Nonunitary optics for irreversible propagation

The time reversibility of optics is a basic feature and of importance in many schemes. However the time reversibility is a major drawback for some optical devices, e.g. an optical isolator. In the followings we discuss the breaking of time reversal, by adding controlled positive and negative dissipation. The basic ingredient for implementing a waveguide isolator is a dual mode waveguide where two optical modes are unidirectionally coupled by periodic perturbation. The reversibility of is manifested by the fact that the Fourier transform of real functions (transparent optics) are symmetric (antisymmetric). We test first a pure imaginary (dissipative) perturbation function with transversely antisymmetric perturbation (necessary for coupling between two consecutive modes).