The combination of a fiber laser system with recent advancements in multipass cells has enabled a German research team to create a laser with a unique combination of few-cycle pulses at high average power, pulse energy, and repetition rate, and with a stable carrier envelope phase (CEP) operation. With these characteristics, such a laser is ideal for driving next-generation attosecond sources, such as those at the Extreme Light Infrastructure (ELI) in Europe. ELI, the world’s largest and most advanced high-power laser infrastructure, will be used to investigate light-matter interactions at the highest intensities and shortest timescales. The beam lines of ELI’s Attosecond Light Pulse Source will provide unprecedented performance in generating isolated attosecond pulses and thus have very demanding laser system requirements. “The development of such a unique laser system opens up new prospects for secondary sources, e.g. for the future generation of isolated attosecond pulses with unprecedented parameters,” said Steffen Hädrich of Active Fiber Systems GmbH in Germany; he presented the laser at the Optica Laser Congress. “These in turn hold promise to advance the understanding of electronic processes on fundamental length and timescales and contribute to new discoveries in biology, chemistry, physics, and medicine.” The researchers developed a fiber-chirped pulse amplification system that coherently combines eight amplifier channels. This system emits 300-fs pulses that are then compressed to the few-cycle regime using two multipass cells. The first multipass cell uses standard dielectric mirrors to achieve 1.7-mJ pulses with a duration less than 35 fs. The second cell uses metal-based mirrors to achieve a pulse duration of just 5.8 fs at a pulse energy of 1.1 mJ, 110-W average power, and repetition rate of 100 kHz. Stable CEP operation is needed to fully utilize the high average power and fast repetition rate of the laser. The researchers measured the CEP of each pulse using a single stereo-ATI phasemeter that can characterizer noise over the full frequency spectrum. The CEP measurements were sent to a PID controller, creating a feedback loop that enabled ~400 mrad of CEP noise. “We demonstrated the shortest pulses and highest compressed average power that has been achieved for few-cycle MPCs with 110 W at the 100-kHz repetition rate,” Hädrich said. “With further improvements, we hope to also achieve less than 300 mrad of CEP noise soon. The presented laser system meets the requirement for the HR1 laser of ELI-ALPS. We are in the process of scaling this approach toward the HR2 parameters, that is, to implement this concept for the demonstration of a 500-W, 5-mJ, 6-fs, CEP-stable laser system.”