Dynamic waveguide properties of active fibers under high power operation

Large Pitch photonic crystal Fibers (LPFs) have enabled the output power increase of fiber lasers with mode field diameters exceeding 50 μm. However, these fibers need strict refractive index matching between the glass matrix and the doped material in the signal core. On the one hand a higher refractive index for the doped material will lead to a step-index guiding structure, on the other hand a strong depression results in huge deformations of the modes. This effect is even more critical in double clad structures, necessary for high power diode pumping. In this case, core guided modes can interact with the modes of the highly multimode double clad structure. An index depression of the doped area can lead to mode anticrossings, as the effective refractive indices of both modes converge and both modes deform strongly. In this contribution the refractive index change in high power fiber lasers is investigated. Therefore, a fiber with a mode anticrossing is experimentally studied. By pumping this fiber, the mode anticrossing can be passed and a quantitative estimate for the refractive index change can be given.