Spectral Broadening of a 500W, 5mJ Femtosecond Laser

Summary form only given. The ELI-ALPS facility requests lasers with parameters beyond current state-of-the-art [1]. One of the most challenging ones is the high repetition rate laser 2 (HR2), a carrier-envelope-phase stable laser system delivering 5mJ pulses at 100kHz repetition rate and with pulse durations of 6fs, which is currently under development at Active Fiber Systems GmbH. It uses a kW-class ytterbium doped femtosecond fiber-chirped-pulse amplifier system. To achieve the bandwidth required for few-cycle pulses significant nonlinear spectral broadening is necessary. The latter technology has seen rapid scaling both in terms of average power [2] and pulse energy [3,4]. Furthermore, by the introduction of stretched hollow capillaries [5] high-quality long waveguides became available, which is inevitable for upscaling the pulse energy. Despite first promising experiments with ytterbium-based laser systems the average power used in combination with stretched-hollow-core fibers has not exceeded a few Watt yet [6,7]. In this contribution, we present a significant up-scaling of the average power of stretched hollow-core-fibers by spectrally broaden 5mJ, 500W, 280fs pulses in a 4m long, 450μm inner diameter fiber to a bandwidth supporting sub-17fs pulses. The laser system used for this experiment is a fiber CPA that incorporates coherent combination of multiple main amplifier channels achieving 5mJ pulse energy at 100kHz repetition rate corresponding to 500W of average power and 290fs pulses [8]. The output beam was sent through a combination of a half-wave plate and a thin-film polarizer to provide arbitrary attenuation of the power and then coupled to the 4m long capillary with an inner diameter of 450μm. The capillary had a pressure gradient with vacuum at the entrance and 600mbar Ar at the output side. The fiber transmission was nearly constant over the full power range up to 500W. At the maximum power the spectrum was significantly broadened to a bandwidth supporting sub-17fs pulse duration at 3mJ energy and 300W average power (Fig.1). In conclusion, we have shown significant scaling potential of stretched capillaries by demonstrating the highest combination of average power and pulse energy in any spectral broadening experiment presented to date. Further power scaling and pulse compression experiments are underway to achieve 500W, 5mJ, 6fs at 100kHz.