A Sub-Picosecond Thin-Disk Laser Oscillator

A commercial thin-​disk head. The disk is surrounded by monolithic prisms that route the pump beam between reflections. (Source: Trumpf GmbH + Co. KG / ETHZ)

Ultrafast laser sources are at the heart of an ever-​expanding range of funda­mental scientific studies and industrial applications, from high-​field-physics experi­ments with attosecond temporal resolution to micro­meter-​preci­sion machining of materials. In order to push the envelope even further, repetition rates of several megahertz and average output powers of hundreds of Watt are required. A parti­cularly compelling route to realizing such high-​power laser pulses is to generate them directly by scaling up the power output from laser oscillators, rather than relying on multi-​stage ampli­fier systems.

The latter approach adds a high degree of complexity, whereas the former leads to robust and poten­tially cost-​effec­tive devices. The group of Ursula Keller at the Institute of Quantum Electronics has now taken the power-​scaling approach to a new level. They present a source that combines the simplicity and high repe­tition rates of oscil­lators with record-​high average output power from this type of laser. The team worked with a thin-​disk laser oscil­lator, where the gain medium – the material in which the quantum processes leading to lasing take place – is shaped as a disk, typically some 100 μm thin. This geometry affords a relatively large surface area, which in turn helps cooling. Still, thermal effects remained a major bottle­neck, and since 2012 the record output power stood at 275 W.

Until now. Combining several advances in thin-​disk laser technology developed in the Keller group, PhD student Fran­cesco Salta­relli, senior research scientist Christopher Phillips and colleagues took a decisive step and achieved an average output power of 350 W, with pulses that are only 940 femto­seconds long, carry an energy of 39 microjoule and repeat at a 8.88-​megahertz rate – values that are of immediate interest for appli­cations both in science and industry.

A key aspect of the work is that the researchers found a way to enable several passes of the pump beam through the gain medium without inflic­ting detrimental thermal effects, and so to reduce the stress on the relevant compo­nents. The ability to control effects due to heating opened the gate to go firmly beyond the 275-​W level and to set the new benchmark. The approach now developed can be taken even further though, and output powers beyond 500 W seem realistic. With further improvements, the researchers estimate, the kilowatt level might come into sight. (Source: ETHZ)

Reference: F. Saltarelli et al.: Power scaling of ultrafast oscillators: 350-W average-power sub-picosecond thin-disk laser, Opt. Exp. 27, 31465 (2019); DOI: 10.1364/OE.27.031465

Link: Institute for Quantum Electronics, ETH Zurich, Zurich, SwitzerlandTRUMPF Laser GmbH, Schramberg, Germany

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