We demonstrate a versatile femtosecond power amplifier using a Yb:YAG single crystal fiber operating from 10 kHz to 10 MHz. For a total pump power of 75 W, up to 30 W is generated from the double-pass power amplifier. At a repetition rate of 10 kHz, an output energy of 1 mJ is obtained after recompression. In this configuration, the pulse duration is 380 fs, corresponding to a peak power of 2.2 GW. The M 2 beam quality factor is better than 1.1 for investigated parameters. © 2013 Optical Society of America OCIS codes: 320.7090, 140.3615.Ultrafast lasers are now a common tool for scientific and industrial applications. Over the past decade, many technological developments of diode-pumped solid-state laser systems have allowed tremendous improvements of their performance, reliability, and cost. In order to achieve high energy per pulse in the femtosecond regime, master oscillator power amplifier systems are commonly used. Regenerative amplifiers based on bulk or thin disk Yb-doped crystals can amplify ultrashort pulses to several tens of millijoules [1,2]. They can provide high gain and high output energy at a low repetition rate, but they are limited in terms of repetition rate to a few hundreds of kilohertz due to the high-voltage-driven switch speed. Ytterbium-doped optical fibers can also be used to amplify ultrashort pulses. Their high surface-to-volume ratio provides good thermal management and allows attainment of high average powers of several hundreds of watts [3]. However, the signal confinement in smallcross-section cores induces nonlinear effects, such as self-phase modulation and self-focusing, which limit the peak power and the pulse energy. Femtosecond pulses with an energy of 2.2 mJ were obtained using the well-known chirped pulse amplification technique together with large-core-diameter photonic crystal fibers [4]. Another approach consists in amplifying femtosecond pulses directly in Yb-doped crystals using multipass amplifiers without active elements. Although it requires quite complex systems, the slab geometry has proven to be a very successful approach. Up to 1.1 kW [5] average power and 20 mJ [6] energy were obtained with an Yb:YAG Innoslab amplifier. Significant improvement of the emission cross section and the thermal conductivity can be observed at cryogenic temperatures in most Ybdoped crystals. 40 mJ output energy was obtained using a cryogenic Yb:YAG double-pass amplifier [7]. However, stronger spectral narrowing at low temperatures induces longer optical pulses of several picoseconds. Finally, the single crystal fiber (SCF) concept lies between fibers and crystals and can contribute to original performance for femtosecond systems. SCFs are long, thin crystal rods with a diameter lower than 1 mm and a typical length of a few centimeters.

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