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A new method for locking the longitudinal modes of a laser resonator and generating ultrashort pulses of light is described. The cavity modes are coupled together when a medium with a refractive index nonlinearity is placed in the cavity. A theoretical study is presented which analyzes the mode structure of a laser resonator containing a cell filled with an anisotropic molecular liquid. It is found that under certain conditions the energy exchange between the modes gives rise to a mode-locked spectrum and to the attendant generation of ultrashort pulses of light ( second for a ruby laser, 10-12second for a Nd3+: glass laser).