Nd:YAG laser - terminology and functionality

An Nd:YAG laser is a solid-state laser whose main emission line is near infrared at 1064nm. The host crystal is yttrium aluminum garnet. Typically, 0.5% - 3% of the yttrium atoms are replaced by neodymium (“doping”). This type of laser was developed in 1964 in Bell Laboratories. In addition to CO2 and fiber lasers, it is one of the most widely used types of lasers for industrial material processing.

For lasers with average output power up to about 100 watts, the excitation takes place via diode lasers with a wavelength of 808nm (diode-pumped laser). The neodymium atoms are excited electronically, i.e. their electrons reach the higher energy level through absorption of the pump light. The energy stored in the excited atoms is released again by stimulated emission.

In the realization of a solid-state body as a laser medium, usually the biggest technological challenge is: This solid-state body must be high-purity but selectively doped and, at best, monocrystalline. It should also implement supplied energy with the highest possible efficiency, thus minimizing heat loss. A solid-state laser can be destroyed by its own gain at high pumping power. Therefore, the laser medium must withstand the highest possible energy density.

The notation Nd:YAG means that in an yttrium-aluminum garnet crystal (chemically: Y3Al5O12) yttrium ions (chemical symbol: Y) have been replaced by neodymium (chemical symbol: Nd). With the Nd:YAG laser, the doping (degree of replacement) is between 0.5% - 3%. The higher the degree of doping, the higher the laser power, but the lower the beam quality. This conflict of objectives is true for all lasers.

Energy is supplied optically, i.e. the Nd:YAG crystal is illuminated. The neodymium ions (Nd3+) are excited electronically. Krypton arc lamps, halogen lamps, xenon flash lamps, light diodes or laser diodes are used as pump sources.

In short, the latter are suitable light emitting diodes whose interfaces have been mirrored so that they form a resonator. Laser diodes are thus electrically pumped and extremely compact semiconductor lasers (these are a subgroup of the solid-state lasers). The high efficiency of the diodes themselves (from now on this designates LEDs and laser diodes), the precise controllability and the much longer service life compared to lamps, have greatly increased the popularity of the Nd:YAG laser, displacing the previously used lamp pump sources, at least in the lower power range.

The optical efficiency indicates the ratio of radiated laser power to supplied light power. While this is only about 2-4% for lamp-pumped YAG lasers, diode-pumped systems achieve optical efficiencies between 30-50%. The lower heat loss of the diode-pumped systems increases the service life of the crystal and reduces the cooling effort. In addition, they enable higher repetition rates (number of pulses per second) in pulse mode (e.g. Q-switch) and pulse peak powers.

Newer disc-shaped resonator geometries (“disc lasers” and “slab lasers”) are increasingly replacing “conventional” cylindrical Nd:YAG rods (“laser rods”), wherein the diameter of the cylinder is much smaller than the length. Therefore, these rods have a much smaller surface area than a disc (i,e, diameter is much larger than the thickness) of equal volume. Cylindrical resonator rods are therefore much more difficult to cool. As heat can be dissipated much more efficiently with disc lasers than with rod lasers, higher pump powers are also possible which ultimately lead to higher laser powers. Currently, Nd:YAG lasers with a continuous output power of more than 10 kW are in industrial use.

The wavelength of the Nd:YAG laser is almost 10 times lower than that of the CO2 laser (1,064 microns compared to 10.6 microns). The shorter wavelength allows significantly smaller foci and thus higher intensities at the same power compared to CO2 lasers. This - and the shorter wavelength absorbed by metals - is advantageous for metalworking.

The Nd:YAG laser in the field of marking:

The high peak power makes the laser ideal for marking many different materials. The main areas of application include the laser engraving of workpieces, tools or devices. Particularly with many plastics, laser marking causes a very visible color change (dark) or foaming of the polymer (light marking).

Furthermore, the light emitted by the Nd:YAG laser (near infrared) can be guided in a glass fiber, facilitating the integration into many machines e.g. welding robot. One economical advantage is that inexpensive optical elements made of quartz glass can be used. In addition to laser marking, Nd:YAG lasers are mainly used for laser welding, cutting and micromachining.

Disadvantages include high investment costs and low beam quality at very high powers.

The wavelength emitted by the Nd:YAG laser can be relatively efficiently halved by means of non-linear crystals, i.e. the frequency of the laser light is doubled. This gives visible green light with a wavelength of 1064nm / 2 = 532nm. Frequency tripling or wavelength thirding leads to UV emissions of 1064nm / 3 = 355nm. This wavelength allows the marking of virtually all plastics.

Other areas of application for the Nd:YAG laser include research, optical flow technology, the removal of tattoos as well as dazzler weapons in the military. 

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