Frequently Asked Questions
Disk (or disc) lasers are solid state lasers, (see also FAQ on 'What is a solid state laser') in which the lasing medium generally consists of a thin slice or disk of Ytterbium-doped Yttrium-Aluminium Garnet crystal. This lasing medium produces laserlight at 1030nm wavelength, very close to that generated in the more common Nd:YAG laser. There is a big difference, however, in the shape of the crystals used in these two types of solid state laser.
In the case of traditional Nd:YAG lasers, the lasing medium is in the shape of a cylindrical rod of several millimetres in diameter, and hundreds of millimitres in length. It is pumped via flashlamps or diodes, and the generated laser beam is parallel to the axis of the rod. Because the conversion of pump light into laser light occurs at a low optical efficiency, the rod heats up and cooling is required. Coolant flows along the outside of the rod, giving rise to a radially parabolic temperature profile, perpendicular to the direction of the laser beam. This leads to what is known as thermal lensing, and causes divergence of the laser beam, and thus a reduction in beam quality, which ultimately affects the available power density in the focused laser beam.
To overcome this problem, the disk laser was developed. In it, instead of a crystal rod with a low surface to volume ratio, a very thin crystal disk is used as the lasing medium. The disk, generally of only a few hundred microns in thickness but several millimetres in diameter, is coated at one end with a reflective surface (which acts as the rear mirror in the laser resonator) and mounted to a heat sink. The very small thickness of the disk (and therefore high surface to volume ratio), its contact with a large heat sink, and the fact that the diameter of the laser light source used to pump the crystal is much greater than the disk thickness, means that, in effect, axial cooling can be achieved. This axial temperature profile reduces thermal lensing to a minimum, thereby promoting good beam quality. In addition, the level of pumping (and thereby the power level of the laser beam generated) can be regulated through simultaneous variation of the pumping power level and the area of the disk that is pumped. This means the crystal can be pumped at a constant intensity whatever the power level, further improving the beam quality. A laser beam generated axially in a disk laser can have a very high beam quality (as low as 4mm*mrad for the lower power materials processing lasers).
Although the fact that the disk is very thin allows very efficient cooling, it also means that only a small fraction of the pump light is absorbed as it passes through the disk. Therefore, to increase the efficiency of the pumping action, the pump light is reflected at the coated rear face of the disk. Around the disk, a set of mirrors and retro-optics cause the pump light to pass through the disk many times (generally ~16 times), increasing the absorption of the pump light. In this way, powers generally up to ~1kW may be achieved from a single disk (at a reduced beam quality). To generate multiple kilowatt sources, several disk units can be combined.
The higher beam quality of disk lasers compared to the more traditional Nd:YAG rod and even diode pumped lasers has a number of advantages, including:-
- For the same output power, smaller fibre diameters and smaller focus diameters ('spot sizes') can be used, thereby increasing the power density in the spot.
- Greater working distances can be used for the same spot size, at the same time giving a greater depth of focus.
- For the same working distance, smaller diameter optics can be used, decreasing their weight and improving accessibility.
- The required threshold power density for materials processing can be achieved at lower laser power, thereby reducing overall heat input.
Disk lasers have become available recently from several suppliers, with power levels ranging from a few hundred watts to a few kilowatts. It is expected that higher power units will become available very soon, in competition to the high beam quality fibre lasers. (see FAQ on ' What is a fibre laser?'). Such high quality lasers can be used for cutting, welding, direct metal deposition and other materials processing applications.