NUBURU released its first industrially relevant blue laser in 2017. Many metals absorb blue light much better than other wavelengths, so the blue laser was rapidly adopted in diverse industries. As the power and brightness of blue lasers increased, a new range of applications became available.

The blue laser’s chip-based design allows increased performance without degrading beam quality. For materials processing applications — laser welding is the flagship example — increased power density allows both increased penetration depth and weld speed.

The unique interaction between blue laser light and various materials also adds new capabilities to additive manufacturing. These examples demonstrate how increased laser capacity has enabled, and will continue to enable, new manufacturing possibilities.

The Absorption Advantage

Copper is the pre-eminent example of an economically important material not amenable to processing with traditional infrared industrial lasers, typically around 1 μm. The benefits of blue stem from basic physics. As shown in Figure 1, copper absorbs only about five percent of incident power at IR wavelengths.

Figure 1. The primary advantage of the blue laser is its high absorption in a variety of materials. Copper absorbs over 10 times more blue light as infrared, and 20 percent more blue than green light.

Consider the implications for welding. Welding takes place when the metal(s) of a workpiece are heated above their liquid phase transition temperature to create a “melt pool” with dynamics that blend the base materials. When the energy source is removed, the material solidifies. If a material absorbs only five percent of the incident energy, that means the source must supply 20 times the energy required to actually do the melting. That’s inefficient, but the real problem is poor quality material processing.

The melt pool differs significantly from the base material. Once the melt pool is initiated, it suddenly absorbs much more infrared — much more of that 20 times “extra” power. The result is a series of violent microexplosions, spattering material from the weldment. The spatter also leaves “bubbles” behind, empty spaces in the final joint called voids. Voids and spatter degrade both physical strength and electrical performance. When copper is the material in question, both physical strength and electrical performance are critical for system performance.

Compare this with blue light. Copper absorbs more than ten times as much blue light than it does infrared. More importantly, the base material and melt pool absorb the same percentage of power in the blue. When the blue laser delivers enough power to initiate melting, it automatically delivers just enough power to maintain the melt pool. The resulting joints are produced quickly, without voids and without spatter.

The Brightness Advantage

Absorbed power density is critical. If a beam carrying power just sufficient enough to initiate melting were expanded to twice its diameter, it would need to carry four times as much power. A convenient metric to capture this is the beam parameter product (BPP). Power density delivered to the workpiece is a function of the beam power and the BPP. The absorbed power density has one more key element: material absorption at the laser wavelength.

For two lasers, one with a BPP of 30 mm-mrad and the second a BPP of 60 mm-mrad, the second provides only one-fourth the power density of the first. Remember, also, that the BPP of an overall system cannot be decreased — that is, it can’t be improved — by external optics. The BPP can only be degraded from its initial value.

This brings up one more essential point. Power density that is too high will also lead to poor quality processing. The trick is to match the power density to the application’s needs. For example, a 1-kW blue laser with a 400 μm spot delivers about 800 kW/cm2 to a workpiece, while that same 1-kW laser in a 200-μm spot results in a power density around 3 kW/cm2. For copper welding that’s too high, resulting in a poor-qualty weld.

The Path to Higher Power

Conceptually, obtaining a high-power industrial laser is straightforward: take the output from many individual lasers and deliver it to the same workpiece. To get a 500-W laser, route the beamlets from 500 separate 1-W sources to the same spot. To double the power, you can either double the number of 1-W lasers or you can double the power of each individual source to 2 Watts.

The trick is, though, to condition the individual sources in a way that optimizes beam quality and then combines them without degrading the beam. Imagine a situation with a 20-diode bar source, each diode emitting perpendicular to the plane of the bar. If those beams are collimated through a single lens, they will necessarily be conditioned differently, with distinct spatial and angular power distributions. That makes it nearly impossible to combine them without degrading beam quality.

The only way to maximize beam quality, match beamlet profiles, and combine for optimum performance is to individually condition each beam. The NUBURU AO-150, for example, starts with chip-based arrays, each with 20 diodes, and colli-mates them individually with actively aligned micro-optics. Outputs from several arrays are integrated through spatial interleaving and polarization filtering. The beamlets are then coupled into a 200-μm diameter fiber.

The architecture is scalable. The AO-500, for example, combines the outputs from four AO-150s to achieve a 500-W output. One of the pathways to higher power is through continued spatial and polarization interleaving, but another parallel path is also built right into the design. GaN diode technology is relatively immature, with efficiencies in the mid- to upper-30 percent range. GaN efficiency is expected to approach the current GaAs level of around 70 percent. That will double the output power without changing the system architecture at all.

Expensive single-mode green lasers are now coming to the market. Because they’re single-mode, their BPP can be on the order of 5 mm-mrad, but that gives a power density well over the threshold for poor quality copper welding. It’s necessary then to intentionally degrade the BPP to get to a regime where good quality welding is possible. You end up paying a lot for laser characteristics you aren’t going to use, and you add to the overall complexity of the system by needing to incorporate additional elements to reach the optimum power density.

The Performance Advantage

Variations in absorption curves and brightness specifications for lasers are interesting talking points, but the key is leveraging these factors for enhanced performance. The example of copper welding highlights how those factors improve performance.

An infrared laser has a very narrow process window between enough power to melt copper and the threshold to create voids and spatter. By “wobbling” the beam — rotating it around the desired weld joint — the effective power density can sometimes be reduced to minimize defects, but not eliminate them. Sometimes there’s no viable process window. The blue laser eliminates the problem. No special exposure is necessary. The blue laser is faster, and the welds are defect-free. There’s even a significant difference between blue and other visible wavelengths. For the example of copper, the blue laser is nearly 20 percent more efficient than a green laser. That is, for a blue and a green laser with the same BPP, the green laser has a built-in handicap of 20 percent — wasted energy that serves only to reduce the process window.

Figure 2. Power density is critical in materials processing applications. With copper welding, for example, a factor of two improvement in beam parameter product leads to increased penetration depth and/or faster welding speed.

The blue laser’s application-matched high brightness provides an equivalent performance advantage. Consider the two identical 500-W blue lasers represented in Figure 2: one a 60 mm-mrad BPP and the other a 30 mm-mrad. At a weld speed of 5 m/min the 60 mm-mrad system will only weld a thickness less than 250 μm, while the 30 mm-mrad system has a penetration depth of more than 350 μm. Similarly, for welding the same thickness, say 300 μm, the 30 mm-mrad system is well over twice as fast as the 60 mm-mrad system.

The Near Horizon

The blue wavelength physical advantage in absorption leads directly to the ability to perform otherwise extremely challenging welds. This buttweld of pure copper and stainless steel has historically been a nearly intractable problem.

The blue industrial laser is relatively new, but the performance advantages in copper welding have already been demonstrated in industries such as battery fabrication, mobile device assembly, and e-mobility manufacturing. Laboratory tests already indicate the blue laser might solve some otherwise intractable problems. Welding dissimilar materials — copper and steel or copper and aluminum, for example — is extremely challenging because of the materials’ different thermodynamic and mechanical properties. It’s difficult to avoid the formation of “intermetallics,” regions of varying composition with equally varying mechanical and electrical properties. The blue laser’s inherently wider process window makes it feasible to minimize the formation of intermetallics and maximize weld quality.

In addition to welding, processes such as cutting, etching, and cladding are natural fits for blue laser materials processing. More intriguing is the extension of additive manufacturing (3D printing) capabilities by incorporating the blue laser. The blue laser offers both enhanced performance for existing material deposition processes and opens the door to new materials.

As the power and brightness of the blue laser improves, welding will be extended to thicker materials and higher speed, and new applications will undoubtedly arise.

This article was written by Jean-Michel Pelaprat, Matthew Finuf, Robert Fritz, and Mark Zediker, NUBURU Inc. (Centennial, CO). For more information, visit here .

Photonics & Imaging Technology Magazine

This article first appeared in the January, 2020 issue of Photonics & Imaging Technology Magazine.

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