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.

KBBF crystal gives direct access to DUV

Researchers in China have created a tunable all-solid-state laser that emits milliwatt power levels in the deep ultraviolet (DUV). Applications requiring light around the 200 nm mark, such as photoemission spectroscopy and photolithography, could benefit from this work (Applied Physics B 93 323).

"Our source tunes from 175 to 210 nm via fourth harmonic generation from a Ti:sapphire laser," Zuyan Xu of the Chinese Academy of Sciences told optics.org. "The highest output power is 2.23 mW at 193 nm but the power is above 1 mW between 182 and 210 nm. This is the first demonstration of a milliwatt-level widely tunable all-solid-state laser below 200 nm by direct second harmonic generation."

The team's set-up can essentially be broken into three stages: the initial nanosecond-pulsed Ti:sapphire laser, optics to generate the second harmonic in the UV and additional components to generate the fourth harmonic in the DUV.

The output from the Ti:sapphire (more than 3W across the range of 690–840 nm) is focused into a set of BBO crystals to generate UV light between 340 and 415 nm. This light is then passed into the KBBF crystal to generate the DUV wavelengths.

When it comes to producing tunable DUV light, one alternative approach is sum-frequency mixing. This however uses two laser beams making the system complex and of limited practical use. To remove this complexity, Xu and colleagues use a KBBF crystal that offers a direct route to DUV light below 200 nm using just one beam.

Low Evolution X output power

The Evolution -- the DPSS Q-switched green laser is used to pump the Ti:sapphire crystal for regeneration amplifier of 30 fs laser pulses. However its output power was dropped dramatically when we operated it this morning. The power could not be changed very much when we tuned two cavity mirrors. Later we adjusted the diodes temperature between 80F and 83F, the power just fluctuated from 22mW to 18mV when the output was set at 23%. In the normal way, the power should be around 600mW.