  We propose a mechanism for realization of exact control of parity-time (PT)
symmetry by using a periodically modulated nonlinear optical coupler with
balanced gain and loss. It is shown that for certain appropriately chosen
values of the modulation parameters, we can construct a family of exact
analytical solutions for the two-mode equations describing the dynamics of such
nonlinear couplers. These exact solutions give explicit examples that allow us
to precisely manipulate the system from nonlinearity-induced symmetry breaking
to PT symmetry, thus providing an analytical approach to the all-optical signal
control in nonlinear PT-symmetric structures.
