Fiber lasers combine active (laser-gain) optical fiber with one or more pump lasers, usually laser diodes. The many types of fiber lasers include low-power continuous-wave (CW) and low- and high-energy pulsed, including ultrafast fiber lasers. But what comes to mind for many people are the “big guns”—the kilowatt-class CW fiber lasers that are predominantly used in materials processing, including cutting, welding, brazing, surface treatment, and other applications, but are also being developed for the military as directed-energy weapons. A fiber is essentially a very skinny, long rod—its configuration makes it one of two types of lasers that have an especially high surface area-to-volume ratio, making them easier to keep cool (the other is the disk laser—a wide, extremely short rod). Fiber lasers are relatively simple in their construction and are easy to maintain. They are compact and, because they are pumped with laser diodes, rugged and long-lived. Alexei Markevitch, market development manager at IPG Photonics (Oxford, MA), outlines the range of wavelengths and powers available for kilowatt-class fiber lasers. “IPG manufactures standard kilowatt-class CW las...