Disk Lasers Facilitate Cutting and Remote Welding

Tim Morris

One of the newest laser technologies of note is the Yb:YAG disk laser, with commercial systems that can deliver 1 kW of power through a 150-µm-diameter fiber. End users can expect to see 4-kW systems, such as the one demonstrated by Trumpf Laser GmbH at Laser 2003 in Munich, by next year. The technology can offer higher beam quality and better electrical efficiency than lamp-pumped rod Nd:YAG lasers. The higher beam quality allows higher energy densities, which in many applications translate directly into faster processing speeds and/or higher precision for welding and cutting applications, albeit at a higher initial system cost. As an example, cutting 1-mm mild steel with the 1-kW disk laser was accomplished at nearly twice the speed achieved by a conventional 1-kW Nd:YAG.

To understand why, it helps to first examine the cooling efficiency of both disk and rod lasers. Conventional Nd:YAG laser technology generates lasing action by side-pumping each rod in a series using lamps or diode lasers. A typical 300-W laser would require only one rod, but a 4-kW device would use eight rods in a series. A typical setup has either two lamps or five diode stacks pumping each rod. Here, a move to diode pumping promotes increased electrical and laser efficiency for obvious reasons -- the diode emits at a specific lasing wavelength, whereas a lamp emits incoherent light at multiple wavelengths, only a few of which contribute to the laser's population inversion. The rest simply generate excess heat.

With disk lasers, diodes surface-pump a disk-shaped crystal that is roughly the size of a nickel. When coupling two or more disks, output power can be expanded without a significant loss of beam quality. The 4-kW system, for instance, pumps four disks using four diode modules (one per disk). Such systems can be pumped a little harder without reducing beam quality because of increased cooling efficiency related to diode use and the high ratio of surface area to laser active volume, which allows more heat to dissipate. The thermal gradient is parallel to the direction of light propagation, so there is virtually no thermal lensing.

A 4-kW prototype disk laser system pumps four disks using one diode module per disk. Beam quality is 7 mmµmrad, even in remote processing applications.

In general, wall plug efficiency (electricity usage vs. laser power output) is around 3 to 4 percent for a lamp-pumped Nd:YAG system. It increases to somewhere in the midteens with diode-pumped Nd:YAG systems and Yb:YAG disk lasers. This may be beneficial in places where electricity is relatively expensive, but it will not provide the major justification for use of disk lasers in the US.

In most cases, improved beam quality will be the major selling point. Trumpf's 4-kW system, for example, can deliver beam quality in the range of 7 mm·mrad through a 200-µm fiber. A traditional lamp-pumped Nd:YAG would require a 600-µm fiber to deliver 4 kW to the workpiece, with beam quality of 25 mm·mrad.

One of the most interesting advantages of the new technology is that the beam quality translates to the ability to use longer-focal-length optics, particularly scanning optics, which can provide precision beam motion at extremely high speeds, especially in remote welding. Until now, however, the limiting factor had been the relatively small operating field size available with Nd:YAG systems, related to the available beam quality. The small-diameter, high-quality beam that is generated with disk laser technology translates to significant improvements in the size of the operating field. For example, the 4-kW prototype laser used in combination with a scanner optic produces a 600-µm spot size with an operating field of 200 X 300 mm. By comparison, a typical diode-pumped Nd:YAG system would have approximately 60 percent less of a useful operating field, using the same optical arrangement.

Higher energy density (energy per unit area) focused on the workpiece essentially translates to faster processing. If higher precision isn't the goal, a switch to a disk laser will allow the user to do the same process faster. When precision is more critical than higher speed, the better beam quality translates to the ability to cut very fine detail, including sharper inside corners (a smaller spot size translates to less rounding with corner cuts). In sheet metal cutting, the small focus diameter translates to higher cutting speeds and shorter cycle times.

On the welding front, disk technology has several benefits:

• It allows precision welding of smaller welds with less heat input.
• In aluminum welding, the system reaches threshold intensity with lower power and a smaller focus spot size.
• Using scanner optics with larger operating fields of view, end users can weld even complex parts without moving them mechanically.
• The use of more compact welding optics can be beneficial when welding must take place in confined or difficult-to-reach spaces.