Friday, January 22, 2010

Disk Laser Technology

A disk laser or active mirror is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. It was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany.

The gain medium of a thin-disk laser is a laser crystal (often Yb:YAG) in the form of a disk with a thickness of 100-200 µm, which is fixed on a water-cooled heat sink. The cooled end face has a dielectric coating that reflects both the laser radiation and the pump radiation.

The heat is extracted dominantly through the cooled end face, and because the disk thickness is considerably smaller than the laser beam diameter, the heat flow is largely in the direction of the beam, rather than in a transverse direction, as for a laser rod. As a consequence, thermal lensing is weak. The figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with Disk Lasers can be much higher than that of a rod system, improving the Beam Parameter Product (BPP) up to 6 times.

The small disk thickness, as required to limit the heating, leads to incomplete pump absorption in a double pass. Therefore, one usually uses some multipass pumping scheme, which can be realized with very compact optics.

Due to improvements in the area of semiconductor pumping diodes the potential of Disk Lasers is not exhausted. While the first generation "only" extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and expected to increase to 4kW per disk towards the end of 2008. Further, by combining several individual disk cavities, as illustrated in Figure 2, the total available laser power of a Disk Laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk "converts" the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this Disk Laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.

It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: Disk Lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser are falling because less cavities are required to generate the same laser power.

Q switching is possible with high pulse energies but not with very short pulses because the laser gain is quite limited.

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