  Amplitude and phase control of gain without inversion is investigated in a
four level loop-structure atomic system. Two features are presented. One is
that gain without inversion can be obtained through the amplitude control of
the applied fields. The other is that gain without inversion show a
phase-dependence on the relative phase between the fields applied on the two
two-photon transitions. Gain and phase-dependence originate from interference
induced by such a loop-transition structure.
