Mode-locking based on a zero-area pulse formation in a laser with a   coherent absorber
  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. 
Mikhail V. Arkhipov, Alexander A. Shimko, Alexey A. Kalinichev, Ihar   Babuskin, Nikolai N. Rosanov, and Rostislav M. Arkhipov
physics.optics