Observation of dark-pulse formation in gain-clamped semiconductor optical amplifiers by cross-gain modulation

We have measured the ultrafast simultaneous cross-gain and laser mode dynam ics in a gain-clamped semiconductor amplifier perturbed by an intense detun ed 150 fs pump pulse. Besides relaxation oscillations, we demonstrate the i nstantaneous formation of a dark pulse in the laser mode that repeats itsel f with a period given by the cavity round-trip time. The dark pulse sequenc e subsequently decays into two-mode beating and is shown to weakly cross mo dulate the amplifier gain. To describe dark-pulse formation a time- and spa tially dependent model based on rate equations is necessary. The experiment al results are in reasonable agreement with numerical simulations. (C) 1999 American Institute of Physics. [S0003-6951(99)02750-3].