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The variation in the output pulse duration of a mode-locked internally frequency-doubled (MLFD) laser was experimentally studied. The dependence of pulse duration on second-harmonic conversion rate and modulation index in a Nd:YAG laser is reported. The laser was frequency doubled by a barium sodium niobate crystal located inside the laser cavity and was mode locked by a quartz acousto-optic loss modulator. The observed variation of pulse duration with modulation depth and second-harmonic conversion rate is in good agreement with theory. The techniques reported should allow continuous variation of 1.06-μm pulse duration from about 50 ps to greater than 900 ps to be obtained.