Recent advances in laser technology have resulted in the development of multi-kilowatt lasers which have been engineered for industrial applications. This paper summarizes some of these technological developments, and describes the characteristics of the laser systems. Emphasis has been placed on the description of multi-kilowatt processing systems which have been delivered to industry.
The NASA Lewis Research Center has designed and fabricated a closed-cycle, continuous wave (CW), carbon dioxide (CO2) high-power laser to support research for the indentification and evaluation of possible high-power laser applications. The device is designed to generate up to 70 kW of laser power in annular-shape beams from 1 to 9 cm in diameter. Electric discharge, either self-sustained or electron-beam-sustained, is used for excitation. This laser facility can be used in two ways. First, it provides a versatile tool on which research can be performed to advance the state-of-the-art technology of high-power CO2 lasers in such areas as electric excitation, laser chemistry, and quality of output beams, all of which are important whether the laser application is government or industry oriented. Second, the facility provides a well-defined, continuous wave beam for various application experiments, such as propulsion, power conversion, and materials processing.
Two high power (3kW) CO2 laser welding systems have recently been placed in production at separate locations welding a new cylindrical design lead-acid industrial type battery for Bell System use. The applications are for the joining together the positive plates of the cell and are believed to be the first in industry for multikilowatt CO2 laser welding on a full production basis.
Product marking is frequently necessary for identification, product information, or theft prevention. The unique characteristics of lasers which lend themselves to product marking are indicated. Three techniques of laser marking are discussed -- engraving, dot matrix marking, and mask imaging. Types of laser equipment, cost considerations, and applications for laser marking are discussed in the paper.
Significant design improvements in the pulsing capability of CO2 lasers have led to improved machining results not achievable in CW operation. Pulses on the order of 8 times the CW rating lead to efficient material processing while cutting heat input significantly. CO2 lasers with lower-than-expected CW ratings can be used to provide for more economical machining.
Aerospace components are becoming increasingly costly to fabricate and difficult to machine as we continue to increase the usage of high strength and toughness titanium, steel, and superalloys. Since 1971, the Air Force Materials Laboratory has successfully established laser cutting equipment and processes that have demonstrated significant fabrication cost savings. These equipment and processes are now being used in practical, economical, production operations.
The use of high-speed lasercutting developed for apparel manufacturing applications is now finding a place in the industrial manufacturing sector. Non-metal structural materials such as boron epoxy, graphite epoxy, Kevlar, etc., difficult to cut by conventional means, can be efficiently cut into uniquely shaped parts with an edge quality previously not possible. The advent of high-powered industrial lasers combined with advanced positioning techniques to opening a vast new market in metals fabricating. In many applications, lasercutting can be economically justified to replace conventional stamping or routing.
Although the pulsed YAG laser has distinct advantages as a precision welding tool, limited power and pulse rate capability has restricted its applications. A 500 Watt average power, 300 pulse per second system is described which makes possible conduction seam welding penetrations as great as .06 inch and welding speeds as high as 150 inches per minute. Penetrations and welding efficiencies for pulsed YAG are compared to published data for CO2 and CW YAG lasers. Penetration and welding efficiencies are substantially higher for pulsed YAG up to speeds of 120 inches per minute. YAG laser penetration welds are reported for the first time. Hermetic seal conduction laser welds are also reported for the first time.
For more than a year, the Space Division of Rockwell International has been laser-stripping single-conductor, Kapton-insulated wire, such as that used in the Space Shuttle orbiter. To perform this work, the Division designed both bench-model and hand-held laser wire strippers. In this stripping process, an optical-mechanical system first rotates the focus spot around the wire, making a circumferential strip. The focus beam is then translated axially to slit the slug, which can be easily removed by hand. While Kapton is highly absorptive of the 10.6-μm and 1.06-μm laser wavelengths, the nickel coating of the copper wire is highly reflective, and the residual heat input is rapidly conducted away. Thus the integrity of the conductor itself is not compromised.
The thermal response of an opaque solid slab exposed to intense laser radiation is described using the heat-balance integral method. Approximate analytical solutions are obtained for the one-dimensional temperature distribution T(x,t), for the velocity xs(t) of the moving vapor-phase boundary, and for various derived quantities such as the front-surface vaporization time tv, the time to back-surface heating tℓ and the sample burnthrough time tBT. Account is taken of the influence on tℓ and tBT of partial vaporization of the target material. The thermophysical properties of the target are assumed to be constant independent of temperature, but the theory accommodates a time-varying laser intensity and a temperature-dependent surface absorptivity. Illustrative burnthrough-time calculations are presented for aluminum, stainless steel, and carbon phenolic targets, covering a wide range of values of target thickness and laser intensity.
The adjustment of film resistors to specified value by selective removal of film material with a programmed laser beam has been practiced since 1966. Evolution of systems to perform not only passive resistor trim but adjustment of operating circuit parameters has led to sophisticated multipurpose machines. This paper describes such a trimmer and correlates the utilization of its various configurations with trim requirements of several types of product. Precautions and suggestions on adjustment of system parameters are discussed to optimize results with respect to accuracy and stability of trimmed resistors.
The use of nanosecond dye laser pulses to form low resistance reliable ohmic contacts between conductors on integrated circuit chips after fabrication is described. Possible applications which show the potential importance of this process to integrated circuit personalization are discussed.
A variety of lasers have been applied to trimming thin and thick film resistors in the last ten years, ranging from pulsed argon lasers for thin film cermet resistors to CO2 lasers for thick film resistors. At present, the Q-switched YAG laser dominates the resistor trimming technology because of its high repetition rate (5-10 KHz) and its relatively high pulse power (2-20 Kwatts). The xenon laser, on the other hand, with its 300 watts pulse power and 200 Hz repetition rate, is taking over the thin film semi-automatic resistor trimming technology. At an output of 535 nm, the wavelength of the xenon laser is half of the 1060 nm of the YAG laser and thus provides a diffraction limited spot size which is one half as large. With a one inch focal length objective lens the YAG laser offers a spot size of .0005 inch where a xenon laser provides a .00025 inch spot size. As thin film resistor geometries approach a .0005 inch width, it is becoming evident that a spot size of .00025 or less is essential. Certain substrates such as circuit boards are not damaged (scarred) by the xenon wavelength as readily as with the YAG. Thin film resistors on substrates of glass, ceramic and circuit board have been trimmed in a variety of geometries with the xenon laser. As the xenon laser does not require special power or water facilities, it forms the basis for a cost effective resistor trimming system in thin film resistor production.
Pulsed lasers are being used to generate high amplitude stress waves in metals and change their mechanical properties. Peak pressures greater than 5 GPa are generated in a metal or alloy when it is covered with a transparent material. These pressures exceed the Hugoniot elastic limit of most metals and produce networks of tangled dislocations in the metals substructure which is the source of the observed change in material properties. The strength, hardness, and fatigue properties of 7000 series aluminum alloys are improved in this manner. Weld zones in aluminum are strengthened up to the bulk level and the surface hardness of stainless steel is increased.
A discussion of the laser system requirements for fusion applications is presented. A comparison between the photolytic atomic iodine and electron beam pumped rare gas oxide laser systems are made in the context of a fusion laser system. The use of spontaneous and superfluorescent emission from selected rare gas halogen exciplex systems to pump high average power atomic iodine lasers is explored with emphasis on the pump source kinetics in relation to amplifier design.
A unique laser assisted fusion approach is under development at Mathematical Sciences Northwest, Inc. (MSNW). This approach captures one of the most developed aspects of high energy laser technology, the efficient, large, scalable, pulsed electron beam initiated, electric discharge, CO2 infrared laser. This advanced technology is then combined with the simple geometry of a linear magnetic confinement system. The laser solenoid concept will be described, current work and experimental progress will be discussed, and the technological problems of building such a system will be assessed. Finally a comparison will be made of the technology and economics for the laser solenoid and alternative fusion approaches.
The laser system requirements for isotope enrichment are presented in the context of an atomic uranium vapor process. Coherently pumped dye lasers using as the pump laser, either the frequency doubled Nd:YAG or copper vapor are seen to be quite promising for meeting the near term requirements of a laser isotope separation (LIS) process. The utility of electrical discharge excitation of the rare gas halogens in an LIS context is discussed.
The Los Alamos Scientific Laboratory is conducting research on uranium enrichment. All processes being studied employ uranium molecules and use lasers to provide isotopic selectivity and enrichment. There are four well-defined infrared frequencies and two ultraviolet frequency bands of interest. The infrared frequencies are outside the range of the available lasers and an extensive research and development activity is currently underway. Lasers are available in the uv bands, however, much development work remains. The specification for the commercial uranium enrichment plant lasers will depend upon the results of the current enrichment experiments, the laser capital cost, reliability; and maintenance cost. For the processes under investigation there are specific photon requirements but latitude in how these requirements can be met. The final laser selections for the pilot plant need not be made until the mid-1980's. Between now and that time as extensive as possible a research and development effort will be maintained.
Research from all over the globe has recently brought attention to the laser as a tool for isotope enrichment. So far the main thrust of this effort has been toward uranium enrichment; however, numerous successes in other areas have been demonstrated. Isotopes of boron, sulfur, chlorine, and carbon have been separated. A new technique is proposed for laser isotope enrichment. The technique, referred to as photodesorption, involves selective isotopic excitation of molecules adsorbed on a surface such that an enrichment results from subsequent physical or chemical events undergone by the excited molecules. The specific processes of concern here are the physical photodesorption enrichment of heavy water from light water and tritiated water from heavy water. The ability to work directly with water molecules has significant advantages for a commercial process. A photodesorption enrichment process has been formulated and some analyses have been performed. This process is described and some preliminary cost estimates are made which assume successful accomplishment of the major R&D objectives of the new process. The results indicate that the process has the promise of a significant reduction in the cost of heavy water and that further study is warranted.