  A study on the effects of optical gain nonuniformly distributed in
one-dimensional random systems is presented. It is demonstrated numerically
that even without gain saturation and mode competition, the spatial
nonuniformity of gain can cause dramatic and complicated changes to lasing
modes. Lasing modes are decomposed in terms of the quasi modes of the passive
system to monitor the changes. As the gain distribution changes gradually from
uniform to nonuniform, the amount of mode mixing increases. Furthermore, we
investigate new lasing modes created by nonuniform gain distributions. We find
that new lasing modes may disappear together with existing lasing modes,
thereby causing fluctuations in the local density of lasing states.
