Fiber lasers are well-known for their excellent heat dissipation and ruggedness, although stable, mode-locked operation of high-average-power fiber lasers has not been readily obtained. But the output of such lasers — stable, ultrashort pulses with tens of watts of average power — would be useful in applications from ophthalmology and waveguide writing to nonlinear conversion. Figure 1. The all-fiber oscillator-amplifier system consisted of a mode-locked oscillator (defined by the dotted line), a preamplifier (bottom right) and a holey-fiber power amplifier (bottom left). Images ©OSA.Recently, researchers at Friedrich Schiller Universität and at Fraunhofer Institut für Angewandte Optik und Feinmechanik, both in Jena, Germany, demonstrated a stable, polarized, mode-locked fiber oscillator-amplifier system that generates 240-fs pulses at 17 MHz, with 21 W of average power.Figure 2. The polarizing photonic crystal fiber in the power amplifier had an ~700 μm2mode field area and a pump absorption of 14 dB/m. The all-fiber system comprises a master oscillator, a preamplifier and a power amplifier based on Yb-doped holey fiber (Figure 1). The oscillator is mode-locked with a semiconductor saturable absorber mirror and pumped with a single-mode 976-nm diode laser. The gain medium is a 31-cm length of polarization-maintaining Yb-doped fiber, which, together with ~5.3 m of undoped, single-mode polarization-maintaining fiber in the resonator, assures that the laser polarization remains stable despite environmental changes. The oscillator’s output consists of linearly chirped, parabolic pulses containing a few hundred picojoules.The 7.2-ps pulses from the oscillator are subsequently amplified in a preamplifier, boosting the average power from 2 to 50 mW and stretching the pulse duration to 10 ps. The 55-cm-long Yb-doped fiber, which serves as the preamplifier’s gain medium, is of the same design as that in the oscillator.Figure 3. The power amplifier generated up to 29 W from 30 mW of seed power. The 10-ps pulses were compressed to 240 fs in a grating compressor, resulting in 21 W of average compressed power.The power amplifier is based on Yb-doped, single-mode, polarizing, double-clad photonic crystal fiber (Figure 2). The advantage of photonic crystal fiber is that the core can be arbitrarily large — reducing the power density and the likelihood of deleterious nonlinear effects — while still supporting only a single mode. In this case, the mode field area of the core was ~700 µm2, and the air cladding diameter was 170 µm with a numerical aperture of 0.6. The resulting pump absorption was ~14 dB/m, so a 1.2-m length of the single-polarization fiber provided sufficient gain for the amplifier.When seeded with approximately 30 mW of input, the power amplifier produced as much as 29 W of 1035nm output (Figure 3). The 10-ps chirped output pulses were compressed to 240 fs in a transmissive grating compressor whose throughput was 73 percent, resulting in 21 W of average compressed power, or 5 MW of peak power. Optics Letters, March 1, 2006, pp. 574