  The concept of quasi-PT symmetry in optical wave guiding system is elaborated
by comparing the evolution dynamics of a PT-symmetric directional coupler and a
passive directional coupler. In particular we show that in the low loss regime,
apart for an overall exponentially damping factor that can be compensated via a
dynamical renormalization of the power flow in the system along the propagation
direction, the dynamics of the passive coupler fully reproduce the one of the
PT-symmetric system.
