Christoph Stihler

Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.More power to fibre optic lasersResearchers have cracked a ‘did the chicken come before the egg’ dilemma in fibre optic lasers. The new understanding of ‘transverse mode instability’ could lead to even stronger lasers. Jens Limpert and colleagues at Germany’s Friedrich-Schiller Universität Jena provided experimental evidence that increasing the power of an optic fibre beyond a certain point degrades laser output because interference patterns between light waves within the fibre core become ‘out of sync’ with the core’s refractive index grating; called a phase shift. Researchers had not been sure if the instability of light waves within the fiber core was initially triggered by this phase shift or by the growing core’s refractive index grating with increasing power. A poor understanding of transverse mode instability has halted further improvements in optic fibre technologies, which produce high power lasers for industry, medicine and defence. The new information could lead to strategies that mitigate the phenomenon.

Transverse mode instabilities in burst operation of high-power fiber laser systems

We propose, to the best of our knowledge, the first mitigation strategy for TMI based on controlling the phase shift between the thermally-induced index grating and the modal intensity pattern. In particular, in this work we present a study of transverse mode instabilities in burst operation in a high-power fiber laser system. It is shown that, with a careful choice of the parameters, this operation regime can potentially lead to the mitigation of TMI by forcing an energy transfer from the higher-order-modes into the fundamental mode during the burst.

Towards the control of the modal energy transfer in transverse mode instabilities

Thermally-induced refractive index gratings (RIG) in high-power fiber laser systems lead to transverse mode instabilities (TMI) above a certain average power threshold. The effect of TMI is currently the main limitation for the further average power scaling of fiber lasers and amplifiers with nearly diffraction-limited beam quality. In this work we experimentally investigate, for the first time, the growth of the RIG strength by introducing a phase-shift between the RIG and the modal interference pattern in a fiber amplifier. The experiments reveal that the RIG is strong enough to couple energy between different transverse modes even at powers significantly below the TMI threshold, provided that the introduced phase-shift is high enough. This indicates that, as the strength of the RIG further increases with increasing average output power, the RIG becomes more and more sensitive to even small noise-induced phase-shifts, which ultimately trigger TMI. Furthermore, it is shown that a beam cleaning also occurs when a positive phase-shift is introduced, even above the TMI threshold. This finding will pave the way for the development of a new class of mitigation strategies for TMI, which key feature is the control of the introduced phase-shift.

Dependence of the mode instability threshold of high-power fiber laser systems on core co-dopants

The phenomenon of Transverse Mode Instabilities (TMI) [I] has become one of the effects more discussed about in the field of high-power fiber laser systems. The reason is that TMI results in the sudden degradation of the quality and stability of the beam emitted by a fiber laser system once that a certain average power threshold has been reached, which seriously threatens the future applicability of this technology.

Mitigation of mode instabilities in high-power fiber laser systems by active modulation of the pump power

In recent years the evolution of the average output power emitted by high-power fiber-lasers and amplifiers has reached a level at which the onset of thermal effects has been observed. The most detrimental of these effects is the phenomenon of transverse mode instabilities (TMI) [1]. These instabilities are characterized by a sudden onset of spatial and temporal fluctuations in a formerly stable beam once a certain average output power threshold has been reached [2]. TMI are currently the most limiting effect for the further average output power scaling of fiber-laser systems and amplifiers with nearly diffraction-limited beam quality. Therefore, the development of mitigation strategies for TMI is critical to further enhance the performance of this technology.

The impact of core co-dopants on the mode instability threshold of high-power fiber laser systems

Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.

Controlling mode instabilities at 628 W average output power in an Yb-doped rod-type fiber amplifier by active modulation of the pump power

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.

Experimental investigation of transverse mode instabilities in a double-pass Yb-doped rod-type fiber amplifier

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.

Thermal optimization of high power fiber laser systems

This work presents an overview on the latest advancements in the understanding of transverse mode instabilities (TMI) together with guidelines to optimize high power fiber laser systems from the thermal point of view.