to be able to realize a system with >10 000lattice sites each with more than 100 atoms, and the facility to laser cool all theatoms into the vibrational ground state using the intercombination line. Theconfiguration allows the production of an array of small Bose–Einsteincondensates, enabling an investigation of the build-up of phase coherence as afunction of atom number.

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.