Illuminating high power lasers

Modern Machine Shop,  March, 1996  by Chris Koepfer

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The industrial laser is no longer a "gee-whiz" technological wonder. It has become firmly established in metalcutting as the tool of choice for many applications. So established, in fact, that many fabrication shops are bringing back work they once sent out to laser specialty houses for processing.

Many familiar industry themes are driving this trend in shops. They include shorter lead times, quality control and demanding customer delivery schedules. But not insignificant in this trend is an increasing understanding, call it demystification, of the laser as a manufacturing tool. Consequently, shops are making the capital and personnel investments necessary to take control of their laser cutting applications.

For new laser users and those more experienced, continuous development of the laser itself has brought to market machines with increasing capabilities. At the same time, reductions in operating costs, made possible by technological developments, make a laser of 3 kilowatts (kW) comparable in operating costs to a laser of 2 kW. How is that possible?

In laser technology, it's not an apples-to-apples comparison. Moving above a laser power output of 2kW is technologically a bigger leap than many shops may realize. This article focuses on the technological issues that come into play with higher powered lasers. To find out about these lasers, we talked to Brian Jarvis, national product manager for LVD Corporation (Taylors, South Carolina), which markets high powered laser machine tools worldwide.

In The Beginning ...

The first laser (Light Amplification by Stimulated Emission of Radiation) was developed in the late 1950s. It consisted of the same components and operated on the same basic principles as lasers made now.

Fundamentally, a laser consists of a cavity (glass tube) with mirrors at either end. One of the mirrors is 100 percent reflective, and the other is less than 100 percent reflective (usually 70 percent). Gas to be lased is pumped into the cavity and circulated by a turbine.

For our discussion, this gas is a mixture of helium, carbon dioxide and nitrogen gases - commonly called a C[O.sub.2] laser. There are many other lasing materials available but, for higher powered, industrial lasers, the above C[O.sub.2] mixture is the most common.

Without getting too deep into the physics of what's happening in a laser, lasers work because the atoms in the gas mixture are excited by an electrical power source or generator. When that happens, a photon of light is given off by stimulated atoms in the lasing gas mixture. These photons excite other atoms that then give up a photon, and very shortly, a chain reaction is formed.

The photons produced in the tube oscillate back and forth between the two mirrors until a portion of the power is allowed to pass through the partially reflective mirror. The beam that escapes from the partially reflective mirror becomes the cutting medium.

The nature of the photons (light) given off in a laser makes it a practical tool for manufacturers. Laser produced light is monochromatic - meaning it has the same frequency - and coherent. This means the frequency is in phase. Focusing the beam gives the laser its power.

Beyond 2 kW

In industrial metalcutting applications using C[O.sub.2] lasers, two basic types come into play. These are C[O.sub.2] lasers excited by direct current (DC) and C[O.sub.2] lasers excited by radio frequency (RF). For industrial lasers up to approximately 2.2 kW, a DC excited laser is most commonly used. It's generally less expensive to manufacture and generally consumes less power than an RF excited laser.

However, once a laser moves into power outputs above about 2.2 kW, an RF excited device is recommended and has become more commonplace in the industry, says Mr. Jarvis. The primary difference between the two types is the location of the electrodes that excite the C[O.sub.2] gas mixture. In a DC laser, the anode and cathode are located inside the glass plasma tube. An RF laser excites the gas with electrodes mounted externally to the plasma tube.

At the upper end of its power curve (approaching 2.2 kW), a DC excited laser gas mixture becomes more unstable. Pumping that much current directly through the cavity causes gas to begin to dissociate (unmix), which degrades the beam being output by the laser. This results in poor cutting performance by the laser machine tool.

It's this small difference in excitation method that allows the RF C[O.sub.2] laser to produce usable and stable beams up to 10 kW.

Weighing The Costs

Traditionally, a DC laser had several things going for it in the marketplace compared to RF excited designs. Typically, DC lasers use less electrical power to excite the lasing gas. On the down side, it consumes more gas. With the electrode inside the tube, heat and dissociation require replenishing the gas more often. A fair approximation of run cost for a 2 kW DC excited laser is about $8 to $12 an hour, says Mr. Jarvis.

Until recently, an RF laser generated power with vacuum tubes. It cost more to run than a DC design - even in the same output range - because it used significantly more electricity. What's changed is the use of solid circuitry to produce the RF excitation in place of vacuum tube technology.