Typical main medium for high-power single-mode fiber lasers. But beyond a certain threshold, they will show a completely new effect, the so-called lateral mode instability (TMI). At certain power levels, high-order modes or even cladding modes suddenly appear, energy is dynamically transferred between these laser pointer modes, and the beam quality is reduced. The beam starts to fluctuate at the output.
Since the discovery of TMI, TMI has been observed in various fiber designs ranging from step-index fibers to photonic crystal fibers. Only its threshold varies with geometry and doping, but it is roughly estimated that this effect only appears after the output power exceeds 1kW. At the same time, this effect is combined with the thermal effect inside the fiber and has a strong correlation with the photo-darkening effect. In addition, the sensitivity of fiber lasers to TMI seems to be affected by the core modal composition.
The geometry of the step index fiber produces many parameters for optimization. The diameter of the core, the size of the pump cladding, and the difference in refractive index between the core and the pump cladding can be adjusted. This tuning depends on the doping concentration, that is, the concentration of Yb ions can be used to control the absorption length of the pump radiation in the activated fiber. Other dopants can be added to reduce thermal effects and control the refractive index step.
But there are some contrary requirements. To reduce nonlinear effects, the fiber should be shorter. However, to reduce thermal load, the fiber should be longer. Photodarkening increases with the square of the doping concentration, so longer fibers with lower doping will be better.
The initial recommendations for these parameters can be found in the simulation. Some parameters, such as thermal behavior, can be simulated but are difficult to predict, especially because photo-darkening is very low and cannot be measured by accelerated testing. Therefore, directly measuring the thermal behavior in the fiber will help plan the red laser pointer experiment.