Direct diode lasers today: better integration drives applications growth

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Coherent’s Dr Jörg Neukum highlights how the ease of integration and maintenance of direct diode lasers has contributed towards their increased uptake in industry

The use of high-power, direct diode lasers is increasing in many manufacturing applications, including those for producing automobiles, medical devices, and consumer products. Typical tasks include metal cladding and other additive applications, soldering, surface heat treatment, brazing and plastics welding. 

Several factors contribute to this growth, most notably the availability of integrated product platforms – sub-systems – equipped with processing heads for optical beam shaping and other application-specific accessories, together with advanced cooling schemes that do not need de-ionised water and deliver 24/7 reliability in any manufacturing location.

Sub-systems optimised for integration

The advantages of diode lasers include compact size, high electrical efficiency, and scalable architectures. But diode laser output is less ‘well behaved’ than other lasers, thus requiring more complex beam shaping and delivery optics. Consequently, after many years of emphasis on the development of higher brightness devices, it is the availability of lasers that include integration-friendly features that has proved most critical to penetrating real-world manufacturing applications. A typical integrated system, therefore, includes a beam shaping module, often with a fibre-based processing head, a power supply, and possibly a chiller. The goal is to simplify life for the user, allowing them to just specify the beam, rather than all the photonic components required to generate that beam. In automotive manufacturing applications, these sub-systems are then integrated into a robotic tool by a speciality tool builder or by the automotive manufacturer, for example, for applications such as brazing two galvanised steel body components.

Scalable power – from plastics to metal applications

Direct diode lasers can range from single emitters outputting a few watts, to bars at hundreds of watts, and stacks of bars up to a kilowatt and beyond. This broad power scalability enables diode laser systems to be optimally configured for a wide range of applications. 

In the tens to a few hundred watts range, diode laser systems are used mainly for plastics welding in numerous applications, where they compete with traditional bonding technologies, such as adhesives and ultrasonic welding. Here, medical devices represent a fast-growing market segment because ultrasonic welding creates particles that require cleaning, and adhesives need regulatory approval. Another interesting segment is smart car key fobs, where ultrasonic welding might damage the sensor electronics. Laser welding is also used for sealing devices with plastic coatings to make them immune to damage from liquid (for example, hot tea/coffee) as in the illuminated exit strips on the aisles of commercial aircraft.

Figure 1: Cladding is optimally performed using a line focused beam shape

Process development is key in many of these applications, starting with the choice of the optimum wavelength – this is particularly important for transparent and white plastics, for example. Direct diode lasers are currently available over a broad range of wavelengths from 1,400 to 1,500nm, 780 to 1,060nm, and even blue diode lasers at 450nm have begun to emerge in recent years – for processing highly reflective metals such as copper, gold and aluminium. Extensive experience in a variety of applications can also be pivotal in product selection, and as such Coherent maintains a network of global applications facilities for this purpose.    

At hundreds of watts, diode laser systems are used in brazing/soldering applications. And at the kilowatt level, diode laser systems are used for additive manufacturing and heat treating (case hardening) applications where large areas must be rapidly processed, and where the fine resolution of a fibre laser, for example, is not required. Such applications include the processing of heavy machinery for shipping, military, and mining and drilling (gas/coal/oil) applications. 

Customised beam shaping

Diode lasers have a large area output profile that is ideal for use in applications that need illumination over a specified broad area, and additive manufacturing (such as cladding) provides a simple illustration of the value of beam shaping. As shown in the simplified schematic in figure 1, in cladding the output is often shaped to a line focus of 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.

Figure 2: Powder streams and the glowing powder when hit by the laser beam (left) and the generation of a ring structure layer by layer via additive manufacturing (right)

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 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 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. 

Dr Jörg Neukum is the director of product marketing for high-power diode lasers at Coherent