Fibre sources target automotive industry

07 Jan 2003

Advances in the reliability of laser diodes mean that IPG Photonics's fibre lasers now offer a real alternative to solid-state and carbon dioxide lasers for industrial materials processing. Michael Hatcher finds out more.

From Opto & Laser Europe January 2003

The boom years of optical telecoms may be long gone, but there's no doubt that some of the technological progress made during that time is now benefiting laser applications in some unexpected ways. One example of this is in the field of fibre lasers.

In better times, high-power, reliable 980 nm diode sources were rapidly developed by companies such as JDS Uniphase (JDSU) to satisfy the demand for sources to pump erbium-doped fibre amplifiers (EDFAs). Now that components like EDFAs are no longer the flavour of the month, those diode sources are being used instead to drive high-power fibre lasers, which are rapidly finding industrial applications in car-part medical device manufacture.

Fast-track development "The telecoms hiccup has allowed us to fast-track high-power fibre lasers," commented Bill Shiner, business development manager of IPG Photonics's industrial laser group. In a fibre laser, a doped silica fibre is excited by a diode source. Two Bragg gratings written into the fibre act like the mirrors of a "normal" laser cavity to generate the laser emission, which results in a compact source with excellent beam quality.

Until 2000, the highest power output seen from a fibre laser stood at 50 W. IPG then introduced its 100 W diffraction-limited source, which is based on ytterbium-doped fibres emitting at around 1.08 µm. By bundling these fibre lasers together, systems emitting 1, 2, 4 and - most recently - 6 kW have now been produced.

According to IPG, customer demand was so strong following its announcement of a 700 W system at last year's Photonics West show that the high-power lasers were fast-tracked into the development process. Having shifted its highly technical workforce from telecoms-focused work into high-power fibre laser development, IPG is already seeing its technology making inroads into the automotive sector.

Shiner says that just last month, IPG installed a 6 kW fibre laser unit at an undisclosed automotive plant in Germany. "Three months ago industry experts forecast that a multi-year development would be required to convince automotive and other major industries to accept this unknown technology," said Shiner, adding: "Large numbers of prospective customers are now lining up for pre-production tests."

During trials, the 6 kW air-cooled unit was integrated into a robot and used for welding and cutting steel and aluminium alloys used in car parts. According to IPG, the tests showed that fibre lasers could cut and weld faster than comparable YAG sources. Shiner is confident that these lasers will go on to make a big impact in a variety of applications: "The lasers have a 20% wallplug efficiency and are ideal for marking, cutting and welding," he said. "I really believe that they will revolutionize the industrial laser market."

The major advantages of fibre lasers correspond directly with the inherent weaknesses of the flashlamp-pumped solid-state Nd:YAG lasers that are widely used in industry today. For instance, whereas flashlamps need to be replaced on a regular basis, fibre lasers are pumped with such reliable diodes that it is rarely necessary to open them up and halt production. Because the laser cavity in a fibre laser is incorporated with the fibre, there are no problems with realigning the optics. And their use of air-cooling, rather than water-cooling, systems, their relatively compact size and their high-quality output beam all add to the list in favour of fibre lasers.

Klaus Kleine is head of the laser group at US-based medical device manufacturer Guidant Corp. Guidant makes vascular "stents" - metal implants that are used to hold open damaged arteries in patients with cardiovascular disease. Although some other stent manufacturers use a chemical etch to create the microstructures, Guidant, along with 95% of the industry, currently uses flashlamp-pumped Nd:YAG lasers to micromachine its stents.

The devices start off as simple stainless-steel tubes. Laser pulses are used to machine each tube into a finely-structured mesh. A balloon placed inside this mesh can then be inflated to hold a coronary artery open and restore normal blood-flow to a patient.

Kleine has been trialling an IPG fibre laser in stent manufacture to find out how it compares with the traditional flashlamp-pumped laser method. "The goal was to find a more reliable source," explained Kleine. He adds that the fine structures of the stents require an extremely small kerf width. "Ideally, you want a 10 µm kerf width with a large depth of focus to cut through up to 400 µm of stainless steel. The beam quality also needs to be good, with an M² value of 2 or better."

Other key laser attributes for the application are high peak-power pulses and a high repetition rate. Kleine points out that diode-pumped solid-state lasers are generally unsuitable because they tend to be Q-switched. This means that their pulse duration tends to be too short for micromachining the stents, which is best performed with 50-100 µs pulses.

Kleine used a 50 W IPG fibre laser to produce vascular stents. At a repetition rate of 1500 Hz and a 0.1 ms pulse duration, this produced a cutting rate of 4 mm/s. Surface roughness and quality inspection showed that the fibre laser performed as well as other lasers. The beam waist was 16 µm and the M² value of the beam quality was 1.1, corresponding with a kerf width of 18-20 µm in the stainless-steel samples.

According to Kleine, the fibre laser certainly has the upper hand in terms of reliability: "We have to replace the lamp in our Nd:YAG laser once a week on average. With the fibre laser, we think we should only have to replace the pump diode once in the equipment's lifetime."

Having discovered that the fibre laser produces stents of equivalent quality to those produced by other laser sources, the question for Guidant now is whether to replace its production-line Nd:YAG with a fibre laser. The outlook seems promising, since the initial capital outlay and maintenance costs certainly favour the fibre source. Kleine says that fibre lasers could also turn out to be useful in some microwelding applications, such as pacemakers, in which a number of tiny metal components need to be joined together.

The only technical drawback appears to be that the fibre laser is not particularly well suited to machining relatively thick samples. For this, Kleine needs a fibre laser that offers more power while retaining excellent beam quality and a longer depth of focus. "This would allow us to micromachine different materials, and also to machine thicker pieces of stainless steel - for example, to use in stents in different parts of the body." But IPG's sources show decreasing beam quality at these relatively high powers. The 1 kW source shows an M² of 8, whereas at 6 kW the M² increases to 22.

Another stumbling block could be the dearth of suppliers in the business. Of the few fibre laser manufacturers, it is only IPG that makes such high-power versions of the technology. Denmark-based Koheras, JDSU and IMRA America currently offer fibre lasers with sufficient power for small-scale micromachining applications and printing, but IPG is the sole producer of bundled systems that can reach the kilowatt power levels needed for heavy industrial applications. Customers may be reluctant to purchase equipment in a market of just one supplier.

Driving down costs JDSU, while not making such systems itself, is still very much involved with IPG's fibre lasers: broad-stripe JDSU diodes are used to pump the fibres. JDSU's fibre-laser product marketing manager Ruediger Hack told Opto & Laser Europe that the company is also working to increase the power output of its fibre lasers. "The next step is 50 W and 100 W models with an M² of 1," he said.

Although Hack also believes that fibre lasers will revolutionize industrial laser applications, he says that costs are currently too high. "Manufacturers need to work on driving down component and manufacturing costs, especially for high-volume applications," he said.

Shiner's view appears to dispute this. He says that the prices of IPG's fibre sources are comparable to those of Nd:YAG sources up to around the 4 kW mark. At higher powers, however, he admits that fibre lasers do become the more expensive option.

Not that this is dulling Shiner's optimism. "I think [fibre lasers] will become huge in the cutting market," he told Opto & Laser Europe. "I don't really see how we can lose - in a few years it should really dominate the YAG business, especially in areas like automotive welding. We will take on YAGs first, and then carbon dioxide lasers."

On top of this replacement market, IPG believes that the technology will also open up completely new application areas that are untapped by current YAG and CO₂ sources, including the aerospace, shipbuilding, oil and gas pipeline industries.