The direct approach
Fibre lasers, some would argue, are now the dominant laser technology in metal cutting, but take away the fibre architecture to leave the diode pump source, and this should be a much less expensive and more efficient way of producing laser light, right? In principle, yes, but it depends on the beam quality required, according to Dr Jens Biesenbach, CTO of Coherent-Dilas – the company makes laser diodes and was part of Rofin until the group was purchased by Coherent earlier in the year.
Diode lasers are generally considered to have poor beam quality – at least compared to a fibre laser – so putting too much effort into combining diode lasers and maintaining the highest possible beam quality will add cost, for example in the beam shaping optics. ‘There’s a certain break-even point where beam quality and power is more economical to produce in a direct diode laser than a fibre laser,’ Biesenbach observed.
Poor beam quality has been a feature of the diode laser in the past, but now higher brightness systems, operating at kilowatt powers, are finding their way into the marketplace and beginning to compete with fibre lasers in terms of beam quality. Teradiode, a start-up from MIT in the US, is one company producing these high brightness diode systems. It claims a beam parameter product of 3mm-mrad from a fibre-coupled direct diode laser with an output power of 2kW, equal to a brightness of 2,293 MW/cm2–sr. The company also offers kilowatt-class direct diode lasers for metal processing with a brightness of 6,800MW/cm2–sr, which equals that of comparable fibre lasers, according to its website.
Japanese machine tool builder Mazak is one of the early adopters of Teradiode’s technology for sheet metal cutting. At Fabtech in Las Vegas in November, Mazak was demonstrating a direct diode machine – using Teradiode’s laser technology – side by side with a fibre laser.
In the demonstration, a 4kW Mazak direct diode system was able to cut 9mm stainless steel 44 per cent faster than a 4kW fibre laser. The same diode system cut 1mm copper 50 per cent faster than the fibre laser. It was 10 per cent faster cutting 9mm mild steel; 18 per cent faster cutting 1mm aluminium; and 14 per cent faster cutting 1mm stainless steel.
‘Across all material types and thicknesses up to 9mm, the 4kW direct diode laser was faster than a 4kW fibre laser,’ commented Rajiv Pandey, senior product line manager at Teradiode.
The Mazak Optiplex 3015 direct diode machine was also displayed at Euroblech in Germany in October.
Panasonic adopted Teradiode’s technology around two years ago for its welding machines; metal welding has been earlier in adopting high brightness diode technology, because the process doesn’t require as high a beam quality as cutting. ‘By expanding into the cutting and welding markets, we’re taking market share from fibre lasers and, as the market grows, we will grow with the market,’ Pandey said.
Teradiode’s technology is based on wavelength beam combining. It uses a diffraction grating to merge multiple wavelength sources into a single, high-intensity beam.
The company is currently shipping 6kW diode systems; it introduced an 8kW laser at Euroblech that will begin shipping next year, and Pandey said the technology can power scale, and will reach 10kW in the near future.
Competing with fibre
Mazak adopting Teradiode’s systems is testament to direct diode lasers’ ability to cut sheet metal, but it’s still an incredibly young technology. ‘Right now, in my eyes, it’s still a little too early for high brightness direct diode technology for cutting,’ commented Biesenbach. Coherent’s product line manager, Klaus Kleine, added: ‘Diode laser systems made for very high brightness applications less than 20mm-mrad look very complex to me.’
Kleine continued: ‘I believe that the best brightness converter is still the fibre laser. The fibre laser seems to be the more elegant solution for now. I expect that to change, but it will take time.’
‘Fibre lasers have a hold on the 2D cutting market,’ said Pandey. ‘What we learned from our customers – machine builders – is that moving to a different technology, and even to a different power level, requires a lot of effort from their side – all the different cut parameters and recipes that they have to test. It’s a fairly big investment for machine builders and they have to do a lot of qualification work, building machines in the lab and testing prototypes. It’s probably a six-month-to-a-year runway to adopt this technology.
‘If you look at the installed bases using direct diode lasers, it’s very small,’ he continued. ‘Diode reliability is at the heart of our whole system. We think reliability is good enough to be made at an industrial scale. After that it’s the type of application that really matters, as to where the technology fits in.’
Hardening, cladding and brazing
At the moment, the dominant applications for industrial direct diode laser systems are areas like hardening, cladding and brazing. These fields need very high power, up to 10kW, and use low beam quality in the range of 100mm-mrad.
‘Laser cladding and hardening, a big market in Asia, is also growing in Europe,’ Biesenbach said. Hardening and cladding need specific homogenised beam shaping. In addition, the wire or powder supply has to be integrated with the laser beam in cladding applications. By contrast, brazing has a similar beam parameter product as cladding, but the systems contain more instrumentation; they need a scanner and more process experience to operate. These are the classic fields for diode lasers, for which Dilas has been supplying lasers since the year 2000.
Coherent-Dilas offers 3-10kW direct beams for cost efficient tooling for the big cladding markets. Higher quality diode laser systems are fibre delivered – Coherent-Dilas offers 6kW and 8kW fibre-delivered units. In China, for example, certain galvanic plating technologies are being substituted with laser cladding in the mining industry, which is a big market.
Coherent-Dilas uses standard 10mm high-power diode bars in its highest power diode systems. ‘This is the cheapest way to generate photons in the laser business,’ commented Biesenbach. For higher brightness between 10-30mm-mrad, the company uses a different laser diode chip, called a Tailored-Bar or T-Bar.
‘The T-Bar sits between an economical, powerful but low brightness bar, and the high brightness single emitters,’ explained Biesenbach. It is a combination of several emitters on one monolithic chip – Coherent-Dilas puts five emitters on one chip. This means all the processes involved in producing T-Bars, such as incorporating optics, is a factor of five more cost-efficient compared to a single emitter. A diode laser system made of T-Bars can give 1kW single-wavelength laser power at 20mm-mrad beam quality, which is a good tool for sheet metal welding, according to Biesenbach.
‘The charm of welding with a diode laser is that the seam is cosmetically almost perfect,’ Biesenbach said. Welding with a diode laser is very forgiving in terms of tolerances. Kleine added that Coherent’s 4kW diode laser using a 600µm fibre at 0.1 NA will ‘perfectly replace lamp-pumped lasers for many welding applications, even keyhole welding.’ The laser typically has a working distance of around 200mm with a spot size of around 0.6mm to 1mm, which allows room for the tooling to hold the materials together. ‘You need a bigger spot for welding, because too much power density will result in a poor join,’ Kleine said. He added: ‘Remote welding needs a fibre or disk laser, but for many welding applications direct diode will be enough.’
Coherent-Dilas’ technology is modular – it is based on the same diode platform as that used to pump fibre lasers, ‘which is a very cost-sensitive market’, noted Biesenbach. Power can be increased with several modules to 1kW, which is ideal for sheet metal welding. Using several wavelengths with the same automated technology, Coherent-Dilas can offer up to 4kW with 25mm-mrad beam quality. ‘Application testing has shown that welds made with Coherent-Dilas’ direct diode laser systems are comparable with those made with a fibre laser,’ Biesenbach said.
To reach higher brightness, Coherent-Dilas uses T-Bar single emitters superimposed on a monolithic chip. This means optically more effort is put into the design, which increases the cost. The company has achieved 8mm-mrad using the same platform as the 25mm-mrad T-Bar unit. ‘This is a nice result, but it is not yet a product,’ Biesenbach said. ‘We’ve developed this technology to be ready for the day – and the day will come in my eyes – when high brightness diode lasers will be used for tasks like cutting sheet metal. We are driving the T-Bar to the maximum in terms of brightness.’
A beam parameter product of 5mm-mrad can also be achieved by halving the size of the T-Bar chips from the standard 100µm to 50µm. ‘That’s the beam quality of industrial multimode fibre lasers today,’ Biesenbach noted.
Coherent-Dilas offers 3-10kW direct beams, as well as 6kW and 8kW fibre-delivered units.
However, since the diodes are only half the width, they only have half the power. ‘You need double the number of chips to get to the same total power, compared to the standard 100µm wide emitter used as a pump source for fibre lasers. Smaller and more numerous emitters will cost more to produce, which makes diode lasers more expensive than fibre lasers,’ Biesenbach added.
Speeding additive manufacturing
‘In the long run, diodes will be bright enough without needing an active material – fibre or disk – to achieve very high brightness, but we’re still a few years away from that,’ commented Kleine. Teradiode might disagree with that statement, but the high brightness diode laser is still a very young technology. Direct Photonics from Germany was another company developing high brightness direct diode lasers, but it has since been acquired by II-VI.
Another potentially big market for diode lasers is for additive manufacturing, according to Biesenbach. ‘Today’s selective laser melting machines for 3D printing use precise fibre lasers to make very fine structures, but these machines are slow because of the single laser. If you combine a fine focus from a fibre laser and a rougher focus from a diode laser, the process can be engineered to run faster,’ he said.
‘[Diodes that are] small and inexpensive – and if they have the right beam quality – would be a very interesting new market for diode lasers,’ Biesenbach added. Fraunhofer ILT is already working on this concept using diode lasers for SLM.
‘Direct diode lasers are definitely here for real; it’s the next big thing after fibre lasers,’ Pandey at Teradiode commented. Teradiode’s lasers are immune to back reflection. In addition, the beam combining process that forms the heart of the company’s technology produces an output beam with a wider spectral bandwidth, which can be controlled by design. ‘Using the broad wavelength range emitted by our direct diode systems gives a better absorption when cutting alloys [compared to cutting with the fibre laser wavelength],’ explained Pandey. ‘That’s an advantage for direct diodes that hasn’t been talked about as much, but is the reason direct diode lasers can cut faster and at higher quality, because these little composition changes in the alloy don’t all respond well to 1µm from a fibre laser. Fibre lasers get the job done, but a broader wavelength range hitting the material gives better absorption.
‘Reliability and other factors have to continue to improve,’ Pandey continued, ‘but with companies like Mazak, Panasonic and others adopting the technology… direct diode is the future of laser processing beyond fibre lasers. Even big companies like Trumpf and others are very interested in this technology; it’s just that no one has really power scaled reliably to a point where it can be a contender to fibre lasers. We think we have done this.’