High-Power Laser Applications

CHART* R HIGH-POWER LASER APPLICATIONS ABOUT THIS CHAPTER Chapter 11 described major uses of low-power lasers, loosely defined as those that do not change objects not intended to be light- sensitive. This chapter covers major applications of high-power lasers, which cause significant changes to the materials they illu- minate. The applications practical today include cutting, welding, drilling, and marking in industry, some specialized tasks in the manufacture of electronic components, and a variety of medical treatments. Several other applications for high-power lasers remain in various stages of development, including causing and controlling chemical reactions, laser weapons, and thermonuclear fusion. 12.1 HIGH- VERSUS LOW-POWER LASER APPLICATIONS The line between high- and low-power laser applications is based on how the laser beam affects materials. If a laser cuts or welds a material, it clearly is making a major change, whether the material is sheet metal in a factory or living tissue during surgery. Marking serial numbers or product codes on equipment is not quite as clear, but is considered materials-working if the laser beam etches away surface material to mark the object. On the other hand, it is reasonable to consider laser applications low-power if they change properties of light-sensitive materials, such as laser printing and optical data storage. Part of our rationale is that ordinary light can make similar changes, like the Understanding Lasers: An Entry-Level Guide, Third Edition. By Jeff Hecht 377 Copyright © 2008 the Institute of Electrical and Electronics Engineers, Inc. 378 HIGH-POWER LASER APPLICATIONS exposure of photographic film. Part is that some of these applications are closely related to other low-power applications; for ex- ample, optical data storage by writing on disks is one of a family of optical disk applications that logically belong together. A final ele- ment of the rationale is the need to make some separation lest these two chapters merge into a single monstrous and unmanage- able whole. The last chapter showed the diversity of low-power laser applications. Less diversity is evident in the use of high-power lasers. Many applications are variations on the basic theme of laser materials working—heating an object to temperatures suffi- cient to vaporize and remove its exposed surface. From this stand- point, many medical applications are merely specialized materials working (a technically accurate view, if a bit callous to the pa- tient), and laser weapons perform materials working on unfriend- ly objects. Many interesting high-power lasers remain in development. Although some ideas are relatively new, some high-profile ideas have been in development for decades, notably high-energy laser weapons and laser-induced nuclear fusion. Research and development on those two long-running efforts have paid important divi- dends for other high-power laser applications by increasing power levels and steadily improving technology. 12.2 ATTRACTIONS OF HIGH-POWER LASERS In Chapter 11, we listed attractions of low-power lasers. Let us take another look at the attractions of lasers, this time concentrating on those important for high-power applications: • Lasers produce well-controlled light that can be focused pre- cisely onto a small spot. • Lasers can generate light in short pulses, concentrating energy to produce very high peak power levels for short intervals, af- fecting materials more strongly than steady power. • Power densities can be tremendous if high-power pulses are focused onto small areas. • Laser beams do not push the objects they strike, allowing noncontact processing. • Lasers can be controlled directly by robotic systems. 12.3 MATERIALS WORKING 379 • High powers are available at various wavelengths, so laser types can be picked to match the strongest absorption by various materials. • Laser light travels a well-defined straight line. This list is not identical with the one in Chapter 11, but some points are quite similar. On the other hand, the list of disadvan- tages of high-power lasers is rather different: • High-power lasers can be very expensive. • Metals strongly reflect light at many wavelengths. • Powerful lasers are not available at most wavelengths, so they cannot be matched to the strongest absorption bands of all materials. • Laser light cannot cut deeply into materials because ablated material gets in the way. • Interaction with the air limit transmission of high-power beams over long distances. • Laser light lacks the momentum of a physical projectile like a bullet. These advantages and limitations shape the high-power laser applications covered in the rest of this chapter. 12.3 MATERIALS WORKING Materials working involves cutting, welding, drilling, and other- wise modifying industrial materials, including both metals and nonmetals. The 1960s engineers who played at measuring laser power in gillettes were, at least in some primitive sense, exploring one aspect of materials working (drilling holes in razor blades). Materials working has become the largest single market for lasers (in total dollars), exceeding $1.68 billion in 2006 according to Laser Focus World. To understand how lasers work on materials, we will start with two of the earliest applications, which used lasers to drill very different materials, then move on to other applications. 12.3.1 Drilling Diamond Dies One way to make thin metal wires is by pulling the metal through tiny holes in diamond "dies." Diamond is an excellent die materi- 380 HIGH-POWER LASER APPLICATIONS al because it is the hardest substance known. It also conducts heat well, so it can dissipate the frictional heat generated by pulling metal through the hole. The problem is that diamond is so hard that conventional drills will not penetrate it. Only diamond bits can drill into diamond, and those diamond bits get dull as they drill. Laser light, however, does not get dull. If a laser beam is focused onto a tiny spot in the diamond, it heats carbon atoms so they evaporate or combine with oxygen in the air. Deposit enough laser energy, and you can make a hole through the diamond. Engineers discovered that lasers could drill holes in diamond dies during the 1960s, and ever since this has been a standard ex- ample of laser applications. It is not a high-volume application, but it is one where there are no attractive alternatives. 12.3.2 Drilling Baby-Bottle Nipples The rubber used to make baby-bottle nipples is on the other end of the hardness scale from diamond, but that doesn't make it easy to drill holes in it. The holes in a nipple must be small for milk to flow properly, so tiny wire pins used to be pushed through the molded nipples. However, that's like trying to punch holes in Jell- O with a thin soda straw. The flexible rubber can catch, bend, and break the fine pins. A laser beam is an excellent alternative because nothing solid contacts the nipple to push it out of shape. The laser pulse simply burns a tiny hole through the rubber, without touching anything to the nipple. Like drilling holes in diamond dies, this laser application has been around for many years because it is the best solu- tion to a tricky problem. 12.3.3 Noncontact Processing As we have indicated, an important factor in drilling both diamond dies and baby-bottle nipples is that laser drilling is a noncontact process. There is no physical laser "bit" that can get dull while drilling diamond. Nor can the laser beam distort the rubber nipple or be caught while drilling the hole. The noncontact nature of laser processing is important for many types of laser drilling, cutting, and welding. Interestingly, many materials most often machined by lasers tend to fall into cat- 12.3 MATERIALS WORKING 381 egories similar to diamond and rubber—either very soft or very hard. Titanium, a light metal used in military aircraft, is too hard to cut easily with conventional saws, but can be cut readily with a carbon dioxide laser. Other advanced alloys and hard materials also are often cut with lasers. On the other extreme are some rub- bers, plastics, and other soft materials that usually are cut with lasers. 12.3.4 Wavelength Effects and Energy Deposition The fundamental interaction in laser materials working is the transfer of laser energy to the object, causing a change such as evaporation of surface material. The details are complex, but we can describe in general two key factors: the effect of laser wavelength and how energy deposition affects the material. Energy transfer to the object depends on absorption at the laser wavelength, which varies considerably among materials. Table 12-1 lists absorption of some common metals and a few other materials at some laser wavelengths. In general, metals tend to reflect more light at longer wavelengths, so they absorb a larger fraction of laser energy at shorter wavelengths, and most metals are easier to cut with shorter wavelengths. Titanium is an excep- tion to the pattern because is absorbs relatively high 8% of light from a carbon dioxide laser, and is almost impossible to cut with conventional saws. In contrast, skin and white paint absorb more Table 12-1. Surface absorption (percent) at important laser wavelengths Laser and wavelength Argon-ion, doubled Nd or Yb Ruby Nd or Yb co2 Material (near 500 nm) (694 nm) (1064 nm) (10.6 μι Aluminum 9% 11% 8% 1.9% Copper 56% 17% 10% 1.5% Human skin (dark) 88% 65% 60% 95% Human skin (light) 57% 35% 50% 95% Iron 68% 64% -35% 3.5% Nickel 40% 32% 26% 3% Seawater low low low 90% Titanium 48% 45% 42% 8% White paint 30% 20% 10% 90% 382 HIGH-POWER LASER APPLICATIONS light at longer wavelengths, so carbon dioxide lasers cut such materials better, along with plastics.

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