We propose what is, to our knowledge, a novel method of generating tunable narrow-bandwidth picosecond pulses in which the chirp of equally stretched pump and signal is canceled by difference-frequency mixing. The effect of the group-velocity mismatch on the output pulse bandwidth is analyzed. The method is proved experimentally by generation of pulses of 3.3-cm~(-1) bandwidth and 10-ps duration in a type I #beta# barium borate crystal, starting from 1-ps pulses.
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