Thin disk sources get powered up

21 Aug 2003

After successfully entering the ophthalmology market, the next challenge for thin-disk lasers is materials processing. Trumpf and Rofin-Sinar are backing the technology with multikilowatt-scale systems, as Michael Hatcher reports.

From Opto & Laser Europe September 2003

Ever since Adolf Giesen and his colleagues at the University of Stuttgart's Institut fur Strahlwerkzeuge (IFSW) in Germany first developed them in 1994, thin-disk lasers have been touted as the ideal method of generating scalable high-power laser emission with excellent beam quality.

Companies such as Jenoptik, Nanolase and Elektronik Laser System have been selling relatively low-power versions of these lasers for more than five years, but only recently have systems builders at the high-power end of the market got in on the act.

This increased interest was confirmed at the Laser show in Munich earlier this year, where both Trumpf and Rofin-Sinar were eagerly pushing their new Yb:YAG-based thin-disk laser systems. Peter Leibinger, Trumpf's chief executive officer, believes that thin-disk lasers are the way forward: "We are absolutely convinced that thin-disk lasers will replace lamp-pumped rods in many applications," said Leibinger as he unveiled the company's 4kW prototype.

A key characteristic of thin-disk lasers - and an attractive one for systems builders like Trumpf - is their scalability. "We can build on the platform and add more and more power while maintaining high beam quality," commented Leibinger.

Kilowatt power Trumpf launched its 1kW disk laser in late 2002. This source, which has an M² value of 6 (Trumpf's continuous-wave lamp-pumped and diode-pumped 1kW Nd:YAG lasers both have an M² of 12), couples into a glass fibre with a core diameter of 150µm. The more recent 4kW version, which incorporates four separate disks, illustrates one of the problems facing manufacturers of high-power disk lasers.

Because a number of separate disks are required to produce such high powers, their cavities must be combined. However, to maintain good beam quality (the quoted M² of Trumpf's 4kW laser is 7), the cavity length must be extended. This was amply illustrated by the large size of the 4kW prototype on display in Munich.

Not to be outdone, Rofin introduced its own thin-disk sources at the show. Rofin's single disk produces 750W with an M² of 7 and the company has also developed a 1.5kW laser with an M² of 12 and a 3kW source that is delivered via a 300µm fibre.

The application areas that Trumpf and Rofin are targeting for their thin-disk lasers are traditionally the territory of lamp-pumped Nd:YAG lasers. Their move into higher-power systems means that the advantages of thin-disk lasers can now be exploited in a huge range of applications, from those that need only a few watts of power right up to 4 kW requirements.

New field of applications Rofin is aiming for both small- and large-scale materials processing applications with its sources, specifically welding of thin materials and use in conjunction with scanner optics for high-speed remote operation. The company's macro division has built a robot-controlled scanner welding system that incorporates a thin-disk laser and takes advantage of its high beam quality over a large working distance. This system was developed in collaboration with the Fraunhofer Institute for Material and Beam Technology in Dresden.

Trumpf is similarly keen to exploit the beam quality of thin-disk lasers in this way: "The laser opens up a whole new field of applications in remote welding," commented Leibinger in Munich.

Thin-disk lasers should be of particular use in welding aluminium and cutting thick sheet metal. This is because in aluminium processing the welding threshold can be reached with a lower laser power than is offered by conventional sources, as the photons are concentrated onto a smaller focus. For cutting sheet metal, the higher beam quality translates to a faster cutting speed with thin sheets, while for thick sheets there is an improvement in working distance and welding depth.

Rofin and Trumpf's lasers are based on Yb:YAG disks, as are almost all such sources. But one company, Jenoptik, is not using this material.

Jenoptik, which showed the first thin-disk prototype in 1997, has enjoyed great success in ophthalmology for retinal photocoagulation with the green source that it launched in 2000. More than 2000 of these lasers are now in the field, and Jenoptik donated its 1000th source to Giesen himself.

Jenoptik's lasers feature neodymium doping rather than ytterbium, in either a YAG or YVO₄ matrix. According to Günter Hollemann, product manager in the laser business unit, this is because neodymium doping allows the laser to be air-cooled, whereas ytterbium lasers must be water-cooled. "Ytterbium is a quasi-three-level laser with reabsorption losses at high temperatures," he explained. "By contrast, the neodymium-doped material used in thin-disk geometries can be conductively cooled by simple, robust air cooling."

In the Nd:YAG scheme, a reasonably high pulse energy can be achieved in Q-switched mode, at a range of repetition rates that are useful for marking applications. The main application of the Nd:YVO₄ crystal, which emits at 1064 or 914nm, is in frequency conversion: "Its linearly polarized output beam and very high stimulated emission cross-section lead to a low laser threshold and high optical [conversion] efficiency," said Hollemann.

Jenoptik's JenLas.D2 source can produce up to 12W at the 532nm second harmonic, with a remarkable diode-green efficiency of 30%. Because of the high absorption, efficient optical conversion is possible using only a four-fold pumping scheme (Yb:YAG lasers usually use 16-fold pumping), resulting in a simpler optical configuration.

At this year's Laser show, Jenoptik introduced its new blue laser based on a frequency-doubled Nd:YVO₄ disk. It features an 800mW output at 457nm, and has been designed for laser show and display applications. The firm has also enjoyed some success in the marking field and special printing applications, with the German company Tampoprint using Jenoptik's JenLas.D8 laser in one of its computer-to-press printing machines.

Also launching its thin-disk sources in Munich this year was PrenovaTec of Germany, which sells lasers with output powers of 10-100 W. The firm is targeting OEMs, end users and systems builders alike for applications such as microwelding, laser sintering and micromachining.

Company representative Thomas Kaiser says that different pulsing options are a key feature of PrenovaTec's lasers. Continuous-wave, diode-modulated and Q-switched lasers are all on offer. "Applications include welding of thin metal plates for the watch industry, where it is necessary to use diode modulation with long starting and finishing ramps for pre- and post-warming of sections. This reduces the occurrence of microcracks," Kaiser told Opto & Laser Europe.

Another area that PrenovaTec is targeting is the medical device sector. It claims that its Q-switched lasers are ideal for cutting and structuring the metal alloys used in stents. Stents - devices used to aid blood-flow by holding open arteries - take the form of a complex mesh, usually made from stainless steel. Fibre laser manufacturers are also targeting this application area and the clash is indicative of the competition between these two new forms of solid-state laser in certain sectors.

Kaiser believes that thin disks have some key advantages over their fibre rivals: "Thin-disk lasers don't have the problems with back reflections that fibre lasers do, especially in microcutting and microwelding applications," he said. "These reflections are amplified inside the fibre, which can cause damage."

Ultrafast challenge next One of the next steps for thin-disk technology is to move into the ultrafast arena. Hollemann says that Jenoptik would be interested in commercializing such lasers, and this is precisely the area on which Giesen is now concentrating. In collaboration with Ursula Keller and her team at ETH Zurich, Giesen recently made the world's first passively modelocked thin-disk source, based on Yb:KY(WO₄)₂. It produces diffraction limited (M²=1.1) 240fs pulses at 25 MHz with an average power of 22W.

Thin disks certainly sound like a major part of Trumpf's plans: "The disk laser is the solid-state laser concept of the future," claimed the firm. That such a wide range of these sources is now available is good news for Giesen and the IFSW, who stand to reap financial rewards through technology licences. It also demonstrates the quality of innovation in European optics.