Tiny and cost-effective laser diodes have been around for 20 years. If we are successful in achieving the beam quality necessary for key industrial applications using diode lasers, everything will change.
To explain the significance of this technology, let’s take a look at a salt shaker. It is shaped like a cylinder, is about 30 millimeters in diameter and about 70 millimeters high. It has a volume of about 50 cubic centimeters. Now let’s shake the salt out and fill the shaker with high performance laser diodes. Sitting on the table encased in this salt shaker is about one megawatt of laser power that we can simply stick in our pockets and take with us.
Theoretically, of course. We would also need to include in the shaker the power supply, cooling system, beam guidance and all the other unavoidable trappings. One megawatt in a salt shaker. No wonder the laser diode has kept researchers and developers in the field of materials processing busy since its inception. This is because this tiny laser holds a mighty potential: miniaturization.
Each diode is an independent laser source. If the diodes are successful in eliciting enough power to weld and cut sheet metal, a new beam source would emerge for these key industrial applications. Even with all those invisible components, this would be one source that would be so uncomplicated, compact, efficient and inexpensive that it would call into question many of the prevalent conventional methods and even existing laser technologies.
The charm of the diode
Laser diodes are the most compact design for a laser beam that there is. They generate laser light from within themselves and directly from the electrical power. With modern chip production, they can be manufactured inexpensively in large quantities. They are solid-state and therefore robust.
They have an immense service life that is continuing to lengthen. Laser diodes are tremendously efficient because they convert the power used directly into laser light. Fifty percent energy use is the rule, but over 80 percent is possible. A lamp-pumped rod laser converts about 5 percent of the power from the socket into performance on the workpiece. The very efficient disk and fiber lasers create 30 percent power, mainly due to the diodes that pump these laser types.
Two disadvantages come up directly against these benefits which are, unfortunately, critical for industrial materials processing. One is that the output of the diodes cannot be increased arbitrarily. Like all semiconductor components, they heat up. All at once, along with the emissions output, the charge from the resonator mirror dissipates directly onto the tiny diode surfaces.
Though they are the only power emitters, the high performance laser diodes nevertheless achieve up to 20 watts of output. If the diodes are arranged in bars and stacked together, they can be used to construct high performance beam sources; these function primarily as the pump sources.
In niche applications, as direct diode lasers with several hundred watts of power, they can also weld plastics or remelt metal surfaces — with a broad focus mark and a high output — as well as solder sheet metal.
But for deep welding or even cutting metal sheets, diodes have not worked very well so far. Their beam quality is not good enough. As a rule, deep welding requires about 30 millimeter *millirad or more.
For precise cutting, single digit beam parameter products also are a requirement. The beam of a high performance laser diode, however, has two different beam parameter products, depending on the viewing direction, of which one amounts to several hundred millimeters *millirad.
The crux of beam quality
The “difficult” beam quality of the diodes is directly rooted in its wonderful, miniscule size. The view through the magnifying glass shows a narrow strip of gallium arsenide (GaAs) that has the approximate proportions of a package of chewing gum.
Like that package, it consists of several layers. On the border between two layers — between the silver gum wrappers, so to speak — positive and negative charge carriers meet and release the photons.
These “flow” along the layer borders through the GaAs strips, are partially reflected and exit on the face as a laser light. The efficiency with which the power is converted into light greatly depends on the thickness of these layers.
For high performance diodes that are required for direct diode lasers, the entire active range of the diode is only a maximum of one micrometer high; where it stretches along the entire range horizontally the diode is from 50 to 500 micrometers high — depending on design.
This allows the beam to develop an elliptical cross section: It has two different diameters and two different qualities as a result. In the vertical short “fast axis,” though the beam fans out with an opening angle of about 45 degrees, the very low height of the active zone has the effect of a pinhole camera.
At the same time, this very low height also means a very small beam diameter and a resulting beam parameter product of only 0.3 millimeters*millirad. The horizontal “slow axis,” on the other hand, barely fans out with about six degrees. In this case, the beam diameter corresponds to the width of the active zone. Despite the low angle opening, this results in a very unfavorable beam parameter product of often several hundred millimeters *millirad.
Macaroni and flashlights
The arrangement of individual emitters to bars and, if applicable, stacks adds an additional challenge to the equation. On the one hand, there is a multitude of individual beams of unsymmetrical quality that have to be shaped into a single symmetrical beam. One way is to use the energy of the diode beams to generate a new higher quality beam.
This is the concept of the diode-pumped solid state laser. However, because the process costs energy, it reduces the efficiency. Another way is to have direct diode lasers form the light of the diodes directly on the fiber optics. However, those who want to transport the light of a bar or even a stack using fiber optics are confronted with the same challenge as someone who directs a square flashlight beam on macaroni.
The macaroni may be illuminated, but some of the light around the macaroni is lost. On the other hand, the end face of a bar has a lot of “dead” area meaning non-radiating space between the emitters. If these dark spots are also shown on the fiber optics, this reduces the efficiency and brilliance.
The higher the output quality of the beam and the less the emitters are reproduced on a fiber, the more the “macaroni” fiber optic absorbs the light in relation to their diameter and the smaller these turn out. But along with the number of displayed emitters, the output of the beam drops, too.
Only those who can make do with low output can achieve high beam quality. With high output, though, users also must accept a thick fiber, reduced efficiency and put up with mediocre to bad beam quality.
However, there is no longer any question that the large key market of welding and cutting applications in sheet metal processing is on the verge of a breakthrough. First of all, diode direct lasers are penetrating into higher and higher output classes and are increasingly becoming a point of interest for sheet metal processing. That is how in June 2008, 10 kilowatts of power output was produced from a 1.5 millimeter fiber with a beam quality of about 165 millimeters*millirad for the deposition welding of a laser in the Fraunhofer IWS.
The deep welding of sheets requires a higher brilliance by a factor of 25. In early 2009, TRUMPF demonstrated a diode laser module with only five millimeters*millirad beam quality and 100 W output. However, this is only one hundredth of the output of the Fraunhof laser. Yet the module can be grouped with additional larger units.
This makes it the foundation for future multiwatt diode lasers whose beam qualities make it possible to deep weld sheet metal and even, to a limited degree, cut thin metal sheets.
The first victim of this breakthrough is already certain: the lamp-pumped solid state laser. Diode technology may replace it in a few years, and as diodes develop further they may even encroach upon the market shares in the fiber and disk laser segment. The diode laser has the potential to evolve into the dominating laser technology among solid state lasers in the long term.
Long before we reach that point, the charm of diodes will shuffle the cards mainly among the competition with conventional systems for materials processing. All the benefits of the laser as a tool combined in a compact, robust and highly efficient system. And this is already available for simple applications at incredibly competitive investment costs.
TRUMPF Laser GmbH + Co. KG
This article was first published in spring 2009.