Single-modeness of short large mode area fibers: an experimental study

We experimentally investigated the limits to the endlessly single-modeness of large mode area photonic crystal fibers. We analyzed two versions of a commercial large mode area fiber with equal structural parameters but five or seven confining rings, and in both cases we found that the single mode regime becomes critical for short (<1 m) lengths. In these cases we report on the excitation of a second-order mode, whose microphotographed structure is very well reproduced by the results of numerical simulations. The multimodal behavior is further confirmed by polarization measurements. Also we found that the single-modeness is maintained in longer (>1 m) fibers. The same results were observed in large mode area fibers with different core sizes.