  The squeezing spectrum of the resonance fluorescence is studied for a
coherently driven four-level atom in the Y-type configuration. It is found that
the squeezing properties of the fluorescence radiation are modified
significantly when quantum interference of the spontaneous decays channels is
included. We show a considerable enhancement of steady-state squeezing in
spectral components for strong and off-resonant driving fields. The squeezing
may be increased in both the inner and outer sidebands of the spectrum
depending upon the choice of parameters. We also show that the interference can
degrade the spectral squeezing by increasing the decay rates of atomic
transitions. An analytical description using dressed states is provided to
explain the numerical results.
