Instant and efficient second-harmonic generation and downconversion in unprepared graded-index multimode fibers

We show that germanium-doped graded-index multimode silica fibers can exhibit relatively high conversion efficiencies (similar to 6.5%) for second-harmonic generation when excited at 1064 nm. This frequency-doubling behavior is also found to be accompanied by an effective downconversion. As opposed to previous experiments carried out in single-and few-mode fibers where hours of preparation were required, in our system, these. chi((2)) related processes occur almost instantaneously. The efficiencies observed in our experiments are, to the best of our knowledge, among the highest ever reported in unprepared fibers. (c) 2017 Optical Society of America