Preparation of long-coherence-length second-harmonicgenerating optical fibers by using mode-locked pulses.

We have prepared an optical fiber that produces efficient (0.24%) second-harmonic conversion by using modelocked 1.064-microm laser pulses along with a harmonic seeding beam. The effective coherence length increased to 35 cm as compared with only a few centimeters in fibers prepared with mode-locked Q-switched pulses. We show that the input bandwidth of the pump pulse imposes a fundamental limit on the effective coherence length and that this length can be severely reduced by competing nonlinear effects.