OBSERVATION OF SPECTRAL BROADENING CAUSED BY SELF-PHASE MODULATION INHIGHLY MULTIMODE OPTICAL-FIBER

We observed spectral broadening caused by self-phase modulation in 400 - and 600-mu m core diameter fibers using amplified, Q-switched, Nd:YA G laser pulses with peak powers to 150 kW. The degree of spectral broa dening was not dependent linearly on the fiber length as in single-mod e fibers because of the more complicated modal evolution in highly mul timode fiber. Furthermore, even slight stress near the input end of th e fiber reduced the observed broadening. The results have significant implications for the delivery of high-peak-power laser beams through o ptical fiber with high-output beam quality for industrial applications . (C) 1997 Optical Society of America.