Lasers & Sources

Few-cycle laser pulses break the 300 W barrier

Nov. 25, 2019

Using stretched flexible hollow-core fiber technology to compress pulses, multimillijoule 10 fs pulses at a 100 kHz repetition rate and an average power of 318 W were generated—potentially useful for materials processing.

John Wallace

Full characterization of the 10 fs NIR pulses by a dispersion scan measurement. First, the spectral phase of the pulses is varied (chirping) by inserting glass with gradually increasing thickness into the beam. Then, in a thin nonlinear crystal, the second harmonic of the chirped pulses is generated and its spectrum recorded for the series of different glass insertions. In this way, a 2D trace is recorded (shown at top left), from which the missing phase information can be extracted by using an iterative numerical algorithm. The simulated trace given by the phase-retrieval algorithm is shown at top right, exhibiting a striking similarity with the measured trace. The measured spectrum of the pulse together with the retrieved phase are shown at bottom left, while their Fourier transform giving the pulse shape (red curve) is displayed at bottom right. The black curve in this panel corresponds to the shortest possible pulse for the measured spectrum.

A team led by researchers from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), Laser-Laboratorium Göttingen (LLG), and Active Fiber Systems (AFS) has generated multi-millijoule three-cycle pulses at a 318 W average power level.¹ These results mark a significant milestone in few-cycle laser technology, paving the way towards industrial applications.

Extreme short light pulses containing only a few electromagnetic-field oscillations are among the fastest events ever made by humankind. Although the first few-cycle pulses were produced about 30 years ago, they could only be used in cutting-edge science, for example for time-resolved studies or attosecond pulse generation. To find their way into industrial applications, a number of major challenges need to be addressed, such as turn-key operation and energy and power upscaling of the few-cycle sources.

The scientists followed a novel approach by directly compressing 300 fs long pulses from a high-energy, high-power laser system to the few-cycle duration. This requires a 30-times compression, which has only recently become feasible by the introduction of stretched flexible hollow-fiber technology, which offers unrestricted length scalability.

In the study, a coherently-combined multichannel fiber laser delivering up to 10 mJ pulses at up to 1 kW average power was used as the light source. This system is currently under development at AFS for the major European laser facility ELI ALPS in Szeged, Hungary. In the pulse compression, a 6-m-long stretched flexible hollow fiber was used, which was developed together by MBI and LLG.

As the pulses propagate through argon gas filled into the hollow waveguide, a nonlinear interaction called self-phase modulation takes place between the intense light and the gas atoms, which makes the spectrum broaden. The pulses with substantially broadened spectrum can then be compressed to a shorter duration by compensating their spectral phase with a set of chirped mirrors. In this way, the team succeeded in generating multimillijoule 10 fs pulses at a 100 kHz repetition rate and an average power of 318 W, which is the highest average power ever achieved for a few-cycle laser.

This shows that, by using stretched flexible hollow-core fiber technology, high-power industry-grade lasers can be brought into the few-cycle regime, opening up new possibilities for industrial applications such as highly parallelized material processing.

Source: https://mbi-berlin.de/research/highlights/details/few-cycle-pulses-break-the-300-w-barrier

REFERENCE:

1. T. Nagy et al., Optica 6 (2019) 1423-1424; https://doi.org/10.1364/optica.6.001423.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.