Variable optics play a critical role in helping high-power diode lasers infiltrate manufacturing applications.
Bodo Ehlers
Today's high-power diode lasers can achieve spots that are approximately 1 mm square. Their intensities can exceed 105 W/cm2. Power levels can reach 6 kW. These specifications facilitate a diverse range of industrial-laser applications from surface treatment to cutting and welding. These applications can be conducted with the same setup because of an optics design, more common in photography, called the zoom lens.
Dissecting diode lasers
Diode lasers create a square spot size because of the nature of their active media; the light is generated in bars of semiconductor material with an emitting surface typically 10 mm wide and 1 µm high. The divergence angle of such lasers is approximately 5° in the direction of the longer extension, commonly called the slow axis, and roughly 30° in the direction of the small extension, known as the fast axis. The laser spot is very asymmetric, as indicated by the different extensions of the emitting facet in both directions.
FIGURE 1. In the optical model of a zoom optic for surface treatment, the laser beams of two diode-laser stacks are collimated in the slow-axis direction by movable cylindrical lenses and are focused in both directions by a spherical doublet.
To increase the power into the kilowatt range, laser manufacturers stack individual bars on top of each other. This decreases the beam cross section in the fast axis and leads to a more symmetric beam. If higher power is required and the output power of a single stack is insufficient, several diode-laser stacks can be stacked next to each other. If the focus geometry requires a rectangular shape with a high aspect ratio, packaging along the slow axis is also pursued.
To maximize the industrial advantages of the diode laser, it is necessary to vary the spot size and the aspect ratio of the rectangular spot. For welding applications, a square spot with small dimensions is preferred. On the other hand, surface applications benefit from a rectangular spot size with varying aspect ratio. Here, the spot size of the laser beam should be adjustable across a wide range to account for the different needs of the individual surface treatment processes.
For transformation-hardening applications, the spot width must be adjustable to allow the laser beam to produce the required track width in one path. This eliminates the need to scan the area to be hardened with a smaller laser beam. Processing time drops, and partial annealing of the previously hardened track is eliminated.
Variable optics that address these diverse processing demands consist of a number of spherical and cylindrical lenses in different positions, depending on the zoom setting.
Exploring optics
Two different variable-optics designs have been realized in industrial applications involving high-power diode lasers (see Fig. 1). The first lens is designed for surface applications such as hardening, cladding, and alloying. Its spot size varies from 6 to 22 mm in one direction and stays constant at 2.6 mm in the other direction. This line-shaped focus geometry exhibits a homogeneous intensity distribution over a large range and thus guarantees high performance in surface-treatment applications. This is important because any intensity peaks or dips in the beam profile lead to localized deviations from the optimum process results.
The second optic exhibits two ranges of continuously variable spot sizes from 1.2 to 6 mm and 2.4 to 12 mm, with its second dimension fixed at 1.2 or 2.4 mm, respectively. This lens works for a more-diverse group of applications ranging from surface treatment, which requires a fairly wide track width of up to 12 mm, to welding applications, which can be performed with spot sizes down to 1.2 x 1.2 mm. At power levels of 2.5 kW, laser intensity can reach 2 x 105 W/cm2, which is sufficient to create a deep-penetration weld. Sheet metal up to 6.0 mm thick has been welded successfully with this laser power.
FIGURE 2. This flexible machining cell has the diode laser and zoom optic attached to the end of the b axis. For easy part handling, the cell has three large doors.
The design of this zoom optic is more sophisticated because it provides a larger zooming range and a much-smaller minimum spot size. The optics use standard lenses made out of fused-silica lenses in addition to customized lenses to increase the performance of the system and reduce the size and weight. All lenses are multilayer-coated to minimize losses.
To enable the use of different optics, the optic attaches to the laser through a universal ring mount. The body of the optics can withstand the relatively high intensity of the stray light and the thermal load due to backscattering from the workpiece and the thermal load induced by the process. The mount also conducts the water through pipes integrated in the body to the lens unit to ensure sufficient cooling.
On the shop floor
Integrating the laser and zoom optics into existing machining systems is a straightforward process because no beam-guiding systems are involved. Only cooling water, electricity for the diodes, and some signal lines must be run to the laser. The laser unit can be mounted to a motion system such as an industrial six-axis robot or can work in a flexible manufacturing cell, such as a flexible laser-hardening cell recently developed to accommodate a large variety of parts (see Fig. 2).
FIGURE 3. Cross section of a hardened track illustrates the uniform depth and large length of hardness possible with a 2-kW high-power diode laser when combined with a zoom lens.
The design goal for the hardening cell, which has a working envelope of 1.6 x 1.0 m and 75 cm in the z direction, was to provide a flexible setup to fully exploit the advantages of the diode laser. The cell includes a rotary table controlled by the machine's computer numerical control and an x-y-z motion system with a motorized b axis to provide continuous 360° rotation. The cell is completely enclosed and equipped with safety features to adhere to industrial safety standards and has received the certification required for imports into the European Community.
One of these cells, which was installed at a job shop in 1997, is based on two different laser sources (diode and Nd:YAG) for materials-processing applications including hardening and welding. A typical diode-laser application is the hardening of profiles for a coordinate measurement machine for the automotive industry. The profiles must be hardened with minimum distortion over a large length, but conventional hardening methods, such as induction hardening, can produce substantially deeper hardened tracks than laser processing, which can increase distortion of the profiles (see Fig. 3). Laser hardening increases the quality of the parts and reduces the cost and cycle time. This is due to the reduction and, in some cases, the elimination of secondary finishing operations often required with conventional processing.
Other applications for the diode laser include welding of various materials such as mild steel, stainless steel, and aluminum. Due to the high surface quality of the weld, this technique can be applied to parts with visible seams that must have a good appearance.
BODO EHLERS is a research scientist at the Fraunhofer Center for Laser Technology, 46025 Port St., Plymouth, MI 48170; e-mail: [email protected].