INDUSTRY PERSPECTIVE TECHNOLOGY FOCUS

THE ND:YAG From a rod to a disk Paul Seiler, Klaus Wallmeroth and Kurt Mann

The Nd:YAG was one of the fi rst ever industrial , and even today it still has many advantages over other laser technologies. Competition from newer laser technologies, however, has made its evolution critical to its survival.

Th e year 1964 is recorded as being the birth In contrast, the active medium of a disk of the Nd:YAG laser. Made from an yttrium laser is a YAG crystal disk that is 0.1 mm aluminium garnet (YAG) crystal doped thick and around 15 mm in diameter. with , Nd:YAG has optical Because the disk is very thin it can be and mechanical properties similar to ruby, cooled effi ciently, and is therefore able to which was used in the fi rst demonstration deliver excellent beam quality. Ytterbium of lasing in 1960. However, Nd:YAG has is used as the doping element because a much lower excitation threshold than it can be strongly concentrated without ruby and can therefore be operated at room causing quenching, which negatively aff ects temperature in continuous-wave (CW) mode. the amplifi cation and occurs for doping Th is represented a huge breakthrough in the elements such as neodymium. Today, disk development of laser technology. Th e optical TRUMPF lasers with 4 kW of power from each disk resonator and pump arrangement (including With its ability to weld and cut many diff erent are commercially available. Th e output the lamp, refl ector and electrical supply) materials, the Nd:YAG laser has become the power of disk lasers can be increased by were well known from earlier workhorse of many industries. putting additional disks into the resonator, technology, and this allowed the ruby rod to and this does not change the beam quality of be simply replaced by the Nd:YAG rod. the system. Today, YAG lasers — doped not only been introduced for applications involving Although fl ashlamp-pumped pulsed with neodymium but also with other rare high-precision processing. Nd:YAG lasers are still widely used for precise earth elements — are used as tools in Advantageously, the use of fi bre-optic spot- and seam-welding, diode-pumped materials processing, ranging from precision guiding made it possible to separate the laser fi bre lasers are now entering this domain. engineering at the microscale to the welding from the processing location. Th is means that For CW applications, diode-pumped fi bre of ship panels. But these lasers are not just one laser can be used in multiple locations, and disk lasers have completely exceeded the signifi cant for materials processing, even either simultaneously, by splitting the laser capabilities of fl ashlamp-pumped rod lasers, though this is their primary focus; they into multiple fi bres, or by time-sharing, with and their performance is improving all the are also used in the medical industry, in the help of an optical switch. Connecting the time. Most applications today need a laser measurement technology and in the military. laser to a robotic head via the fi bre optics power of less than 10 kW, but these new laser Th ere are three properties that make this enables 3D processing, with the laser moving designs are ready for applications that require type of laser particularly useful: the high around the object. power levels of up to 100 kW. For these high amplifi cation of laser light; the option to Th e kilowatt CW YAG laser was adopted outputs, however, the optical components for store energy in the active medium; and the for use in large-scale industrial applications beam transmission and focusing will need wavelength of operation (1,064 nm). in the early 1990s, but this would not have to be improved, and this will happen over Single-rod YAG lasers allow pulses of been possible without the fl exibility of fi bre- time. Fibre and disk lasers have recently been several kilowatts in power and durations of optic delivery. Today, these lamp-pumped fi nding important applications in the domain several milliseconds to be produced. Although multi-kilowatt lasers are still running very of CO2 laser cutting. the fi rst lasers had average outputs of less than successfully in the automotive industry. In the next few years, the diode laser will 10 W, the high level of amplifi cation off ered by Nevertheless, new solid-state laser concepts probably oust the fi bre and disk laser as the YAG made it possible to produce small spot- based on diode pumping — the fi bre laser most popular CW laser, at least at output welds. Today, pulsed lasers for spot welding and the disk laser — have been developed ranges of up to several kilowatts. Investment have average outputs of 20–500 W for a since the mid-1990s, and are now proving costs and energy consumption will be the multitude of applications. Another interesting highly successful. deciding factors here. However, if a high feature of early pulsed lasers was the A fi bre laser generates a diff raction- beam quality and laser power is required, introduction of Q-switching — a shuttering limited beam by using doped monomode fi bre and disk lasers will remain the tools scheme for the creation of large laser silica glass fi bres as its laser active material. of choice. ❐ pulses — for distance measurements in the Th ese fi bres have a core diameter of military. Q-switching is able to provide pulses 10–20 μm, and are doped with rare earth Paul Seiler and Klaus Wallmeroth are at TRUMPF with powers in the megawatt range (with elements such as ytterbium or erbium. Laser, Aichhalderstr. 39, 78713 Schramberg, corresponding nanosecond pulse durations), High-power output is realized through a Germany; Kurt Mann is at TRUMPF Laser and and was later used to write information on combination of oscillators and amplifi ers, and Systemtechnik, Johann-Maus-Strasse 2, 71254 metal/plastic media. More recently, ultrashort- through the parallel coupling (fi bre bundling) Ditzingen, Germany. pulse YAG lasers in the picosecond range have of those arrangements. e-mail: [email protected]

NATURE PHOTONICS | VOL 4 | MAY 2010 | www.nature.com/naturephotonics 285

© 2010 Macmillan Publishers Limited. All rights reserved