  Among the interesting outcomes of the study of the physical applications of
spectral singularities in PT-symmetric optical systems is the discovery of
CPA-lasers. These are devices that act both as a threshold laser and a coherent
perfect absorber (CPA) for the same values of their physical parameters. Unlike
a homogeneous slab that is made to act as a CPA, a slab CPA-laser would absorb
the incident waves coming from the left and right of the device provided that
they have appropriate intensity and phase contrasts. We provide a comprehensive
study of one of the simplest experimentally accessible examples of a CPA-laser,
namely a PT-symmetric optical slab system consisting of a balanced pair of
adjacent or separated gain and loss components. In particular, we give a closed
form expression describing the spectral singularities of the system which
correspond to its CPA-laser configurations. We determine the intensity and
phase contrasts for the TE and TM waves that are emitted (absorbed) whenever
the slab acts as a laser (CPA). We also investigate the behavior of the
time-averaged energy density and Poynting vector for these waves. This is
necessary for determining the optimal values of the physical parameters of the
system that make it act as a CPA-laser. These turn out to correspond to
situations where the separation distance $s$ between the gain and loss layers
is an odd multiple of a characteristic length scale $s_0$. A curious by-product
of our study is that, except for the cases where $s$ is an even integer
multiple of $s_0$, there is a critical angle of polarization beyond which the
energy of the waves emitted from the lossy layer can be larger than the energy
of those emitted from the gain layer.
