A current limit in power scaling of Yb-doped fiber amplifiers is the sudden onset of mode instabilities. We investigated this effect on a single-frequency Yb-doped photonic crystal fiber amplifier with a low mode instability threshold power. By measuring the overlap of the fiber output beam with the fundamental mode of an external cavity to be about 95%, we could exclude significant modal power transfer below a sharp power threshold. Furthermore, we directly measured the frequency resolved intensity noise spectra. No fluctuations in the overall output power were observed, but for the modal content different oscillation regimes were identified.

Gain dynamics and refractive index changes in fiber amplifiers are important in many areas. For example, the knowledge of the frequency responses for seed and pump power modulation are required to actively stabilize low noise fiber amplifiers. Slow and fast light via coherent population oscillations rely on the change of group index to delay or advance pulses, and refractive index changes in fiber amplifiers are a possible explanation for mode fluctuations in high power fiber amplifiers. Here, we analyze the frequency dependent influence of seed and pump power modulation on the fiber amplifier output power and the refractive index. We explain the observed power and refractive index modulation with an analytic model originally developed for telecom amplifiers and discuss a further simplification of the model.

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The Free‐Electron Laser (FEL) FLASH offers the worldwide still unique capability to study ultrafast processes with high‐flux, high‐repetition rate extreme ultraviolet, and soft X‐ray pulses. The vast majority of experiments at FLASH are of pump–probe type. Many of them rely on optical ultrafast lasers. Here, a novel FEL facility laser is reported which combines high average power output from Yb:YAG amplifiers with spectral broadening in a Herriott‐type multipass cell and subsequent pulse compression to sub‐100‐fs durations. Compared to other facility lasers employing optical parametric amplification, the new system comes with significantly improved noise figures, compactness, simplicity, and power efficiency. Like FLASH, the optical laser operates with 10‐Hz burst repetition rate. The bursts consist of 800‐μs long trains of up to 800 ultrashort pulses being synchronized to the FEL with femtosecond precision. In the experimental chamber, pulses with up to 50‐μJ energy, 60‐fs full‐width half‐maximum duration and 1‐MHz rate at 1.03‐μm wavelength are available and can be adjusted by computer‐control. Moreover, nonlinear polarization rotation is implemented to improve laser pulse contrast. First cross‐correlation measurements with the FEL at the plane‐grating monochromator photon beamline are demonstrated, exhibiting the suitability of the laser for user experiments at FLASH.