We experimentally observe mode-locking in a continuous narrowband tunable dye laser with molecular iodine absorber cells, whose transitions have long phase relaxation time T ₂. We show that mode-locking arises due to the coherent interaction of light with the absorbing medium leading to Rabi oscillations, so that zero-area () pulses in the absorber are formed. Such mode-locking regimes are different from most typical passive mode-locking mechanisms where saturation plays a major role.