FEATURE
Issue: 

Solid-state gold

Greg Blackman on the advances being made in solid-state laser technology for materials processing, including fibre, disk, and direct diode lasers

Sales of IPG Photonics’ fibre lasers for materials processing, which account for most of the company’s business, grew by 17 per cent in the third quarter of 2014. IPG has been posting strong financial reports for the last two years, suggesting a healthy market for fibre laser technology for materials processing. Some of the growth will be down to new applications of laser processing, but some will be due to replacement of CO2 laser systems with high-power fibre lasers.

‘Growth in the macro market generally comes from kilowatt fibre lasers,’ noted Markus Röhner, head of product management at Jenoptik Laser. ‘There are new markets for fibre lasers, but the majority of sales come from replacing CO2 lasers.

‘The overall laser market for macro applications has grown by a moderate three or four per cent, not more, in recent years,’ he said.

One example where Jenoptik is seeing new fibre laser applications coming through is by combining fibre lasers with appropriate handling systems like robotics, such as for 3D metal cutting in the automotive industry. These are small markets at the moment, Röhner said, but growing nonetheless.

The growth in fibre systems, however, by no means heralds the death of CO2 lasers. Frank Gäbler, director of marketing at laser supplier Coherent, commented: ‘There are still many CO2 lasers out there in the field; it’s not that fibre lasers have taken over the market.’ However, it does show that fibre lasers are now becoming more widely used for materials processing.

CO2 has generally been preferred for cutting thicker material, but solid-state disk and fibres lasers are now able to do so at good cut quality. Trumpf has released its BrightLine fibre technology, which enables solid-state lasers like the disk laser to cut thick metals at a high quality.

‘The increasing penetration of solid-state lasers in flatsheet cutting applications is remarkable,’ stated Till Schneider, senior manager, product management at Trumpf Laser- und Systemtechnik. ‘With the revolutionary BrightLine fibre technology, Trumpf has significantly expanded the thick sheet quality boundaries of solid-state laser cutting that are set by the CO2 laser.’

Schneider said that Trumpf provides its disk laser for multi-kilowatt materials processing. He said that a combination of advanced power pumping diode concepts, a 6kW per disk resonator setup, the replacement of laser wear parts with long-life components, and further improvements in the laser control, such as the optimised stand-by modes, has all improved Trumpf’s solid-state laser offerings for materials processing.

‘The worldwide solid-state laser market in the last few years has seen growth in almost all applications from micro to marking to low power and to multi-kilowatt applications. But we see a significant growth especially in 2D and flatsheet cutting applications,’ Schneider added.

Direct diodes

There is now another laser technology competing for market share with CO2 lasers – and to a certain extent to fibre and disk lasers – which has previously not been suitable for most materials processing tasks, and that’s the direct diode laser.

‘With increasing average powers and improving beam qualities, the direct diode laser has expanded from surface and brazing applications to industrial welding applications,’ noted Schneider. Trumpf has recently introduced a diode laser platform, the TruDiode 2006-4506, which achieves comparable welding application results to disk and fibre lasers with a beam quality of 30 mm*mrad.

In the past, the beam quality of direct diode lasers was not considered good enough for cutting metals. Now, however, there are direct diode systems being released positioned for thin metal cutting because the beams are bright enough. And, with an efficiency that exceeds even fibre lasers, the technology is very attractive for certain material processing applications.

‘In a couple of years the direct diode technology can also become suitable for cutting applications,’ added Schneider. ‘Some technological concepts of direct diode cutting lasers are on the market already or have been announced but are still waiting for broader industrial adoption.’

Gäbler at Coherent commented: ‘At Euroblech [the trade fair] we could observe direct diode laser manufacturers positioning their kilowatt devices as a replacement for CO2 lasers. A 2kW direct diode system would cut thin metals faster than a CO2 laser, because the metals absorb at the diode laser wavelengths much better than with CO2 beams.’

He noted that the direct diode’s beam quality is not equivalent to that of a fibre laser yet. ‘We’ll see how diode lasers for cutting develop,’ he said. ‘There is a debate that diode lasers are going to replace fibre lasers in the future.’

Coherent uses stacks of laser diodes to pump an active fibre in its fibre laser technology. It is focusing a bar of diodes producing not very bright light into a fibre to get a bright beam. The direct diode companies, on the other hand, take the bar and focus the poor beam quality into optics to get a bright beam.

‘The optics for mode conversion have a cost,’ commented Gäbler. ‘It therefore is yet to be seen if in the end direct diode technology is more cost effective than an active fibre. Fibre-coupled direct diode lasers typically have an energy efficiency in the range of 40 per cent. Standard fibre lasers are around 35 per cent efficiency – we’re speaking about a five per cent difference.’

He added that it’s really the performance of the machine that counts for a sheet metal fabricator buying a laser system. ‘They [end-users] care about cost of ownership, reliability, the service, processing speed, etc. The end-customer doesn’t care what kind of laser is inside the machine. In the end, if it is a direct diode laser or a fibre laser, it doesn’t really matter.’

Micromachining

The micromachining market is a growing area for laser processing with the improved reliability of ultrashort pulsed (USP) lasers operating in the picosecond or femtosecond regime. ‘With the industrialisation of ultrashort pulse technology, applications are no longer restricted to a laboratory environment,’ commented Dr Nick Hay, director of business development at Powerlase Photonics, a provider of pulsed diode-pumped solid-state (DPSS) lasers. The company offers ultrashort pulsed lasers with average powers around 100W and pulse energies of 100µJ.

There are applications of USP lasers in high-throughput surface processing, such as annealing, cleaning and selective thin film removal, noted Hay, while Gäbler gave the example of picosecond lasers replacing electrical discharge machining (EDM) tools, such as in producing car injection nozzle drilling. There are also some surface structuring applications, such as making microchannels in metals or engraving the mould forms for automotive dashboards, for example. Another application Gäbler noted for USP lasers is for producing medical devices, such as for roughening the surface of implants to encourage tissue growth in the body.

High-power fibre lasers are the perfect choice for macro material processing, whether the laser is operating in a continuous wave or modulated mode. Also the production technology is well established. ‘Fibre lasers are easy to build because the fibre technology of splicing and cleaving, etc has been well developed over the last 30 years in the telecommunications sector,’ commented Röhner at Jenoptik.

‘Moving to ultrashort pulsed lasers in the pico- or femtosecond range, fibre laser technology is limited due to the constraints around peak power and pulse energy with excellent beam quality,’ he continued.

Peak power in the range of megawatts, sometimes gigawatts for femtosecond lasers, exceeds the damage threshold that can be passed through a standard optical fibre. ‘The fibre technology is perfect for the oscillator that generates the pulses, but you need a different technology to amplify those pulses. We think the disk laser is the perfect choice for achieving high peak power, high pulse energy, combined with an excellent beam quality,’ Röhner stated, adding that rod and slab lasers are limited in their beam quality over the full power range.

One of the boundary conditions for USP lasers is cost of ownership. ‘Picosecond lasers are expensive compared to regular nanosecond lasers,’ commented Gäbler.

Coherent’s Rapid NX is a new picosecond laser designed to be more cost effective than standard picosecond lasers. ‘At 6W or 10W you’re creating enough pulse energy to engrave on metal surfaces without post processing,’ Gäbler said. ‘Staying below a certain energy level, you can ablate surfaces well without needing post processing.’

The cost of ultrafast lasers is relatively high, according to Röhner. ‘There is a big market in the micromachining industry for femtosecond laser applications, but the number of systems sold into this market is very low at the moment,’ he said. ‘This is due to the lower productivity and higher investment price for the system. It always comes down to the question: will a customer accept the higher price for much better cut quality?’

Hay at Powerlase Photonics noted that progress has been made in UV pulsed solid-state lasers. Powerlase offers its Rigel u180 system, which operates at 355nm and provides power scaling to around 200W. This performance improves the throughput of existing solid-state UV processes and means some high volume excimer applications can be replaced.

‘High energy ultrashort pulses are much more straightforward to produce in a solid-state design, with uptime increased and cost of ownership greatly reduced compared to excimer lasers,’ Hay commented, adding that the combination of both high average power and high peak power yields results that are not achievable with fibre designs.

In terms of productivity, this comes from speed and speed comes from the repetition rate of the laser. ‘For most systems, repetition rate is limited by the motion system, by the linear or rotating stages,’ commented Röhner. ‘Normally, the cutting geometry is not in a straight line; it’s a complex 2D structure which limits the processing speed. With scanning heads, you can achieve much higher speed and productivity but you are also limited to the acceleration of the scanning head.’

Applications with USP lasers operate by a different mechanism to thermal processes where the material is melted. Therefore, for USP lasers, fluence is more important than power. ‘A certain fluence level is required, but too much fluence can also reduce the quality,’ Röhner explained.

Customer service

Solid-state lasers are now reliable and robust enough to be considered as industrial processing tools. ‘All solid-state platforms – disk, fibre or diode lasers – of the well-established laser suppliers are recognised as industrially proven,’ commented Trumpf’s Schneider.

Now it’s a case of making the devices easy to use and maintain. ‘Laser system end-users want to treat the laser more or less like a drilling machine,’ remarked Röhner. ‘In the last five years the end-user expects to have a machine they can plug in, begin using, and get good results. This is a huge challenge for laser and machine suppliers.

‘It’s not only the laser or control, but also the application,’ Röhner continued. ‘Drilling or cutting with a laser at the wrong frequency or power level won’t produce good results. You have to understand the application.’ Jenoptik has 14 years of experience producing disk lasers and has delivered more than 20,000 disk laser systems in the field to date.

Trumpf is also improving the usability of its lasers. The remote services functionality of the company’s TruControl platform, for example, provides an internet service facility for troubleshooting issues with the system and therefore ensuring a high laser up-time.

Gäbler at Coherent commented that customer experience is important. ‘We’re delivering just the laser component, which compared to the full machine is a fraction of the cost, but still a big item on the bill of materials. In the end the business model of our customers is billed such that they want to have full responsibility of serving the end-customer. That’s important, because it’s a revenue stream for them. We want to make sure that our customers are educated and capable to work on our [Coherent’s] lasers by themselves. This is something that’s important for integrators, because that makes their life easier.’

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