## 4 High Average Power Short-Pulse Thin Disk Lasers at HiLASE
### Fiber Oscillator and Front-End  
Within the HiLASE Centre, we are developing fiber laser systems, the setup of a mode-locked oscillator, a pulse picker and consecutive amplification stages. These systems are not only used as the seed source for the thin disk amplifiers; also, other applications, such as laser micromachining and a laser driver of a photocathode electron source, are possible.  
As a gain material, ytterbium-doped single-mode fibers are used. These are pumped at a wavelength of 976 nm and emit around 1030 nm. The fiber oscillator is based on the ANDi scheme (all normal dispersion) shown in Figure 10 [47], which means that the system fully works in the normal dispersion regime, without any dispersion compensation inside the cavity. Passive mode-locking is achieved by saturable absorption based on non-linear polarization evolution.  
The oscillator generates pulses of 1.9 nJ of energy (90 mW average power) and a pulse width of 1.5 ps (compressible to less than 90 fs) at a repetition rate of 48 MHz. A pulse picker, based on a synchronized pulse-pumped semiconductor optical amplifier (SOA), is used to vary the repetition rate between 10 and 1 kHz. If the SOA is unpumped, the pulses are attenuated, and if the SOA is pumped, the pulses are amplified. To optimize the pulse picking performance and to reduce undesired non-linear effects inside the SOA and following amplification stages, the pulse length is increased to 1 ns, while the average power is reduced to less than 10 mW (0.2 nJ). The pulse energy behind the pulse picker depends on the repetition rate (_i.e._, the gain of the SOA is repetition rate and polarization dependent) and reaches from 0.89 nJ at 1 kHz to 1.03 nJ at 200 kHz (Figure 11a). Hence, the SOA is not only working as a pulse picker, it also amplifies the selected pulses at a 4-5-dB gain.  
The SOA-based pulse picker is followed by a first pre-amplifier, operated at low pump currents to keep the noise (_i.e._, amplified spontaneous emission) at a minimum level. This pre-amplifier is a setup of a core-pumped ytterbium-doped single-mode fiber in the single-pass geometry. Depending on the  
Figure 11: **(a)** Pulse energy at the output of the SOA pulse picker in dependence of the pulse picking frequency; **(b)** output pulse energy of the single-mode fiber amplifier in dependence of the repetition rate.  
Figure 10: All normal dispersion (ANDi) mode-locked fiber oscillator. LD, laser diode; WDM, wavelength division multiplexer; YDF, ytterbium-doped fiber; SMF, single-mode fiber; COL, collimator; QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarizing beam splitter; BPF, band pass filter; M, mirror; FI, Faraday isolator; color lines: black, fibers; green, active fiber; red, free-space propagation. The arrow at one collimator indicates the possibility to change the collimator position and, thus, the cavity length [48].  
repetition rate, the output energy after this pre-amplifier stages varies between 40 and 50 nJ (Figure 11b). After the amplification in the first stage, the pulses pass the power-amplifier, an ytterbium-doped double-clad fiber, which is supposed to boost the pulse energy to the \(\upmu\)J-regime in the near future.