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.
Highlights
Fiber laser systems are able to simultaneously deliver high average powers and excellent beam quality
In the experiment an artificially applied pump intensity-noise of 2.9% led to a reduction of the transverse mode instability (TMI) threshold of 63%, whereas a similar seed intensitynoise decreased it by just 13%
Experimental results and discussion As detailed in “Frequency region of interest” section, the frequency region of interest for both pump and seed intensity-noise was found to be in the range from 1 Hz up to 2 kHz for the active fiber used in these experiments
Summary
Fiber laser systems are able to simultaneously deliver high average powers and excellent beam quality. This is mostly because they can handle high heat loads due to the efficient heat extraction granted by their large surface-to-volume ratio [1, 2]. Even the fiber-laser technology has suffered from thermal effects during the last decade. The most detrimental of these effects is transverse mode instability (TMI), which discovery was first published in 2010 [3]. TMI manifests itself as a dynamic energy transfer between different transverse modes, which occurs sharply above a specific average-power threshold [4]. Above this TMI threshold the beam profile exhibits spatial and temporal fluctuations on a millisecond timescale [5], which prevent the utilization of fiber lasers and amplifiers for many applications
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