Modular Diode Lasers Stack Up as Quality Beam Tools

Dr. Friedrich Bachmann

Diode lasers with total power ranging from several hundred watts to more than a kilowatt began to play an important role in materials processing applications in the mid-1990s. The German government recognized that diode lasers could revolutionize that market and, in 1997, launched a national project called Modular Diode Laser Beam Tools.

This endeavor's goal was to improve high-power semiconductor laser elements. The project investigated mounting, cooling and beam-forming technologies and their integration into high-power diode laser systems for industrial applications, such as welding, brazing, surface treatment and cladding.¹Better high-power systems

The project, now in its fifth and final year, was divided into three focuses: chip technology, laser and systems technology, and processes and applications. In the first stage, researchers improved the chip technology. The optical properties of an integrated high-power diode laser system, which consists of several diode bars, are directly related to the beam quality of the individual systems. The maximum power and beam quality of wide-stripe emitter bars were improved by a factor of two to three, to more than 50 W (at a reasonable lifetime) and to an average beam parameter product of 15 mm mrad. Researchers developed high-power bars, with wavelengths other than the usual 808, 940 and 980 nm, for wavelength coupling in high-power devices. The result was a high-power diode laser with an output of 3 kW and a beam parameter product of 114 mm mrad (measured according to 1/e² criterion).

Now tapered lasers at three wavelengths are being qualified.² These lasers will be integrated into a high-power system to reach beam quality below 100 mm mrad at powers as high as 3 kW.

The concept

The use of modular diode lasers is an essential part of this project. Because it is not necessary to stack diode laser bars to increase power, the bars or even the stacks can be assembled in different shapes. This offers the opportunity to match the laser beam or the energy distribution to a desired geometry. An example of this, the simultaneous welding of polymers, is already close to being commercialized.³ Small laser modules, each with about 25 W and a line focus of about 0.1 x 10 mm², can be assembled to a long line, a rectangle or other shape (Figure 1). The entire seam can be welded in a very short time, which is an important advantage if many identical products have to be produced quickly.

Figure 1. Diode laser modules for simultaneous welding of polymers can be configured in a variety of shapes. All images courtesy of Fraunhofer Institute.

The small size of the diode laser bars allows them to be easily integrated into multifunctional systems. One example that resulted from this project is a system for processing electronic surface-mount devices. This unit picks the electronic device from a magazine, positions it onto the printed circuit board, adjusts the position with an integrated image processing system, and solders the device using diode laser radiation (Figure 2).

Figure 2. One example that resulted from this project is a pick-and-join system with four integrated diode lasers for soldering surface-mount devices.

Four perpendicular line-shaped laser beams can be individually adjusted in length and position. The length of each soldering line can be varied between 6 and 24 mm to match the outer dimensions of quad flat packs, small-outline packages and plastic leaded chip carriers. The integrated camera system that is used for positioning can also be applied to process control.

Focusing on applications

Diodes also can be assembled in the desired geometry, such as a line focus or a ring focus.

The basic application for the line focus system is the simultaneous cutting of thin metal sheets by a high-power-density laser line, similar to a mechanical line cutter. Four modules with six bars each were assembled in a line to provide a line focus 47.5 mm long and ~0.3 mm wide. However, despite the fact that each stack was equipped with a homogenizing glass rod, the homogeneity of the line as well as the steepness of the edges of the intensity distribution were not sufficient for homogeneous cutting in the first trial. This was demonstrated in experiments for cutting 0.2-mm-thick stainless steel.

Therefore, a homogenization unit that mixes the light of all four modules was developed and produced the desired result: an almost ideal top-hat profile with a variation of only ±5 percent at an average power of 780 W. The first cutting experiments have shown the functionality of the system and the feasibility of the process. The unit also has successfully been used for simultaneous welding of thin metal sheets on a line.

An annular focus is necessary for applications such as laser-assisted flame cutting, Researchers have developed a focus that has an inner diameter of about 4 mm and a width of 1 mm at a working distance of ~50 mm, and an opening ~30 mm in diameter in the center for a process gas nozzle or other device. Ten diode laser stacks with six bars each are positioned as a decagon. The radiation of each laser stack is collimated in both the fast and the slow directions. It is expanded in the slow direction by a cylindrical telescope and finally focused to a line of about 3.5 x 0.7 mm. By bending mirrors, which are adjustable for each element, the radiation is directed to form a ring focus. Total power is 2 kW (Figure 3).

Figure 3. A total power of 2 kW was reached with this diode laser system, shown with the ring focus cover removed.

The cylindrical head is only 200 mm in diameter and 270 mm high. With the energy distribution reached so far, oxygen-assisted cutting was demonstrated (Figure 4). Mild steel with a thickness of 10 mm was cut symmetrically at the rate of 0.75 m/min at 2 kW and an oxygen pressure of 15 bar. The kerf width, which is mainly determined by the diameter of the oxygen jet, measured to about 2 mm. This unit was originally intended for flame cutting exclusively, but it also has gained interest from the welding group for generating the annular weld seams and from the hardening group for hardening rods, tubes and other cylindrical parts.

Figure 4. Energy distribution in the ring focus.

One sometimes has to control not only the shape of the beam, but also its intensity distribution. During the project, the team developed an experimental modular hardening laser for this purpose. The system has 10 small laser heads, each consisting of a stack with 10 bars with fast axis collimation, providing 400 W of maximum power, so that 4 kW of total power can be achieved. An adjustable cylindrical telescope allows variation of the beam size in the working plane between 2 x 4 mm² and 2 x 10 mm² at a working distance of about 125 mm, which provides sufficient space for most hardening applications.

Each laser head has its own power supply and is connected to a pyrometer, which detects the surface temperature. The temperature at the surface of the workpiece can be accurately controlled for each element by control of laser power via a feedback loop.

Such a system can be used in two ways. First, overlapping the radiation of several heads can generate a large spot with controllable beam shape (Figure 5). If five spots measuring 2 x 10 mm² properly overlap, a 10 x 10 mm² spot can be realized and its intensity distribution can be controlled to some extent because the power of the five stripes can be controlled individually. This might be useful for laser hardening of complex structures.

Figure 5. This diode laser unit has an adaptable intensity distribution.

The second way is to perform the hardening process at several locations simultaneously, which minimizes or even eliminates distortion effects. Tests for such an application were performed using high-precision guiding rails. Improved homogenization of the diode laser beam is a considerable step forward in process reliability.

This project proves that the modular approach is advantageous because the laser diodes can be adapted to a variety of applications through special focus geometry, controllability or simultaneous processing.

Acknowledgment

The project was partially funded by the German Federal Ministry of Education and Research. The author would like to thank the Fraunhofer Institute for Laser Technology in Aachen, Germany, for supplying information for this article.

References

F. Bachmann (2001). Proc. SPIE, Vol. 4762, p. 1.
M.T. Kelemen et al (2002). Proc. SPIE, Photonics West. Vol. 4648B.
U.A. Russek (2003). Proc. SPIE, Photonics West. Vol. 4977.

Meet the Author

Friedrich Bachmann is product manager for high-power diode lasers at Rofin-Sinar Laser GmbH in Hamburg, Germany, and manager of the Modular Diode Laser Beam Tools project.