  Spectral singularities are ubiquitous with PT-symmetry leading to infinite
transmission and reflection coefficients. Such infinities imply the divergence
of the fields in the medium thereby breaking the very assumption of the
linearity of the medium used to obtain such singularities. We identify
saturable nonlinearity retaining contributions from all orders of the field to
limit the infinite growth and regularize the spectral singularity. We present
explicit numerical results to demonstrate regularization. The all order
nonlinear PT-symmetric device is shown to exhibit very effective isolation or
optical diode action, since transmission through such a system is
nonreciprocal. In contrast, a linear system or a system with Kerr nonlinearity
is known to have only reciprocal transmission. Further we demonstrate optical
bistability for such a system with high contrast.
