Theoretical and experimental study of bent fully aperiodic large-pitch fibers for enhancing the high-order modes delocalization

The power scaling of fiber lasers and amplifiers has triggered an extensive development of large-mode area fibers among which the most promising are the distributed mode filtering fibers and the large-pitch fibers. These structures enable for an effective higher-order modes delocalization and subsequently a singlemode emission. An interesting alternative consists in using the fully-aperiodic large-pitch fibers, into which the standard air-silica photonic crystal cladding is replaced by an aperiodic pattern made of solid low-index inclusions cladding. However, in such a structure, the core and the background cladding material surrounding it must have rigorously the same refractive index. Current synthesis processes and measurement techniques offer respectively a maximum resolution of 5×10 -4 and 1×10 -4 while the indexmatching must be as precise as 1×10 -5 . Lately a gain material with a refractive index 1.5×10 -4 higher than that of the background cladding material was fabricated, thus re-confining the first higher-order modes in the core. A numerical study is carried out on the benefit of bending such fully-aperiodic fiber to counteract this phenomenon. Optimized bending axis and radius have been determined. Experiments are done in a laser cavity operating at 1030 nm using an 88cm-long 51μm core diameter ytterbium-doped fiber. Results demonstrate an improvement of the M 2 from 1.7 when the fiber is kept straight to 1.2 when it is bent with a 100 to 60 cm bend radius. These primary results are promising for future power scaling.