Over the lifetime of a fiber laser, the TMI threshold is decreased due to photodarkening of the fiber. Many investigations have been made to model both effects, but the microscopic mechanisms both of TMI and PD are not yet fully understood. The existing models are either comprehensive, but very slow and therefore limited to the simulation of short fibers, or reduced models that do not take transverse effects into account. Furthermore, these models have been applied only to single-pass fiber amplifiers so far.
We present a hierarchical numerical approach that allows to first precalculate the transverse distribution of the photodarkening losses, and then apply the precalculated data to a scalar coupled-mode model of the fiber laser. As a result, it is possible to perform virtual long term tests simulating several 10 000 hours of laser operation in a few hours. The transverse distribution of photodarkening losses in the fiber and the mode coupling gain can be analyzed at any cross section along the fiber.
The simulation results are compared to experimental data, which demonstrates the feasibility of the approach to predict the TMI threshold for different laser setups.