Which laser is the best

The primary focus should be on the application, not the laser technology

by Jochen Deile

A lively dispute exists as to which laser source will own the future of materials processing. In nearly every debate, one thing is clear: There is no single ideal beam source for all applications. There is no doubt that there will continue to be a need for a variety of beam source types in the future. Why? Because the industries and applications that use laser technology are extremely diverse.

FIGURE 1. CO₂ lasers are well known for cutting various types and thicknesses of sheet metal.

Whenever a new type of laser appears on the market, users and experts alike ask if the new technology will replace the old tried–and–true approaches. For example, some experts prophesied the end of the CO₂ laser when the first high–power solid–state lasers were introduced. And, for a long time, the first diode laser in the kilowatt range with its superior efficiency and compact size was considered to be the laser beam source of the future. In retrospect, we can see that not a single new beam source technology has completely replaced existing, proven technologies. Rather, new technologies have found their own niches and have generally expanded the potential of lasers in processing materials. Every technology has its advantages and it is the desired application that dictates which laser is the best laser.

Meeting application demands

A more differentiated consideration of the numerous laser applications for processing materials reveals that there is no such thing as one optimum laser that can perform well in all of them. However, there is an optimum laser for each type of application. Different laser technologies are based on different approaches. Each laser source has its strengths and weaknesses. Consequently, there is a basic principle in laser technology: Different applications put different requirements on the beam source and on the production system. As a result, there are applications that one technology can handle better than another. Take, for example, 2D laser cutting where the CO₂ laser is historically the dominant technology. For sheet metal fabricators, productivity is a must. Therefore, the cutting process has to be very robust and the machine flexible enough to cut all sheet metal efficiently—even short run production. For this reason, a 2D laser cutting system must deliver a broad processing spectrum. In addition, the laser must guarantee high process reliability for the user.

FIGURE 2. Fiber optic cables enable beam sharing in solid–state lasers.

The cutting behavior of the CO₂ laser meets the sheet metal fabricator's demands perfectly. With a 10.6 µm wavelength, the CO₂ laser offers a high degree of flexibility when cutting sheet metal of different types and thicknesses (see Figure 1). In addition, it cuts with high edge quality, which can be of particular importance for parts produced for the food service and appliance industries, for example.

While CO₂ lasers lead the pack in laser cutting of sheet metal, solid–state lasers also have a place in the world of industrial processing. While both cutting and welding applications exist for solid–state lasers, the trend is on the welding side. More and more manufacturers are viewing laser welding as a cost–effective aspect of the sheet metal processing chain, specifically for joining thin– to medium–thickness sheet metal, from 0.02 inch to 0.25 inch.

Sharing of power

Solid–state lasers are a good choice not only because of their processing ability, but also because of their "networking" capability (see Figure 2). This concept adds great flexibility and increases the utilization rate of the lasers. As many as six fiber optic cables can be connected to a single solid–state laser. This means that multiple lasers can be used at the same workstation to provide flexibility and redundancy. For example, during scheduled maintenance in an automotive welding assembly line the laser beam from another laser can be re–routed to the workstation and production can continue uninterrupted. The solid–state laser's ability to time or beam share can also reduce operation costs by increasing the rate of utilization. One laser beam can process for a few milliseconds at one station and, while the robot for this station is repositioning, the corresponding laser beam can be used somewhere else. A simple analogy that illustrates the time sharing/beam sharing concept is the popular time share condo. Each time share participant purchases a specified period of time to use a condo at a luxury resort and when that time is over a new guest arrives. Costs are shared by all time share participants making it an affordable vacation approach for many individuals rather than an expensive investment for one person.

Solid–state choices

There are so many varieties of solid–state lasers available on the market today that choosing the right laser for a specific application can be dizzying for anyone without a physics degree. They use different laser media and have varying levels of beam quality and laser power—each with its own advantages and disadvantages. Two popular examples of solid–state lasers are the fiber and the disk laser. Both technologies feature excellent beam quality.

FIGURE 3. A polycrystalline silicon wafer (used in solar cells) has thousands of holes (diameter typically in the range of 30 to 100 µm) and is processed with a picosecond laser.

An application example for the disk laser in the multi–kilowatt arena is Daimler's "RobScan" technology. RobScan (Robot–guided Scanner for laser beam welding) is a laser welding process developed by Daimler for vehicle body construction. The RobScan process combines the high speed and precision of scanner optics with the flexibility of a robot. The beam source used in the process is a disk laser, which is routed to the laser head by flexible fiber optic cables. RobScan reduces the positioning times between weld seams because the scanner mirror and robot movement are simultaneously superimposed, otherwise known as "welding on the fly." Process advantages include single sided component accessibility and high welding speeds up to 15 times faster than resistance spot welding. Disk lasers are being used to consistently produce more than 900,000 weld joints per day in Daimler's C–Class doors, rear center section, and inner side wall using RobScan technology.

For applications in the lower output range, typically lower than 1kW, fiber lasers can be advantageous. Low–power fiber lasers can easily generate beams with very high beam quality or beam parameter product (BPP). As power levels increase, the BPP of the fiber laser increases, which indicates a decrease in beam quality. This is why the fiber laser is particularly good in lower–power ranges for microwelding and cutting or in thin sheet metal applications where very fine, detailed contours are required such as in the medical industry.

Still another type of solid–state laser are pulsed lasers. A picosecond laser for example, has a relatively low average power output of just a few watts, but can provide a pulse peak power that can exceed its average power by millions of watts for a few picoseconds. These lasers are ideal for processing materials where thermal influence must be minimized such as in the ablation and trimming of solar cells and the cutting of silicon wafers used in the semiconductor industry (see Figure 3). Because of the low heat input associated with pulsed picosecond lasers, their applications are considered "cold" laser processes and are quite different than the CO₂ and solid–state laser processes described earlier.

Direct diode pumping and amplification (power scaling) without "Chirped Pulse Amplification" (CPA) are necessary requirements for the success of ultra–short pulse technology in the industrial market. To ensure a cost–effective application in industrial microprocessing, scaling the average output to values of 50 watts and greater is necessary.

What does the future hold?

Only the future will reveal how many fiber lasers, disk lasers, rod lasers, and CO₂ lasers will be used to process materials. And, there is always a new technology on the horizon that could impact the way we look at laser technology. Given the developments in diode technology, it appears that diode–pumped solid–state lasers and direct diode lasers will play an increasingly important role in processing materials with lasers in the near future. The diode is on track to become the central element for all lasers and just may make the current discussion about the "best" laser source appear irrelevant in a few years.

Jochen Deile is manager development, new laser products for TRUMPF Inc. (Farmington, CT; www.us.trumpf.com).