Increasing femtosecond laser power

Aachen, Germany – Researchers at the Fraunhofer Institute for Laser Technology (ILT) here have developed an amplification concept for the femtosecond laser (fs laser) that enables the achievement of mean power in the kilowatt range for the first time. The short pulses of the fs laser enable ultra-precise material processing. Thanks to the high mean power, the throughput in production can be increased significantly.

Aachen, Germany – Researchers at theFraunhofer Institute for Laser Technology (ILT) here have developed an amplification concept for the femtosecond laser (fs laser) that enables the achievement of mean power in the kilowatt range for the first time. The short pulses of the fs laser enable ultra-precise material processing. Thanks to the high mean power, the throughput in production can be increased significantly.

While lasers with continuous radiation (cw) are often used to cut and weld, pulsed lasers are mostly used to ablate. For the latter, the following generally applies: the shorter the pulse, the smaller the heat penetration depth, the higher the temperature at the material surface, and the higher the precision of the material processing.

Material ablation mainly occurs via vaporization. The shorter pulses of the fs laser enable greater precision when ablating. In addition, materials can be processed that can only be poorly processed with lasers, or not at all.

The fs laser has not yet become widely established in industrial production, a situation that could soon change when new products become readily available, thus making better throughput, lower maintenance costs and easier usability possible.

Advanced amplifier design
The amplifier design is based on INNOSLAB technology. This in-house development of the Fraunhofer ILT is characterized by an especially simple, robust and compact construction. The researchers from Aachen have extended the amplifier design by using ytterbium (yb)-doped YAG as an active medium, which enables the amplification of ultra-short pulses due to its large bandwidth.

Since yb-doped crystals place great demands on the pump beam source, the availability of brilliant pump diodes has made this step first possible. The oscillator-amplifier system enables, in addition, more flexibility with respect to the repetition rate and the pulse duration, since to generate the pulse a variety of commercial fs oscillators are available based on fiber and solid-state lasers with about 2 W of mean laser power.

Dr. Peter Rußbüldt, project leader at the Fraunhofer ILT, has cascaded two INNOSLAB amplifiers. In this way, the amplifier chain was able to generate a mean power of 1.1 kW at a peak pulse power of 80 MW and a pulse duration of 600 femtoseconds. The Aachen researchers have thus reached a new record.

The original purpose of the joint project with the Max Planck Institute for Quantum Optics, funded by the BMBF (the German Federal Ministry of Education and Research), was a scientific application: the generation of coherent EUV radiation. During the project, researchers continued to develop the laser at the Fraunhofer ILT.

"Thanks to this new power output, the fs laser slowly departs from its image as a scientifically complicated toy," said Dr. Rußbüldt. "The higher power means the throughput also increases in production, which, in turn, means an enormous time- and cost-based advantage for manufacturers. Therefore, the fs laser can now be applied in fields in which its throughput did not suffice in the past."

Typical applications of the fs laser in the macro sector are manufacturing processes for fiber-composite lightweight components. An fs laser can process the most varying of materials irrespective of their characteristics. In the micro sector, the applications of this new beam source range from drilling of nozzles to tool engineering, and from solar cell engineering all the way to printing technology.

The Fraunhofer ILT will hold a workshop on this topic on April 13 and 14, 2011 in Aachen. Technological developments and their market potential will be illuminated in this USP workshop.

At SPIE Photonics West (San Francisco, Calif., U.S., January 25 to 27, 2011), the Fraunhofer ILT will be showing high precision assembly technology for optical components in the German Pavilion of North Hall at Stand 4601 and presenting further developments of heat sinks for high power diode lasers. In the accompanying conference (LASE, January 22 to 27, 2011), the Aachen experts will be presenting the new high power lasers to the specialist audience.

For further information, check the ILT web site: http://www.ilt.fraunhofer.de

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