  We observe experimentally a mode-locking in a continuous narrow-band tunable
dye laser with molecular iodine absorber cells, which transitions have large
phase relaxation time T2. We show that the mode-locking arises due to coherent
interaction of light with the absorbing medium leading to Rabi oscillations, so
that zero-area (0{\pi}-) pulses in the absorber are formed. Such mode-locking
regime is different to most typical passive modelocking mechanisms where
saturation plays the main role.
