Ultrafast Lasers

The Fraunhofer ILT experts use a broad portfolio of different laser amplifier designs based on Yb-doped crystals; depending on the average power, this includes rod, multi-rod, INNOSLAB and disk amplifiers. The amplifiers are characterized by diffraction-limited beam quality and bandwidth-limited pulse durations. With pulse durations of several 100 fs, the institute’s developments are addressing average powers from 1 W to 10 kW, repetition rates from 10 kHz to 100 MHz and pulse energies from 10 μJ to 10 mJ. 

One of Fraunhofer ILT’s core competencies is the INNOSLAB technology that it has developed and patented. With Yb:YAG as the active medium, single-stage (two-stage) output powers > 500 W (> 1000 W) with beam qualities M² ~ 1.10 x 1.40 are achieved at pulse durations of 700 fs. These can be improved by spatial filtering of the output beam to M² ~ 1.10 x 1.10.

As part of the Fraunhofer Cluster of Excellence Advanced Photon Sources CAPS, the output power of these laser sources is to be scaled to over 5 kW.

An essential part of the work is the tailor-made adaptation of the INNOSLAB platform to the requirements of customers from the industry and research in terms of compactness, alignment, long-term stability and special pulse parameters.  In addition to INNOSLAB amplifiers, the institute is also  adapting other laser platforms such as fiber, rod or disk  lasers, which are being combined and further developed to customers’ specifications. 

In the Fraunhofer Cluster of Excellence Advanced Photon Sources CAPS, initiated in 2018, the Fraunhofer-Gesellschaft  is currently combining the expertise of 13 Fraunhofer institutes in the fields of laser source development, system technology and applications of ultrafast lasers. Fraunhofer ILT is coordi-nating this project together with the Fraunhofer Institute for Applied Optics and Precision Engineering IOF.

By 2022, ultrafast laser systems with average output powers in the 10 kW range will be implemented for applications in industry and research. For this purpose, laser sources will be developed that exceed the average power of current ultrafast lasers by one order of magnitude. These beam sources enable the institutes to scale ultra-precise manufacturing processes and develop new pulse duration and wavelength ranges for basic research and metrology. 

The centerpiece of the cluster infrastructure at Fraunhofer ILT is the establishment of a user facility in which the worldwide unique beam sources are made available to partners from  industry and fundamental research for application experiments.

While the output pulse durations of Yb-based high-power ultrafast lasers are limited to several 100 fs due to their limited gain bandwidth, shorter pulse durations are increasingly in demand for applications in the industry and research.

With a flexible and compact additional module developed and patented at Fraunhofer ILT, the pulse duration can be significantly reduced by non-linear spectral broadening in a multi-pass cell with subsequent dispersion compensation. The process achieves a high efficiency of over 90 percent without influencing the beam quality and is currently used for output powers of up to 500 W. The process can be scaled in the  pulse energy range from some μJ up to more than 10 mJ.  The shortest pulse durations demonstrated so far are 25 fs with a compression factor of > 20.

Extensive software tools have been developed for design, layout and optimization and can be used to map the effects relevant for ultrafast and high-power lasers. These enable  a complete three-dimensional analysis of the systems under investigation:

• Wave-optical calculation of the propagation of laser radiation and its transformation using active and passive optical elements
• Modelling of 3- and 4-level gain media
• Consideration of gain, pump light distribution, rate equations, heat conduction, Kerr effect, self-phase modulation, dispersion and parasitic lasing