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Laser types for micro welding: selecting the right laser.

Four types of lasers can be used for micro welding: pulsed neodymium-doped yttrium aluminum garnet (Nd: YAG), continuous wave fiber, quasi-continuous wave (QCW) fiber, and nanosecond fiber. Each type offers unique features that work best for specific applications. In some cases, several options may work: that’s when a cost of ownership and service -ability can tip the scales.
With the Nd: YAG laser, the active gain medium is neodymium, which is doped into a host crystal of yttrium aluminum garnet. This solid rod of material is typically 0.1 to 0.2 inch in diameter and about 45 in long. Micro welding Nd: YAG  lasers are optically pumped using flash lamps, they emit light which a wavelength of 1,064 nm, but can be frequency doubled (532 nm) to appear green. With the excellent pulse control, the Nd: YAG laser also offers high peak powers in small laser sizes, which enables welding with large optical spot size. The pulsed Nd: YAG laser is suitable for spot welding applications with less than 0.02-in. penetration and seam welding of heat sensitive packages.
A fiber laser is generated within a flexible doped glass fiber that typically is 10 to 30 feet long and 10 to 50 microns in diameter. Ytterbium is used as the doping element because it provides good conversion efficiency and a near 1-micron output wavelength, which matches well with existing laser delivery components.
The efficient lasing process allows the fiber laser to be small, air-cooled and offer high wall plug efficiencies. The fiber laser unique characteristics are its focusability and it’s beam qualities that can be fine-tuned for each welding application. The two ends of the beam quality spectrum are single mode and multimode. Single mode is defined by a beam quality of M2  less than 1,2, while multimode generally is above M2 of 2.
For high-speed seam welding applications, the fiber laser is operated in CW mode. In other words, the laser output remains on until it is turned off. For spot welding either a single weld or seam, the laser output can be pulsed or modulated, which means the laser is turned on and off rapidly. Cw fiber lasers are suitable for general seam welding up to 0.06 in. deep for a 500-W laser, high-speed seam welding of same and dissimilar materials, and producing spot welds less than 100 microns in diameters.
Quasi-continuous wave fiber lasers peak power and pulse width characteristics are similar to those of the Nd: YAG laser through the parameter range is not quite as broad. Similar to CW fiber lasers, the QCW lasers offer single mode to multimode options with spot sizes from 0.001 to 0.04 in. These lasers also shine in small spot size and small penetration applications, although they do offer fairly comprehensive coverage of many micro welding applications.
Nanosecond fiber laser typically used for laser marking applications can be repurposed for certain welding applications. It provides multi-kilowatt peak power, but with the pulse width of 60 to 250 nanoseconds that can be delivered between 20 and 500 kilohertz. This high peak power enables welding of almost any metal, including steel, copper, and aluminum. The very short pulse widths enable very fine control for welding small parts, as well as the ability to weld dissimilar materials.