Optimizing large-pitch fibers for higher average powers

摘要

The large-pitch fiber design and its fundamental operating principle - the delocalization of higher-order modes - have enabled the current performance records of ultrashort-pulse fiber laser systems. However, further average power scaling is limited by transverse mode instabilities. This effect has been linked to the average thermal load inside the active fiber and, consequently, it is strongly enhanced for short fiber amplifiers that are typically used for ultrashort-pulse amplification. Recently, it has also been discovered that photo-darkening is an additional, very important heat source in an active fiber that can be as strong as the quantum defect. A reduction of the Yb-doping concentration in high-power active fibers is a promising way to mitigate this problem. In this contribution we propose a new large-pitch design which has an increased delocalization of higher-order modes and, at the same time, allows for a larger core to clad ratio that enables reducing the Yb-ion concentration while maintaining a similar amplification efficiency. A comprehensive laser simulation that includes modal properties, thermo-optical effects and photo-darkening is used to highlight the important benefits of this design. Additionally, this novel and compact fiber design can be a promising unit-cell for a multi-core fiber, which should ultimately enable the development of multi-kW average power and multi-mJ pulse energy ultrashort-pulse fiber lasers in the near future.