Fiber Lasers Scale Up
Fiber lasers undergo a series of upgrades in the quest to become the preeminent industrial laser technology.
Long in the shadows of their conventional solid-state counterparts, fiber lasers (FL) have undergone a quiet revolution with respect to their design and overall performance in recent years. First marketed for optical telecommunications in the 1980s, today these compact, silica-based lasers are finding new lifeblood in industrial applications, with a majority finding an audience within original equipment manufacturers' product lines, primarily for marking, welding, and cutting routines.
a) First conceived by Elias Snitzer in 1963, the fiber laser has evolved from its initial applications in optical communications to industrial manufacturing. b) Changes in the diameter of the fibers and actual designs, such as the jacketed air clad fiber from the Univ. of Southampton, UK, are being put forth in an effort to reduce nonlinear effects.
But this may only be a starting point. The most recent optoelectronics forecasts project sales of fiber lasers to nearly double in 2005 to an estimated $120 million. This outlook, coupled with a series of performance milestones, is leading FL developers to think big. "With its inherent qualities over conventional solid state lasers, I cannot see how fiber lasers will not eventually dominate the industrial market," says Bill Shiner, Manager of Industrial Product Development at IPG Photonics, Oxford, Mass.
Fiber up sides
FL developers note that this drive for market position is being fueled by the distinctive features that fiber lasers offer, namely size, beam quality, and optical efficiency. The fact that fiber lasers are able to operate in the 1 µm wavelength range of the optical spectrum, stemming from the inclusion of rare earth elements within their cores, has enabled them to become serious competitors to the larger, more power consuming solid state and CO2 lasers used in manufacturing schemes.
"Fiber lasers are inherently much higher efficiencies than lamp-pumped or even semiconductor- pumped classical lasers," says Martin Seifert, CEO/ President of Nufern Inc., East Granby, Conn., a leading supplier of optical fibers. "Optical efficiencies of ~70% are the norm for ytterbium-doped fiber lasers, with 'wall plug' efficiencies > 30% vs. 1 to 2% for flash lamp-pumped rod lasers."
A far more attractive quality has been the ability to obtain a near-diffraction-limited beam, M2 (beam quality) value nearing 1, from a FL system. While solid-state lasers boast larger amounts of raw power, the beams these systems deliver exhibit higher orders of diffraction, with typical M2 values nearly two orders of magnitude greater than those of FL, resulting in poor beam quality at large distances away from the source.
These combined features have provided the foundation for fiber lasers to claim a reasonable market share. Why they have not been able to take a decided foothold over the market, however, has primarily boiled down to one issue: power.
Resolving the challenges associated with increasing the overall power output while maintaining a single-mode beam has been at the heart of the most recent advancements coming from companies and university labs working in FL. Although multi-mode FL systems have considerably higher power, the development of more powerful single-mode continuous wave (CW) lasers has been a significant source of trial and error.
But progress has been made. In 2002, the highest recorded power output of a single mode CW FL was about 100 W. However, that level has increased by an order of magnitude in only two years, further narrowing the gap between FL and their solid-state counterparts. As recently as this past December, results produced by the Optoelectronics Research Centre at the Univ. of Southampton, UK, with funding by the U.S. Defense Advanced Research Projects Agency's two-yr-old high-power FL program, announced the creation of a single-strand mode, continuous wave fiber with a power of 1.36 kW and beam quality of 1.4. This makes it one, if not the most, powerful single-strand FL in the world with the potential to be scaled even further. According to the Southampton team, led by Johan Nilsson, "Power-scaling beyond 10 kW in a single-fiber configuration looks entirely feasible with our fiber laser design."
Even higher powers are now being realized with small trade-offs in beam quality by combining the outputs of multiple single-mode lasers. "The single mode lasers, with their perfect beams, are the engine for our multi-kilowatt parallel combined fiber lasers," says IPG's Shiner. Multi-kilowatt systems are currently being shipped that have been demonstrated in numerous applications to provide 50% higher processing speeds and depth of penetration over conventional YAG lasers and double or triple those produced by CO2 lasers operating at the same power levels.
The U.S. military's Airborne Laser (ABL) project recently reached a performance milestone with the successful ground-based firing of all six modules comprising its megawatt-class Chemical Oxygen Iodine Laser (COIL). Although the test duration was short (less than a sec), "what's important is that the COIL produced photons and this proves that the laser hardware is ready to go," says Ellen Pawlikowski, ABL Program Director.
It has been eight years since the project's start which began with a $1.1 billion contract being awarded to Boeing, TRW (now Northrop Grumman Space Technologies), and Lockheed Martin to develop a prototype ABL to attack theater ballistic missiles, which was later expanded in 2001 to include all ballistic missiles. Once completed, the laser system (operating at 1.315 µm), will be housed aboard a modified 747-400 freighter aircraft which will fly at an altitude of 40,000 ft. Tests of the COIL will continue for several months as engineers make adjustments to gradually increase the firing time.
>>More info: www.boeing.com/defense-space/military/abl/flash.html
"The fiber laser is achieving welding depths never before reached at the 1-µm level," says Shiner. Within IPG Photonics' line, for example, "our 10-kW fiber laser has achieved depths of penetration in aluminum of 14 mm at a processing speed of 2.5 m/ min. This kind of performance, combined with the portability, opens up a wide range of applications only previously imagined."
What has made the difference?
Industry insiders credit these new performance levels to better semiconductor diodes pump sources, the emergence of large mode area (LMA) fibers, and double-cladded fiber designs. With such high powers, non-linear effects in the fiber can impact the laser performance.
The solution given by the emergence of LMA fibers has been to scale up the physical diameter of the fiber itself to reduce the detrimental effects of various nonlinear interactions such as stimulated Raman and Brillouin scattering.
Nufern, for its part, has "promoted a 20-µm core 400-µm cladding fiber for modest power CW applications into the few hundred watt regime, but we have also made specialized laser fibers for a number of clients up to about 750-µm dia. At this time we feel that other technical issues will create greater limits than diameter beyond this range," says Seifert.
Recently, novel structures like photonic crystal fibers that rely on shaping the core of the fiber to facilitate the single mode operation, as well as reducing non-linear effects, are also being explored as a performance measure.
Down the road
With their performance levels increasing, fiber lasers have indeed carved a place within the industrial laser market. And efforts are continuing to make these systems even more powerful. "We anticipate a single-mode CW fiber with a 2 kW output to be ready by the first or second quarter of 2006," says Shiner.
Ultimately, a reduction in semiconductor diode pump costs and the next wave of available power outputs will dictate the success of this technology.
IPG Photonics, 508-373-1100, www.ipgphotonics.com
Nufern, Inc., 866-466-0214, www.nufern.com
Univ. of Southampton, +44-023-8059-3150, www.orc.soton.ac.uk