What Does It Takes to Change the Game?
Industrial lasers are integrated into manufacturing and fabrication facilities around the world. Every moment, there is an industrial laser somewhere that is cutting, etching, or welding. But the energy from traditional industrial lasers is not absorbed well by copper, so using them for copper processing has been slow and produced inferior quality.
In 2017 NUBURU (Centennial, CO) released the 150-watt AO-150, and in December of 2018, the 500-watt AO-500 industrial lasers, which operate in the blue wavelength region in the vicinity of 450 nm. They have demonstrated excellent performance for copper foil and thin metal welding, up to 1 mm. It was already known that blue laser light performs well with copper but putting it to practical use was a challenge. (Figure 1).
Blue to the Rescue
Industrial lasers are powerful, flexible, reliable, and easily integrated into automated manufacturing systems. But traditional industrial lasers — operating in infrared regions near 1 μm and near 10 μm — cannot efficiently process copper. And with the increasing ubiquity of electronics, copper is unquestionably an essential material for a wide range of applications.
The problem with conventional lasers is that copper absorbs only a minimal amount of incident infrared radiation. Consider copper welding, a desirable process for such applications as busbar welding for electric motors, interconnect joining in portable electronic devices, and foil assembly in lithium-ion batteries. Copper absorbs only about five percent of incident infrared radiation. This inefficiency means that starting a weld — initiating a melt pool — takes a significant amount of excess energy. Once that melt pool is established, though, it absorbs a higher percentage of infrared energy than the base metal. The excess energy in the melt pool causes the metal to vaporize, creating bubbles throughout the weld puddle. Some of these bubbles are at high pressure and escape the puddle, forcing material to be ejected as they escape. Some are at lower pressure and are unable to move the viscous molten metal, so they get trapped as the weld puddle cools. The result is voids in the joint and material spattered on the surrounding surfaces. Trying to weld copper with infrared lasers requires operators to tightrope an almost impossibly thin line between enough energy to initiate a weld and not so much that the joint quality is unacceptable. In many cases that leaves not just a narrow process window, but no process window at all.
The fundamental physics of blue laser light absorption sidesteps that entire problem. Copper absorbs more than ten times as much incident blue laser light as infrared, so it's easy to melt the copper or initiate a keyhole in the material. Once the copper begins to melt, the absorption rate of the blue light increases in proportion to the temperature until the keyhole is initiated, at which point the absorption approaches 100%. Since the absorption of blue light begins at around 65%, the increase in absorption with temperature up to the formation of a keyhole is very stable and provides a wide process window. The result is that blue lasers weld copper faster than alternative methods and the resultant joints are of higher quality.
Putting the Pieces Together
Technological advances on three fronts came together to enable the engineering development of a high-power industrial blue laser. The first was the availability of high-power blue-emitting laser diodes. The second was the development of advanced micro-optics and macro-optics compatible with the high-power emissions. The final piece of the puzzle was the ability to couple high power into an optical fiber and provide users with a beam delivery system similar to traditional industrial lasers. The NUBURU team used off-the-shelf blue laser diodes and developed the supporting capabilities necessary to produce a high-power blue industrial laser.
The beauty of compact multi-die laser diode packages is that electronic connections, mechanical support, and thermal stabilization can be integrated into one assembly. The challenge comes in combining eighty individual 450-nm laser diode outputs into a single optically stable beam. To reach industrially relevant output power, our engineers created an innovative network of micro-optics, interleaving mirrors, and macro-optical beam conditioning optics.
The AO-150 laser (Figure 2) is built around individual packages of 20 blue laser dies, arranged in a five by four diode pattern. The first step in combining these multiple beams is to ensure they are equally collimated. Laser diodes have a highly asymmetric output, with the divergence in the “fast axis” being much higher than that in the “slow axis.” Each diode requires its own set of slow axis and fast axis collimating micro-optics that are actively aligned to ensure that all of the collimated beamlets are parallel to each other with limited pointing errors.
The resulting pattern is a five-by-four array of collimated beamlets. A series of interleaving mirrors combines the output arrays from four separate packages. All of the output patterns are spatially offset from one another, so the intermediate result is a five-by-sixteen array of beamlets (Figure 3). A cylindrical telescope circularizes the divergence of the beamlets so the final focal spot is circular. The module has a nominal output power of 200W. This module can then be used either in our AO-150 system with the output coupled into a 200-μm optical fiber, or it can be integrated with other modules to create higher-power laser systems.
The latest product added to the NUBURU lineup is a 500-watt system that uses four of the same 200-watt optical modules as a building block to create a higher power system. The modules are combined with a series of mirrors and a polarizing cube, as shown in Figure 4. Finally, an aspheric lens focuses the 320 beamlets into a 400-μm optical fiber.
Achieving Industrial-Class Performance
The system is designed to efficiently and reliably deliver industrial-capable optical power at blue wavelengths. The optomechanical systems have achieved sufficient power margins to provide long term reliable operation. Both the AO-150 and AO-500 couple more than 90% of their output light into the QBH fiber. That minimizes the thermal loads on the cladding mode strippers, resulting in a reliable fiber beam delivery system. The thermal management — the result of efficient optical coupling and effective active cooling — adds to the stability and low power degradation.
Still, every system eventually reaches the end of its useful lifetime, so each 200W module of the AO-500 is designed to be field replaceable.
A continuing series of tests are under way to determine the welding parameters for specific assemblies. The output beam profiles of the AO-150 and the AO-500 are smooth, symmetric Gaussian contours. The symmetry means the output can be focused to a small spot with high power-density. As seen in Figure 5, the AO-500 produces lap welds, butt welds, and hairpin welds for a variety of material thicknesses, even in challenging geometries. These joints are all produced at high speeds and are essentially void- and spatter-free.
The advantages for copper processing are repeated for other “yellow metals” as well — good results have been demonstrated for stainless steel welding, and even the notoriously difficult challenge of welding dissimilar materials. The effectiveness of materials processing applications depends upon the amount of energy delivered to a target area. Even if an application does not require highly concentrated energy delivery, optical efficiency is always enhanced by starting from a high brightness source. Brightness is a measure of power per unit area per unit angle. For example, if the same amount of power is delivered through two 0.22 numerical aperture fibers, one with a 600-μm core and the other 400-μm, the brightness of the 400-μm source will be twice as much as the 600-μm source. To maximize operational flexibility and efficiency for industrial processes, higher brightness is always desirable.
A Bright Future for Blue Light Lasers
For many materials processing applications, the physical advantages of blue light have always been evident, but the technology has been out of reach. Multi-die laser diode packages, highpower blue micro-and macro-optics, and powerful design and measurement tools have now enabled development of robust and reliable 450-nm lasers at industrially useful optical output power. The high-power, high-brightness system is already demonstrating the ability to address challenging materials processing applications. The flexibility and power of the system are likely to trigger an ever-increasing range of applications in a wide variety of industrial contexts.