  We measured the ensemble-averaged spectral correlation functions and
statistical distributions of spectral spacing and intensity for lasing modes in
weakly scattering systems, and compared them to those of the amplified
spontaneous emission spikes. Their dramatic differences illustrated the
distinct physical mechanisms. Our numerical simulation revealed that even
without reabsorption the number of potential lasing modes might be greatly
reduced by local excitation of a weakly scattering system. The lasing modes
could be drastically different from the quasimodes of the passive system due to
selective amplification of the feedback from the scatterers within the local
gain region.
