PULSE AMPLIFICATION: Few-cycle pulses get seriously powerful

In a stunning display of the power of a pulse amplification method called optical parametric chirped-pulse amplification (OPCPA), researchers have demonstrated sub-10 fs ultrabroadband pulses at 395 nm with energy up to 250 mJ. The team, with members from the Max Planck Institute for Quantum Optics (MPQ; Garching, Germany), the University of Munich, the University of Jena, and the Russian Academy of Sciences (Novosibirsk, Russia), hails the result as a stepping stone to terawatt and petawatt systems incorporating the same approach.

The research was carried out as a milestone in the development of the Petawatt Field Synthesizer (PFS), a 500-terawatt-class laser that will be constructed at MPQ. The approach has also been earmarked as a promising candidate in a giant European next-generation light-source initiative called the Extreme Light Infrastructure Project, which aims to track and fund light sources to reach the ultrarelativistic intensity regime.

As the name suggests, OPCPA combines the broadband nature of optical parametric amplification (OPA) with the peak power flexibility provided by chirped-pulse amplification (CPA). In conventional laboratory-scale OPA systems, a seed pulse is stretched in time, amplified, and then recompressed before being sent to a separate OPA stage that is pumped with the compressed pulse. In OPCPA, the stretched pulse itself is used in the OPA process, with recompression happening after the OPA stage via a chain of chirped mirrors. In this way, very broadband pulses can be amplified to enormous peak powers without damage to the optics.

Short pulses, thin crystals

While OPCPA has been demonstrated before, high pulse energies have been reached only with high pulsewidths-the shortest demonstrated pulsewidths of about 20 fs had an energy of around 300 mJ. The key step for the new work was to use very short pump pulses and relatively thin beta barium borate (BBO) crystals as the amplification medium. The short pump pulses increased the bandwidth available for amplification as well as reducing constraints on the OPCPA crystals that will be used in future scaled-up versions of the setup. “It became clear early in the project that broadband OPCPA crystals are not available at large enough size to achieve the necessary energy level,” says Stefan Karsch, project leader for the PFS and senior author of the work.

The new work uses the ATLAS system at MPQ, a Ti:sapphire-based CPA laser. Both seed and pump pulses were derived from the ATLAS laser, taking advantage of the inherent pulse synchronization that it allowed. The seed beam was split off by a glass plate, compressed separately from the pump beam, and focused into a gas cell to create a white-light filament of energy of approximately 800 mJ with a transform-limited pulse duration around 10 fs.

The pump pulse was produced by spatially filtering and then frequency doubling the remainder of the ATLAS output. The result was pulses with energy as high as 6 mJ at a pulse length less than 100 fs. Crucially, the beam then underwent a pulse front tilting using either a prism or gratings and a telescope. This tilting resulted in significantly better phase matching in the OPA stage, which was accomplished noncollinearly in BBO crystals of thicknesses between 0.5 and 2 mm. The result was signal pulses with energy between 150 and 250 mJ and pulsewidth of approximately 5 fs.¹

“We believe this study is fundamental for pushing the frontier of few-cycle laser-matter interaction, since to our knowledge this is the only viable way for generating such short pulses with petawatt peak powers and above,” Karsch says.“The main implication is that it might become possible to achieve much higher laser powers in the future with this method as a driver. Coherent focusing of single attosecond extreme-UV harmonic pulses that can only be driven by such few-cycle pulses might increase the intensity even further.”

Future plans to expand on this proof-of-principle result will see two more amplification stages added to the setup. The PFS is currently funded and on schedule for completion in 2010-an OPCPA system that will deliver pulses of similar width at pulse energies in excess of 3 J.

REFERENCE

1. Fülöp et. al., New J. Physics 9, 438 (2007).