Nd:YAG laser - terminology and functionality

An Nd:YAG laser is a solid-state laser whose main emission line is near infrared at 1064nm, with a host crystal that is yttrium aluminium garnet. Typically, 0.5% - 3% of the yttrium atoms are replaced by neodymium (“doping”). First  developed in 1964 in Bell Laboratories, Yag lasers are one of the most widely used laser sources in the industrial sector for material processing, alongside CO2 and fiber lasers. 

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, meaning 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 realisation 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 minimising 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Nd:YAG means that in an yttrium-aluminium garnet crystal (chemically: Y3Al5O12) yttrium ions (chemical symbol: Y) have been replaced by neodymium (chemical symbol: Nd).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 not limited to Yag lasers, it is true for all lasers.

Energy is supplied optically, as 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.