Mode Instability In Fiber Amplifiers Research Articles

Theory of transverse mode instability in fiber amplifiers with multimode excitations

Theory of transverse mode instability in fiber amplifiers with multimode excitations

Accurate Modeling of Transverse Mode Instability in Fiber Amplifiers

Transverse mode instability is a key limit to power scaling of high-power fiber lasers. Accurate modeling efforts have, however, been hampered by a lack of experimental data to verify a model. Recently, there have been some good experimental studies, making it possible to validate a model. In this work, we developed a model by integrating a 3D fiber amplifier and stimulated thermal Rayleigh scattering. Since we are only interested in the regime where the fundamental mode dominates, our 3D amplifier divides the core into many cylindrical shells. This limits the model to situations where bend-induced mode distortion of the fundamental mode is negligible, but it is still applicable for most practical scenarios. The benefit of this model is high computational efficiency; it can run in minutes on a PC. This 3D amplifier model considers various pumping configurations and amplified spontaneous emission. It can simulate most experimental conditions. Excellent quantitative fit to experimental data was achieved. Additional studies were also conducted to show that gain saturation is a dominating effect in understanding the observed behaviors of transverse mode instability.

Intensity noise as a driver for transverse mode instability in fiber amplifiers

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.

Steady-periodic method for modeling mode instability in fiber amplifiers

We present a detailed description of the methods used in our model of mode instability in high-power, rare earth-doped, large-mode-area fiber amplifiers. Our model assumes steady-periodic behavior, so it is appropriate to operation after turn on transients have dissipated. It can be adapted to transient cases as well. We describe our algorithm, which includes propagation of the signal field by fast-Fourier transforms, steady-state solutions of the laser gain equations, and two methods of solving the time-dependent heat equation: alternating-direction-implicit integration, and the Green's function method for steady-periodic heating.