ANALYSIS: DIODE LASERS
Figure 2: powder streams and the glowing powder when hit by the laser beam (top) and the generation of a ring structure layer by layer via additive manufacturing (bottom)
“Broad power scalability enables diode laser systems to be optimally configured for a wide range of applications”
laser utilisation in materials processing. One of them is the extension of the
available wavelengths, which will increase the number of applications that the technology can be used in. The recent emergence of blue diode lasers for processing highly reflective metals is a good example. Another development for direct diode
uniform intensity, which, based on wire-feed or powder-feed, is oriented for optimum throughput. Figure 2 shows a real cladding application in action: the generation of a ring structure on an iron base. The powder nozzle, in this case, generates four streams of powder, which meet in the focal spot of the laser beam (lateral powder nozzle). The average grain size of the iron (Fe) powder is approximately 105µm.
Advanced cooling – avoiding corrosion Lifetime and reliability depend on efficient cooling; most high-power bars (and 2D arrays) utilise some type of microchannel cooling, where the flow of cooling water is very close to the current-driven junction itself. De-ionised water is typically required, since this provides the necessary low electrical conductivity. This adds to the cost of operation. In addition, de-ionised water has to be controlled carefully, to prevent leakage in the cooling microchannels – a leading cause of early device failure. And, for example, if there are 10 bars enclosed in a single housing, a solitary leak can essentially damage all 10 bars.
Coherent has addressed the issue by using a micro-architecture that separates cooling water from the electricity. This allows efficient cooling using only clean, industrial-grade water, lowering the cost of ownership. Avoiding de-ionised water minimises the need for careful control of water condition. This simplifies infrastructure requirements, extends expected device lifetime, and lowers cost.
Service and support Even though diode lasers represent one of the most reliable and long-lifetime photonic technologies, service and support are still critical if a problem arises. The ability to perform remote diagnostics, scheduled maintenance, and fast-response are all key for industrial applications that often run 24/7 and increasingly are located outside Europe and North America. This has proven to be a competitive advantage for high- power diode laser system vendors that can deliver this service.
Where next? For the future, there are a couple of routes that can be followed to increase direct diode
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lasers is ongoing in terms of power. Today’s facet passivation techniques for the semiconductor chip have already enabled increases in the power densities. Currently 200W-300W CW – and well beyond 1kW for short pulse (QCW) operation – is available from a 1cm-wide laser diode bar. Higher CW power supports lower system
costs per kilowatt for applications in the materials processing market. Higher power per facet area also supports higher brightness applications such as fibre coupling for materials processing and fibre laser pumping. Last but not least, with VBG (volume
Bragg grating) filter technology and dense wavelength multiplexing, high-brightness, multi-wavelength, multi-kW systems have been demonstrated, as well as their use for high-brightness direct diode cutting. Currently the cutting market is dominated by fibre lasers, but we have to keep in mind that high-brightness direct diode devices have just been demonstrated and there is a roadmap of further possible development. Overall, direct diode is a versatile
technology which has scaling capabilities in terms of power, and can take advantage of the integration of application-specific wavelengths into existing platforms, and can be utilised across numerous markets. l
Dr Jörg Neukum is the director of product marketing for high-power diode lasers at Coherent
AUTUMN 2019 LASER SYSTEMS EUROPE 15
Coherent
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