Abstract: In this work we present a novel way to manipulate the effect of transverse mode instability by inducing traveling waves in a high-power fiber system. What sets this technique apart is the fact that it allows controlling the direction of the modal energy flow, for the first time to the best of our knowledge. Thus, using the method proposed in this work it will be possible to transfer energy from the higher-order mode into the fundamental mode of the fiber, which mitigates the effect of transverse mode instability, but also to transfer energy from the fundamental mode into the higher-order mode. Our simulations indicate that this approach will work both below and above the threshold of transverse mode instability. In fact, our model reveals that it can be used to force a nearly pure fundamental mode output in the fiber laser system almost independently of the input coupling conditions. In this context, this technique represents the first attempt to exploit the physics behind the effect of transverse mode instability to increase the performance of fiber laser systems.
Abstract: Roughly a decade ago the future of fiber laser technology looked brighter than ever with enticing power scaling prospects  . These predictions seemed to be supported by the unprecedented exponential rate of power increase sustained over the previous two decades  . At those times, multi ten-kW fiber laser systems seemed within reach. So what could possibly go wrong?
Abstract: The effect of transverse mode instability (TMI) is currently the main limitation for the further average-power scaling of fiber laser systems with diffraction-limited beam quality. In this work a main driving force for TMI in fiber amplifiers is identified. Our experiments and simulations illustrate that the performance of fiber laser systems in terms of their diffraction-limited output power can be significantly reduced when the pump or seed radiation exhibit intensity noise. This finding emphasizes the fact that the TMI threshold is not only determined by the active fiber but, rather, by the whole system. In the experiment an artificially applied pump intensity-noise of 2.9% led to a reduction of the TMI threshold of 63%, whereas a similar seed intensity-noise decreased it by just 13%. Thus, even though both noise sources have an impact on the TMI threshold, the pump intensity-noise can be considered as the main driver for TMI in saturated fiber amplifiers. Additionally, the work unveils that the physical origin of this behavior is linked to the noise transfer function in saturated fiber amplifiers. With the gained knowledge and the experimental and theoretical results, it can be concluded that a suppression of pump-noise frequencies below 20 kHz could strongly increase the TMI threshold in high-power fiber laser systems.
Abstract: In this work we present a novel way to mitigate the effect of transverse mode instability in high-power fiber amplifiers. In this technique a travelling wave is induced in the modal interference pattern by seeding the amplifier with two modes that have slightly different frequencies. The interference pattern thus formed will travel up-or downstream the fiber (depending on the sign of the frequency difference between the modes) with a certain speed (that depends on the absolute value of the frequency difference). If the travelling speed is chosen properly, the thermally-induced index grating will follow the travelling modal interference pattern creating a constant phase shift between these two elements. Such a constant controllable phase shift allows for a stable energy transfer from the higher-order modes to the fundamental mode or viceversa. Thus, this technique can be adjusted in such a way that, at the output of the fiber almost all the energy is concentrated in the fundamental mode, regardless of the excitation conditions. Moreover, this technique represents one of the first examples of the new family of mitigation strategies acting upon the phase shift between the modal interference pattern and the refractive index grating. Additionally, it even exploits the effect of transverse mode instability for gaining control over the beam profile at the output of the amplifier. Therefore, by adjusting the frequency difference between the seed modes, it is possible to force that the beam at the output acquires the shape of the fundamental mode or that of a higher order mode.
Abstract: In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the threshold of transverse mode instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed-or the pump intensity-noise, has a severer impact on the TMI threshold. Our experiments have shown that the TMI threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI threshold of fiber laser systems.