Competitive High-Power Laser Technology Overview

Transcription

1 Competitive High-Power Laser Technology Overview by Teresita K. Kolenchak, Michael R. McKinney, and Robert Ireland with contributions from Manfred Berger, Gary Herritt, and Herman Reedy

2 CO 2 Lasers CO2 lasers use a CO2 gas mixture as the lasing medium. Most resonator cavities consist of one fully reflective mirror and one partially reflective mirror. Pump energy is supplied as either direct current or radio frequency. For axial flow CO2 lasers, it is possible to increase power by linking multiple tubes via bend mirrors, thereby increasing the length of the resonator cavity s optical path. Other types of laser cavities require different techniques to increase power, but all of them do so by increasing the volume of the active medium (i.e., the CO2 gas mixture). Power output from CO2 lasers can be as high as 20 kilowatts, with good beam quality. Output wavelength is in the far-infrared, including wavelength bands from 9.3 to 10.6 microns (µm), with the 10.6-µm band being the primary wavelength of operation. At these wavelengths, specialized optics are required to focus the beam. Low-power (50 to 200 watts), sealed-tube CO2 lasers are becoming popular tools for marking a variety of metals, including stainless steel. While Nd:YAG systems also mark such materials, low-power, sealed CO2 systems cost substantially less, making them an attractive alternative. These low-power CO2 lasers can also cut ceramics and plastics, mark wood and glass, and drill micro via holes in printed circuit boards. To facilitate faster processing of these materials, CO2 lasers can be designed to lase at wavelengths in lower ranges such as 9.3 or 9.4 µm. High-power (multikilowatt), flowing-gas CO2 lasers, typically operating at 10.6 um, can be found welding, cutting, and drilling in all manner of industries, from small job shops to automotive facilities. CO2 lasers are also used for perforating, scribing, and micromachining. Materials processed by CO2 lasers include metals, plastics, glass, quartz, fabrics, and others. CO2 lasers are also used for selective laser sintering, one form of rapid prototyping. Compared to other types of high-power lasers, most CO2 lasers have low-order transverse modes. Transverse modes affect the focus properties of the beam. Low-order modes can be focused to smaller focal points. Poor modes or high-order modes cannot be focused as well. For high-power laser cutting, small focal points reduce kerf width (or width of the cut) and increase energy density at the focus, which allows faster cutting speeds. CO2 lasers have a moderate wall plug efficiency of 5 to 15 percent and typical power levels of 2 to 6 kilowatts. Because the higher CO2 laser wavelength does not focus on the retina of the human eye, safety is less of an issue. Therefore, unlike YAG lasers, eyeglass wear is a more effective precaution when using CO2 lasers. Maintenance can be expensive, however, because of the limited lifetime of the gases involved with CO2 lasers. These systems also require management of the plasma (cover or shield gas). In addition, the required tubed laser beam delivery to the work piece is less flexible and more difficult to integrate than other systems.

3 Nd:YAG Lasers Lasing materials of solid-state lasers are synthetic crystal rods. The rods are pumped with energy, typically from xenon flash lamps or laser diodes. Most solid-state lasers use resonator cavities with external mirrors. For industrial applications, the most commonly used laser crystals are made of Nd:YAG. These lasers operate in the near-infrared at a wavelength of 1.06 µm. Power output can be as high as 5 kilowatts. The uses of Nd:YAG lasers include cutting, drilling, welding, scribing, and engraving. Materials processed by Nd:YAG lasers include carbon resin, ceramics, most metals, and most plastics. YAG lasers often are used in industrial welding applications and are power-scaled by placing rods in series. Unlike CO2 systems, YAG lasers can deliver power via a fiber delivery system. While the output fiber (and beam diameter) is larger than with a fiber laser, they still offer more flexible beam delivery than CO2 lasers. Indeed, fiber delivery at high power levels has permitted the Nd:YAG into areas where there is limited overlap with CO2. Integration is also easier than with CO2 lasers because the fiber can be used with robots. Consequently, primary advantages to the YAG are lower wavelength and the ability to use robotics. In addition, YAGs have a low plasma plume and, therefore, require no shield gas. Disadvantages to this type of laser include poor wall plug efficiency of approximately 3 percent and a large footprint (relative to diode and fiber laser systems). This larger footprint is due to its large cooling system. YAGs also have adequate beam quality for welding but not for cutting (since the achievable energy density is lower). One reason for the decrease in cut quality is because the beam quality for the Nd:YAG decreases when coupled into fiber.

4 Diode Lasers Diode laser systems provide power via semiconductors connected to a fiber- or mirror-based delivery system. When these laser diodes are grouped or stacked in one- or two-dimensional arrays, the total power output can extend into the kilowatt range. The laser diode material is cleaved along natural facets, creating mirrored surfaces. Thus, the diode itself acts as the resonator cavity. Electric current provides the pump source, and wavelength is dependent on the semiconductor material from which the diode is made. With typically linear or rectangular beam shapes, industrial diode laser stacks are not suitable for drilling or cutting, where a narrowly focused beam is required. However, they are suitable for applications including continuous seam welding, brazing, cladding, heat treating, and soldering. Materials that can be processed by industrial diode lasers include a wide range of metals and plastics. Also, with shorter wavelengths than typical Nd:YAG (1.06 µm) or CO2 (10.6 µm) lasers, the absorption rate in aluminum and other metals is much higher and does not require the metal to be precoated, as is often the case with CO2 lasers. In addition to industrial uses, laser diodes are also used as the pump source in solid-state (crystal), disk, and fiber lasers. Diode lasers have a much higher efficiency than other types of lasers but also lower energy density (even with stacks and fiber coupling). Premature diode failure and, consequently, reliability concerns still plague diode lasers (and the systems that employ them to pump other materials). Higher costs result as diode stacks must be either replaced when they fail or duplicated in system designs for built-in redundancy. Maximum commercially-available power levels for the diode laser exceed 5kilowatts.

5 Fiber Lasers Fiber lasers have made huge advances in the last 2 years. While they have not gained widespread acceptance as an industrial tool, they show promise for some new applications. Their current primary use is in low-power applications. With a smaller footprint than other laser types, they are also very modular. Because the laser cavity is a conventional multimode fiber, smalldiameter fiber delivery is inherent to the system, and no air-to-fiber coupling losses result. And, at an emission wavelength of 1.03 µm, there is relatively low loss in the fiber. The fiber is typically made of fused silica, doped with Ytterbium (Yb), pumped by diode laser stacks, and capped by fiber Bragg gratings. With multiple layers of (pump) diode laser stacks, the laser is scalable beyond 10 kilowatts. The primary disadvantage to the fiber laser is the high cost of the many diode stacks (since single emitters currently can achieve only about 4 watts of power) and related reliability issues. The key to overcoming uncertain diode lifespan is redundancy, which will mask diode stack failure, although this further increases the cost and complexity of those laser systems. Fiber lasers offer the preferred wavelength range for metal processing due to high material absorption. The 300-µm fiber also provides a very straight-sided beam profile, which is good for welds and extended depth of focus. A primary benefit to fiber lasers is their delivery via thin fiber, which can be manipulated using inexpensive robots. Some anticipate applications where fiber lasers could eventually replace CO2 laser systems. For example, the shipbuilding industry currently uses 4- to 8-kilowatt CO2 laser systems. But, in about 5 years, we could see fiber lasers used in these processes, according to Stefan Heinemann (Fraunhofer USA [Center for Laser Technology]), due to the ability to manipulate the fiber output within confined areas.

6 Disk Lasers In the thin-disk laser system, the laser active medium is a very thin disk with less than 200 µm thickness. The Yb:YAG crystal is stimulated at the frontside via a diode laser in a quasi-end-pumped design. The backside is cooled over the whole area. Due to the small thickness, only a part of the pump beam is absorbed. In an optical system consisting of one parabolic mirror and one retro-reflective mirror (refer to diagram), the not-absorbed power on each disk will be imaged multiple times from each diode laser system to optimize efficiency. Typically, up to 32 passes of each pump beam are realized. A single disk can produce up to 3.5 kilowatts of power. Through innovative designs, such as the one diagrammed above, multiple disks can be cascaded to achieve higher power levels. Because the disk can be cooled over its entire backside, thermal lensing is kept to a minimum. Heat flow and temperature gradients predominantly occur on-axis, resulting in an almost homogeneous distribution of temperature and minimal distortions of the wavefront. Due to these advantages, it is possible to achieve excellent beam quality even at high laser powers. Depolarization caused by the active medium is also very low. Furthermore, simple power scalability can be achieved through variation of the pumped diameter and pump power. Modular design allows the changing of different laser parameters, such as operational mode, power level, and beam quality. The laser module consists of a thin disk module and other components like a resonator or housing. Cooling the thin disk module is done with either a cooling unit or house water supply. The completely assembled module contains a coated and qualified thin disk, bonded on a heat sink and integrated with the optical system necessary for multipass imaging of pump radiation. The crystal can be assembled on a goniometer mount, enabling the use of the thin disk as a mirror in a resonator. Disk lasers offer both higher efficiency and better beam quality than Nd:YAG lasers. The smaller beam, and resultant higher energy density, reduces kerf loss and allows faster cutting on thin steel. (Because thick steel cuts require wider kerfs for improved gas flow dynamics, however, disk lasers are less effective and slower on such samples.) In addition, the smaller-diameter fiber delivery system allows for manipulation of the laser output with inexpensive robots. Disk lasers are more production-ready [than fiber lasers]... [we] will implement [them] in 2005, according to Dr. Klaus Loeffler (Volkswagen). They are considered by many to be the successor to Nd:YAG lasers in most welding applications. Disk lasers with long working distance and high beam quality have excellent long-term potential in high-power applications.

7 Summary Replacement of existing laser systems, such as CO2 and Nd:YAG, likely will occur slowly. As a cornerstone for new processing techniques, the new systems (disk and fiber lasers) are presently not practical, due to cost, complexity, and/or reliability. They do, however, offer new approaches and new capabilities, meet new demands (such as flexibility), and permit new designs. Fiber lasers offer compact design, high efficiency, adjustable beam quality, power scalability, flexible beam delivery, no chiller requirements, and eye safety (no interlocks, enclosures, nor curtains). The fiber cavity needs no alignment, turning, nor polarization and shaping optics. Additionally, disk lasers offer high beam quality at near-infrared wavelengths, low cost, small footprint, power scalability, high efficiency, and fiber delivery. Opinions differ widely and often seem to speak more to vested interests than emanate from hard facts. Nevertheless, when talking to industry experts, certain common thoughts emerge. Most feel that fiber and disk lasers will gradually begin to gain share at the expense of Nd:YAG laser systems in new application areas. Some, like Dr. Klaus of Volkswagen, think that disk lasers (and eventually fiber lasers) will replace Nd:YAG in traditional welding applications, since both offer fiber delivery with high coupling efficiency and good beam quality. Many experts see diode lasers being used predominantly in niche applications or as pump systems for disk and fiber lasers. Meanwhile, CO2 lasers, with high power potential, low cost, good beam quality, sound reputation, and a large installed base, seem poised to remain the king in cutting for many years to come. Nd:YAG lasers, for the time being, will continue to lead the market in welding and marking system installations. According to Peter Leibinger, a Vice President at TRUMPF, the CO2 laser still has a lot [of use] in front of it. However, he sees the disk laser as the [ultimate] high-power solution for powers of 1 kilowatt and above. He projects that the fiber laser will, when laser diodes reach adequate levels of reliability and affordability, become the leading light source for precision low-power (500 watts and below) applications. For a summary of performance parameters for each type of laser system described in this report, refer to the attached chart.

8 References Denney, Paul, Overview of Disk, Fiber and Direct Diode Lasers, The 12th Annual Automotive Laser Application Workshop, March 9-10, Haimerl, Walter, Keynote Presentation, The 12th Annual Automotive Laser Application Workshop, March 9-10, Heinemann, Stefan, Flexible Process for Manufacturing of Self-Fixturing Assemblies A Case Study from the Shipbuilding Industry, The 12th Annual Automotive Laser Application Workshop, March 9-10, Leibinger, Peter, Comments Made to Manfred Berger, Herman Reedy, and Robert Ireland During a Visit to TRUMPF s Headquarters, Ditzengen, April 30, Loeffler, Klaus, Keynote Presentation, The 12th Annual Automotive Laser Application Workshop, March 9-10, McKinney, Michael R., All About Industrial Lasers, FAB Canada, July Morris, Tim, Disk Laser Enables Unique Application Advancement, The 12th Annual Automotive Laser Application Workshop, March 9-10, Penn, Wayne M., The Fiber Laser: Potential and Reality and Applications, The 12th Annual Automotive Laser Application Workshop, March 9-10, Shiner, Bill, Fiber Laser Automotive Applications, The 12th Annual Automotive Laser Application Workshop, March 9-10, Zediker, Mark S., Direct Diode Laser Applications, The 12th Annual Automotive Laser Application Workshop, March 9-10, 2004.

9 Laser Comparison Chart CO2 Nd:YAG Diode Disk Fiber Absorption in (%) Iron 8 >30 >30 >30 >30 Steel 12 >30 >30 >30 >30 Beam Quality Medium-High Low Very Low High High Cost/Watt Low Medium Low High High Beam Delivery Mirror Mirror or fiber Mirror or fiber Fiber delivery; Fiber; smaller fiber smaller fiber Efficiency Wall Plug 5 to 15% 3% (flash lamp) 30% < 20% <20% 10 to 15% (diode) Electro-Opt. 20 to 25% ~20% (flash lamp) - >60% >60% 40% (diode) Footprint Medium Medium Very Small Medium Small Maintenance Optics replacement Optics, flash lamp, or diode replacement Optics or diode replacement Optics or diode replacement Consumables Gas Typical Industrial Available Power 5-kW cut 12-kW weld 2-kW cut 4.5-kW weld NA cut 6-kW weld heat treat 2-kW cut 4-kW weld Fiber or diode replacement New cut New weld Energy Density High Medium Low High High Typical High-speed cutting, High-speed welding Heat treatment, Precision cutting, Precision cutting, Applications welding welding welding welding Summary Many installations, good beam quality, will remain the king for cutting Many industrial installations for welding, lower beam quality Power densities have limited their applications; longer diode life necessary; niche applications Technology Proven and accepted Proven and accepted Limited industrial applications First systems are hitting market now; limited industrial experience; diode life and cost will be factor; high beam quality may open new application areas Very limited industrial experience Wavelength (λ) 10.6 µm 1.06 µm µm 1.03 µm 1.03 µm Promising technology that lacks industrial experience; lower diode cost will improve cost justification; potential in existing laser processing areas Very limited industrial experience