Powering up the Pulse

After the international laser technology congress AKL, in Aachen in May, Matthew Dale learned that dramatic increases in power are on the horizon for ultrafast lasers

Ultrafast lasers are on the verge of a major upgrade – one that will see them reach kilowatt average powers and allow them to enter large-scale, high-throughput industrial applications.

That was one of the key messages at the international laser technology congress, AKL, in Aachen this year.

Speaking at the conference in May, Fraunhofer Institute of Laser Technology (ILT) director Professor Dr Reinhart Poprawe explained that the surface texturing applications of ultrafast lasers are currently costly and can only be carried out on relatively small areas.

‘How can we scale this up to the macroscopic world, is that at all possible?’ he asked. ‘We believe that this is the case. Ultrafast lasers, based on their high precision, will be scaled into the macroscopic world.’

Officially announcing the launch of a new Fraunhofer cluster of excellence called ‘Advanced Photon Sources’ at the conference, Poprawe explained how, through a €20 million project, 12 Fraunhofer institutes, led by the ILT in Aachen and the IOF (applied optics and precision engineering) in Jena, will be dramatically ramping up the power of ultrafast lasers into the multi-kilowatt range over the next four years.

With average powers slightly under 2kW already currently achievable at the institute in the sub-picosecond range, according to Poprawe, in two years’ time the institute plans to have developed ultrafast lasers with 5kW average power, with the ultimate goal being to reach 20kW by 2022. ‘There are no physical laws between us and this aim, so it should be possible to demonstrate this,’ he remarked.

A glass automotive dashboard cut using Coherent's HyperRapid NX industrial 100W picosecond laser

Once the average power of ultrafast lasers has been successfully scaled to the multi-kilowatt range, through the use of innovative multispot technology – in which a diffractive optical element or spatial light modulator can be used to split a single, powerful laser beam into a large array of multiple beams with less pulse energy – ultrafast lasers will be able to process larger surface areas of materials at increased throughputs without inducing thermal damage. This could open up a whole range of applications on the macro scale.

One such application highlighted at AKL was equipping wind turbines with surface structures that prevent the build-up of ice and insects on their blades – both of which have been proven to reduce the efficiency of the turbines. Other possibilities include drilling billions of holes into the aerofoils of aeroplanes in order to reduce drag during flight, creating antimicrobial surfaces on ship hulls to prevent the attachment of large quantities of algae.

Ramping through amplifying

Both Fraunhofer ILT and RWTH Aachen University spin-off Amphos – the manufacturer of high power ultrafast lasers that was acquired by Trumpf at the end of last year – are using InnoSlab amplification technology to dramatically ramp up the power of their ultrafast lasers. InnoSlab amplifier systems are characterised by a very simple beam path, and require no chirped pulse amplification and no regenerative beam path in order to achieve large amplification with high efficiency.

‘The InnoSlab amplifier concept uses a cooled, slab-shaped Yb:YAG crystal that is usually 10 x 10mm in size and offers properties such as low heat generation, high quantum efficiency and the ability to support pulse durations of less than one picosecond due to its broad emission spectrum,’ explained Amphos managing director Dr Torsten Mans in his presentation at AKL. Mans developed the world’s first InnoSlab ultrashort pulse laser system 10 years ago and was a member of the high power fs-InnoSlab team at Fraunhofer ILT.

In order to generate average powers exceeding 500W, the InnoSlab system can be scaled by broadening the width of the Yb:YAG crystal, according to Mans, which can be done without reducing beam quality or increasing the demand on brightness from diode lasers. Power scaling can also be achieved by using multiple InnoSlab amplifiers, with Amphos having used a chain of three amplifiers in the past to produce an ultrafast system for the European XFEL facility, with a burst mode of 20kW. For scientific customers such as these, Amphos combines an InnoSlab amplifier chain with an Amphos 400 pulse source and a compressor particularly suited to kilowatt powers, to deliver systems with average powers exceeding 1kW.

While using multiple InnoSlab amplifiers enables a dramatic increase in the average power of an ultrafast laser, doing so increases the footprint of the system considerably. ‘It’s not very compact,’ confirmed Mans. ‘You need an optical table in order to adjust and align everything accordingly, but it’s rather flexible, so you can fulfil special requests and changes easily. This concept is focussed on offering these [high-power] parameters for users in their labs.’

Amphos aims to reach average powers of 1.5kW using InnoSlab amplification later this year or in 2019. The firm then plans to integrate all the components of its kilowatt average power systems into a single housing – which, according to Mans, is a prerequisite if the laser is to one day be used for industrial applications.

Higher powers for industry

In the meantime, before ultrafast lasers with kilowatt and multi-kilowatt average powers emerge from the lab and become available to the wider industry, ultrafast firms are already in the process of ramping up the average powers available in their product offerings for industrial applications.

Standard ultrafast lasers currently on the market can offer anywhere between 5 and 50W of average power. However, ultrafast company Amplitude Systèmes vice president of sales, Vincent Rouffiange, pointed out that a new peak of sales is emerging in the latest generation of ultrafast lasers capable of offering 100W of average power. Here, the high quality and low thermal damage of ultrafast lasers can be offered in addition to high throughput, making this power range well-suited to high volume industrial applications.

Amplitude Systèmes’s Tangor is such an industrial ultrafast femtosecond laser capable of exceeding 100W average power, and is suited to applications such as micromachining and manufacturing medical devices and microelectronics. The company also has 350W femtosecond lasers in its R&D pipeline, according to Rouffiange, with his colleague Dr Clemens Hönninger adding, in an AKL presentation, that multi-hundred-watt class ultrafast lasers, such as these, are expected to enter mass production very soon. Additionally, in three years, Rouffiange continued, the firm also plans to have used slab technology to achieve kilowatt average powers with its ultrafast lasers, as is being done by the newly announced Fraunhofer cluster of excellence.

The XLScan system enables wide-area marking and processing of large substrates by extending the working field of a galvanometer scanner using an xy-stage

‘There is no doubt that the direction is going that way in the market,’ Rouffiange commented. ‘What drives the market towards higher average power is the increased throughput of these ultrafast lasers.’

Manufacturing consumer electronics and displays are both increasingly being served by high average power ultrafast lasers, particularly for cutting flexible OLED displays. This is because the ultrashort pulse duration limits the heat affected zone during processing, while the increased throughput offered by the high average power enables millions of displays to be cut each year to serve the smartphone market.

‘This is a huge application in terms of quantity,’ Rouffiange remarked. ‘Ultrafast lasers for this type of application tend to be in the UV domain. There is definitely a trend underway in using short wavelengths.’

Coherent’s ultrafast UV offering, the HyperRapid NX, which offers over 100W of average power at picosecond durations, is being used in the production of automotive and smartphone displays to focus beneath the surface of hardened glass substrates and scribe them without causing damage to the surface. ‘In the past, these lasers were limited to “high value” applications that could justify the higher cost of short-pulse lasers,’ said Florent Thibault, product line manager for the HyperRapid NX.

‘However, in recent years there has been continuous progress in the power-to-cost ratio of picosecond lasers, and a corresponding broadening of the market.’ Picosecond lasers can now be used to perform diverse precision applications, such as high-value marking, surface structuring for friction reduction, photovoltaics manufacturing, LED dicing, and thin film applications.

Even with picosecond lasers now driving an extensive range of high-volume applications in industry, a growing number of ultrafast users are switching over to femtosecond lasers for their industrial applications, according to Amplitude Systèmes’s Rouffiange. ‘This is something we have really been noticing over the past year for high average power systems,’ he said. ‘There is a real trend underway. The shorter pulse duration offers better machining quality and a larger parameter range in terms of flexibility of repetition rate and energy than that offered by picosecond durations.’

Michael LaHa, product line manager of Coherent’s Monaco femtosecond lasers, said a completely new generation of powerful femtosecond lasers based on ytterbium fibre – of which Amplitude Systèmes’s Tangor also belongs to – has been developed in recent years, offering higher average power at a lower price.

‘Prior to the development of lasers based on ytterbium fibre, the high cost and lower power of femtosecond lasers confined them to a niche role,’ he said.

‘Now we are seeing tremendous interest in these new femtosecond lasers from several industries, and because ytterbium fibre is a fairly new technology, it has nowhere near reached a performance plateau; output power is continuing to increase, and with it, the application space.’

Such femtosecond lasers can be used for precise cutting, drilling, scribing, marking and surface modification of materials such as glass, polymer, ceramic, thin film and stainless steel. 

Scaling up scanning

One way of boosting the average power of an ultrafast laser is by increasing its repetition rate, for example a 10W system would have to be increased from a 500kHz repetition rate to 5MHz in order to become a 100W system. Although in doing this, the processing throughput of ultrafast lasers can be increased, a suitably fast scanning system must now also be used in order to maintain pulse overlap, and therefore quality during processing.

It becomes clear the scan system is the bottleneck in achieving higher ablation rates [with ultrafast lasers], since the ablation efficiency peaks at a specific energy density and the pulse overlap should not exceed certain values to avoid thermal effects,’ said Dr Holger Schlueter, business developer at scanning solution specialist Scanlab. ‘There is a multitude of solutions being developed, however, that enable higher scan speeds on the work piece.’

High speed galvanometer scanners are recognised as the most flexible solutions currently available on the market, as unlike polygon scanners, which are limited to line-oriented processing, they enable processing to be carried out in any desired shape. According to Schlueter, one of the key advances in scanning technology in recent years has been the development of ‘SCANahead technology’, which pre-computes a set-point trajectory during processing in order to fully exploit the dynamic performance potential of a galvo scanner. With the technology, the axis acceleration of the scanner is constantly maximised, which reduces corner-rounding and unwanted necking effects when producing sharp angles and circles at high speeds.

Although galvo scanners offer high speed, highly flexible laser scribing, they are inherently limited by their field of view, which is often no more than 100 x 100mm. Consequently, there is growing interest in combining high speed laser galvanometer scanners with larger motion platforms to enable flexible laser machining processes over an extra-large field of view.

‘Such systems enable ultrashort pulse processing of large substrates…and enable high throughput in combination with unprecedented accuracy,’ commented Schlueter.

A collaboration between Scanlab, Physik Instrumente (PI) and ACS Motion Control has therefore developed such a combined solution – the XL Scan (extra-large scan). The new process enables simultaneous control and movement of a galvo scanner and an XY-stage, which effectively overlays the small field of view of the scanner with the longer travel of the stage.

Using the new solution, throughput increases of up to 41 per cent have been achieved in processing applications, according to PI.

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