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High-power diode lasers modify surfaces directly

07 Sep 2007

Direct applications of diode lasers for materials processing are receiving a great deal of attention. Tim Hayes spoke to John Haake of Coherent about the company's entry into this market sector and its HighLight high-power diode laser for surface treatments.

Coherent's acquisition of Nuvonyx in May signalled the company's move into the materials-processing market, particularly direct-diode applications. The deal was the perfect marriage, as it combined Coherent's expertise in producing laser diodes with Nuvonyx's knowledge of packaging and assembly of high-power laser diodes. The net result is that laser-based technologies now look set to displace traditional techniques in applications such as welding, heat treatment and surface cladding.

One product that Coherent is now pioneering is the HighLight system, which delivers 1–4 kW of output power. According to Coherent, this is a big step forward from previous levels and comes courtesy of Nuvonyx's specialist knowledge of diode laser technology.

The platform produces a continuous-wave, single wavelength 808 nm beam from a compact laser head weighing just over 7 kg that can be mounted on a robot or a gantry system if desired.

"HighLight uses multiple diode bars, incoherently combined," John Haake, Coherent's director for applications engineering and product management told OLE. "There's no coherency from emitter to emitter and indeed no coherency across the laser cavity." The system includes a beam-shaping free-space optical delivery system, along with a control unit and a chiller.

Reliability and quality
Reliability in a system of such complexity and power was a challenge, Haake readily concedes. By addressing the possible causes of failure at every stage throughout the cooling system, the electrical distribution system and the optical system, the reliability has been raised to levels that high-power diodes have not demonstrated before. "That is HighLight's biggest achievement: making a high-power system that's actually useable and practical on a shop floor," Haake commented.

Diode sources are usually regarded as producing low-quality beams, a view that Haake would not disagree with. But not all applications require a beam of the highest quality or pristine roundness. He points out that some surface-treatment applications can actually benefit from the use of a line source. Most laser specialists would regard such a beam as low quality, but in practice the less-localized irradiation keeps the beam's power density below the value at which the surface will melt.

High-power diodes particularly suit applications such as laser cladding, heat treatment, welding and brazing. These are applications where, in general, the more direct laser power that can be brought to bear, the better, but they do not necessarily require a perfect quality beam. "Laser heat treatment, for instance, is a kinetic, dynamic process that's never depended exclusively on the beam quality," noted Haake. "In practice, the need for a pristine spot becomes less significant in these kinds of applications."

In a laser-cladding operation on a stainless-steel alloy, such as 316L or 420, the beam profile can paint the surface in a continuous line as the cladding material is deposited onto the surface. This provides lower dilution and smoother cladding surfaces compared with other processes, and is said to be significantly more efficient that similar operations performed with a Nd:YAG or CO2 laser because of the natural beam shape and the shorter wavelength.

High-power diodes can provide similar advantages for welding applications, typically achieving process speeds of three times faster than tungsten inert gas or plasma transfer arc laser welding. There is less distortion and no post-processing required, and the line beam allows the filler wire to achieve the optimum weld beam geometry. The result can be a faster production line without changing upstream sampling or fixturing processes.

Economics Coherent claims that taking all factors into account, including electricity usage and mains links, the use of high-power direct-diode lasers can lead to a substantial cost saving – potentially reducing it by a factor of four compared with CO2 lasers. "As well as letting end-users speed up their production lines and save money, HighLight can bring something novel to treatment processes, introducing technological improvements and allowing surface treatments that were not previously possible," said Haake. "Surface modification processes are of great interest since wear, friction and similar issues are usually surface effects in man-made objects. Altering just the surface layers can introduce some novel properties that can be readily exploited."

Other recent developments in industrial production are a good fit with the new laser capabilities, and Haake believes that advances in robotics and automation have been key factors. "Without industrial robots the precision needed for laser processing of materials would be too great, and the delicacy required would be too demanding," said Haake. "It is a combination of elements, not just one development standing alone, that have allowed high-power diode lasers to develop their niche in direct materials processing."

Acquisition The Nuvonyx high-power diode technology behind the HighLight platform represented a good fit with Coherent's plans for business development, but Haake believes that both parties benefit from the acquisition. For Nuvonyx, the biggest advantage is access to the global market, through a network of salesmen and distributors that the company previously lacked, backed by Coherent's experience in operating on an international level. From Coherent's perspective, the acquisition will allow the company to address the high-power industrial laser market in which it has previously had little presence. "Coherent has resources and know-how in the epitaxy and how it affects the beam quality, while Nuvonyx offers expertise on the assembly and packaging side and how to put kilowatt lasers together in a stack to form a multikilowatt source," commented Haake. "I think Coherent will benefit from that knowledge and from our position in the heavy capital market for industrial laser products."

The HighLight platform won't carry the Nuvonyx name but it will be branded as a Coherent product, which Haake believes is proof of Coherent's strong belief in the product. "Carrying the Coherent brand into the industrial markets where Nuvonyx is strong is a significant statement of intent. Coherent want to announce their presence, and HighLight will certainly do that."

• This article originally appeared in the September 2007 issue of Optics & Laser Europe magazine.

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