Fraunhofer ILT notches femtosecond record

29 May 2009

Researchers claim "paradigm shift" in the design of commercial femtosecond lasers.

power play

Engineers from the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, are set to unveil a femtosecond laser boasting a world-beating power output of 400 W at LASER World of Photonics in Munich, Germany, next month. That's a monumental improvement on most commercial femtosecond lasers, which produce an average output power around the single-watt level, while even high-end models are limited to the 50–100 W range.

"The Fraunhofer ILT is introducing a paradigm shift in the design of commercial femtosecond lasers," claimed Axel Bauer, head of the institute's marketing and communications group. "Our laser module holds the world record for average output power among lasers with pulse durations of less than 1 ps."

What's more, says Bauer, the laser design is nowhere near as complex as existing high-power femtosecond lasers. "50 W femtosecond lasers are regarded as high-end," he explained. "Their high price and limited processing speeds are restricting their commercial potential, and until now they have not been widely embraced in production."

In contrast, the new 400 W laser module is based on InnoSlab technology, a diode-pumped slab-laser design that has been under development at the Fraunhofer ILT for the past 10 years or so. InnoSlab technology already forms the basis of a number of industrial ultrafast lasers, and is known to deliver short pulse lengths, high repetition rates and good beam quality – while at the same time boasting an extremely simple design.

The new laser module essentially comprises a single laser oscillator that produces an output power of 1–2 W, combined with a single-pass amplifier that boosts power levels to 400 W. At full output, the laser achieves an optical efficiency of 50% and an almost diffraction-limited beam quality of M² <1.4, which allows a round beam to be produced over the entire propagation range.

Another innovation has been to modify the output intensity to produce pulse energies of up to 100 µJ without the need for complex chirped-pulse amplification. "This represents a further breakthrough for the simplification of femtosecond laser systems and the costs they entail – a key requirement for their widespread use in industrial practice," said Bauer.

Other notable metrics include pulse durations of less than 700 fs – significantly shorter than is possible with existing picosecond lasers – and spectral bandwidths below 2 nm. The design also allows the same optics to be used as for typical nanosecond and picosecond lasers, which avoids the need for special attachments for time compression and so leads to improved performance and a simpler set-up.

According to Bauer, the output characteristics also make the laser ideal for nonlinear frequency conversion, and the team is now working to develop schemes for frequency doubling, pulse compression and the production of high harmonics. At the same time, efforts are ongoing to increase the pulse energy to 10 mJ and above for scientific applications.

Bauer claims that the technology could be pushed to even higher power levels. "According to the theoretical and experimental findings, the practical limits of the ultrashort-pulse laser have not yet been reached," he said. "The Fraunhofer ILT is already working on scaling the laser to output powers of more than 1000 W."