A theoretical model of synchronization of a mode-locked semiconductor laser with an external pulse stream

The frequency range over which a semiconductor laser will synchronize to an external pulse stream is analyzed. The laser is taken to be fully integrated, consisting of an amplifier, a passive waveguide and a saturable absorber. The model is of the lumped parameter type and, in particular, includes the effects of large signal optical pulses, absorption in the passive waveguide and gain dispersion and self-phase modulation in the amplifier. The situation in which the external signal illuminates the saturable absorber without being coupled optically into the laser cavity is analyzed. It is shown that, for synchronization to occur, the external signal must be of comparable intensity to that of the circulating pulse. The range over which locking will occur lies between 0.05% and 0.2% of the mode-locking frequency for external signals of amplitude 0.1 to 0.5 times that of the circulating pulse. It is concluded that integrated mode-locked semiconductor lasers could be used for clock extraction in communication systems