Bill Shiner
Significant numbers of high-power fiber lasers, up to 6 kW, are now being installed in the material processing market by IPG Photonics, which has it's headquarters and factory in Oxford, MA, and two other manufacturing plants in Europe.
The heart of the company's technology is proprietary active fibers and a patented pumping technique that allows the utilization of broad area multimode diodes rather than diode bars. This leads to a projected diode lifetime of more than 100,000 hours of operation.
A device may be made from coils of ytterbium-doped multiclad fiber with an emission wavelength of 1.07 to 1.08 microns. Alternately it may be thulium doped with a wavelength of 1.8 to 2.0 microns or erbium doped with a wavelength of 1.54 to 1.56 microns.
The diode pump energy is delivered to the active medium via multimode fibers that are spliced to the multiclad coil. The laser cavity is created directly in the active fiber. The laser emission exits the fiber laser through a passive single-mode fiber typically with a core diameter of 6 microns (see Figure 1).
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The resulting laser beam is essentially diffraction limited and, when outfitted with an integral collimator, produces a beam that is extremely parallel. For example, the 100-watt single-mode fiber laser has a full angle divergence of 0.13 milliradians at half angle when collimated to 5 mm diameter.
The maximum power from an industrial single-mode IPG fiber laser module is currently 200 watts. Higher powers are produced using multiple modules. The emissions from lasers are collected using a proprietary beam combiner, resulting in a single high-quality beam. For example, a 1-kilowatt unit would be made up of 10 individual fiber lasers integrated into a common cabinet. Although the beam is no longer single-mode, the resulting M2 of 7–10 is better than high-power solid-state lasers. The beam from a 6kW fiber laser can be delivered via a 200- to 300-micron fiber. Different output beam profiles, including a near rectangular shape, can be produced.
The ytterbium fiber laser has a wall plug efficiency of 16 to 20 percent. Erbium and thulium fiber lasers demonstrate lower wall plug efficiency but are still more efficient than typical YAG lasers. There are certain applications where these wavelengths are the best choice. IPG will complete its first 2-kilowatt erbium laser during the first quarter of 2003. The unit is being developed because of industry demand for a laser with the performance of Nd:YAG and eye safety better than CO2.
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The company's single-mode CW systems can be modulated to 50,000 Hz with pulse durations as short as 10 microseconds. Three super-pulsed versions with pulse durations as short as 1 nanosecond or pulse energies up to 1 milliJoule in a 100-nsec pulse and multimode CW versions from 300 watts to 6 kilowatts are now available.
Fiber laser benefits
Fiber laser technology offers several benefits to the industrial user. The footprint of a 4-kilowatt fiber laser unit is 0.5 m2 versus 11 m2 for a conventional lamp-pumped Nd:YAG, and there is no requirement for a chiller. They are essentially maintenance free during their entire lifetime because there is no need to replace flashlamps or diodes. The high electrical efficiency greatly reduces operating costs. Better beam quality allows the user to produce spot diameters substantially smaller than conventional lasers producing high fluence and/or longer working distances (1 kilowatt can be focused to 50 microns with a 4-inch lens).
The cost for fiber laser technology, up to 1 kW output power, is below or comparable with lamp-pumped YAG lasers. At this time the acquisition cost of a fiber laser greater than 1 kilowatt is higher. However, when all factors—floor space, chillers, maintenance and so on—are accounted for these lasers should be more cost effective than equivalent power rod-type Nd:YAG lasers.
Recent developments
During the last six months, several multi-kilowatt fiber lasers have been in Beta environments in European factories. These lasers have operated flawlessly on a multiple-shift basis, demonstrating their reliability and providing performance data only possible with much larger lasers. A 2-kW Beta unit has been lap welding 1.2-mm-.thick, zinc-coated automotive steel at 5 m/.min. The quality and performance is comparable to a 4-kW lamp-pumped rod type Nd:YAG laser.
A 2-kW fiber laser with a 300-micron final fiber diameter cuts 4-mm-thick coated steel at a rate of 10 m/min, with clean sharp edges. The maximum cutting speed attained is 16 m/min.
In the United States, IPG is installing the first two multi-kilowatt fiber lasers at Alabama Laser (Munford, AL) and EWI (Columbus, OH) for cutting and welding applications.
Industry veteran Bill Shiner is industrial market development manager with IPG Photonics, Oxford, MA. Contact him by telephone at (508) 373-1144 or via e-mail at [email protected]