The Miracles of Laser Technology

A femtosecond = one millionth of a nanosecond or 10 -15 of a second and is a measurement sometimes used in laser technology.


CHIRPED-pulse amplification strikes again.

Using it in a high-peak-power mode, Laboratory scientists produced first the 100-terawatt laser and then the petawatt laser, opening up new opportunities for applying laser-matter interactions. Now a Livermore team has won an R&D 100 Award for applying chirped-pulse amplification in a high-average-power mode for cutting and machining materials. The system was developed for disassembling nuclear weapons components, but it has many other uses as well.

The team, led by Brent Stuart, illustrates Livermore's collaborative nature by combining research and development expertise from Laser Programs and Defense and Nuclear Technologies Directorates.

From Demilitarization to Dentistry
----------------------------------
By ionizing the material being cut--removing it atom by atom--the cutting technique allows precise machining of everything from steel to tooth enamel to very soft materials like heart tissue. Each pulse of this machining system is extremely short, lasting just 50 to 1,000 femtoseconds (or quadrillionths of a second). These ultrashort pulses are too brief to transfer heat or shock to the material being cut, which means that cutting, drilling, and machining occur with virtually no damage to surrounding material. Furthermore, this revolutionary laser can cut with extreme precision, making hairline cuts in thick materials along a computer-generated path.

In dentistry applications, the thermal nature of the conventional laser ablation process can heat and crack a tooth and produce a random-shaped hole within a large area of collateral damage. In contrast, at the same ablation rate, Livermore's new laser precisely removes the material and leaves the surrounding areas in their original state (see a and b of the figure, below).

The ultrashort-pulse laser represents a major advancement in cutting technology. Conventional lasers, diamond saws, and water jets are used commercially for a variety of cutting and machining applications. But each one has drawbacks. None of them can achieve the precision of the femtosecond laser machine tool (0.1 millimeters), and most of them damage surrounding material to varying degrees. Because of these shortcomings, no commercial cutting system can be used on the range of materials or applications of Livermore's new tool.

Industrial lasers, which melt and boil material to remove it, are often used in precision cutting. The heat and shock cause considerable damage to the area surrounding the cut that can range from changes in the grain structure to cracking. The damage may extend from a few micrometers to several millimeters from the cut, depending on the properties of the material, the laser pulse duration, and whether a cooling method is used. Very tiny structures only a few tens of micrometers in size, such as biological tissue or semiconductor devices, are extremely fragile. Even the slightest thermal stress or shock creates intolerable collateral damage.

These conventional cutting methods also leave slag around the cut. When material is vaporized, some of it is deposited on the walls or upper surface of the cut. This residue reduces the quality of the cut and the efficiency of the cutting system.

With each short pulse of the Laboratory's new laser cutter, material is heated to temperatures far beyond the boiling point, producing an ionized plasma, while leaving surrounding material cool. The pulse deposits its energy so quickly that it does not interact at all with the plume of vaporized material, which would distort and bend the incoming beam and produce a rough-edged cut. The plasma plume leaves the surface very rapidly, ensuring that it is well beyond the cut edges before the arrival of the next laser pulse. And because only a very thin layer of material is removed during each pulse of the laser, the cut surface is very smooth and does not require subsequent cleanup (see c and d of the figure, below).

Removal of minimal amounts of material makes this new cutting system useful for processing extremely valuable or hazardous materials. If the cutting is done in a vacuum, better than 95% of the removed material can be recovered.
Another Livermore team is building a high-powered femtosecond machining system for the Department of Energy's Y-12 Plant at Oak Ridge, Tennessee, one of this country's primary manufacturers of nuclear weapon components. A second unit at Livermore will be used as engineering support to the Y-12 unit. The high precision of this cutter will maximize the plant's ability to reuse high-value components and minimize the amount of waste generated during the cutting process.

Livermore is studying the use of the Femtosecond Laser to machine high explosives for experiments at its High Explosives Applications Facility. Because so little energy or mechanical shock is transferred to the part being machined, the team has demonstrated that materials such as high explosives or parts containing high explosives can be cut without danger of detonation. The team is also working on the design of a system for demilitarizing chemical weapons.

Other potential applications abound. Using the laser as a surgical tool for soft tissue has already been discussed in Science & Technology Review (October 1995). A semiconductor device producer is exploring the use of the unit for cutting high-value semiconductor wafers. Other major U.S. manufacturers are looking into incorporating femtosecond machining systems into their production lines. In manufacturing, new materials are constantly appearing, and the features on all kinds of devices are becoming smaller and smaller. The femtosecond machining system may be the most effective way to respond to both challenges with its high precision on all materials regardless of composition.

--Katie Walter

[link to www.llnl.gov (secure)]