Study and control of the intermodal energy transfer in high-power fiber laser systems

• Applicant: Dr.-Ing. Cesar Jauregui Misas, Ph.D.
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 416342891

The power extraction in laser systems is accompanied by the unwanted generation of heat, which can lead to the degradation of the laser performance. Fibers are particularly resilient against thermo-optic effects due to their extremely elongated geometry, which allows for an efficient heat evacuation due to the large surface to volume ratio. In spite of this, fibers are not completely immune against thermo-optic effects. In fact, when the output average power of a fiber laser system reaches a certain threshold, the formerly stable high-quality output beam starts to fluctuate and its intensity profile is distorted. The reason for this behavior is the unstable energy transfer between different transverse modes in the active fiber assisted by the creation of a thermally-induced index grating in the fiber. Such an effect is called transverse mode instabilities (TMI) and it has quickly become the strongest limitation for the further scaling of the output average power of fiber laser systems worldwide. This, in part, explains the strong interest that such effect has attracted recently in the scientific community.Very recently, a new mitigation strategy for TMI has been demonstrated by the applicant. It is based on weakening the thermally-induced index grating by modulating the pump power. However, while doing these experiments, it became apparent that the pump modulation also led to a periodic energy transfer between the transverse modes in the fiber. This is an extremely important observation, since this is the first time that it has been demonstrated that the direction of the energy flow can be influenced from the outside. Thus, it is the main goal of this project to learn to control such energy flow. If the project is successful it can be the first step towards turning one of the most damaging problems in fiber laser technology into an advantage, since it will lead to these systems emitting all their energy into a single (selectable) transverse mode regardless of the output power, something extremely interesting for the applications and unheard of in any other laser technology.

• DFG Programme: Research Grants