Optical unidirectional devices by complex spatial single sideband perturbation

Important functionality of photonics circuits is invoking interactions between few optical modes. Intermodal power exchange can be realized by introducing a spatial perturbation of the refractive index. The power transfer is symmetric from mode n to m and vice versa, as well as time reversible-both can impede definite performance of a variety of optical devices. By modifying the coupling concept-i.e., using a matched periodic modulation of both the index of refraction and loss (gain) of the medium, we implement a complex spatially single sideband perturbation (SSP), which breaks the symmetry and allows a unidirectional energy flow from mode m to mode n, while blocking the inverse flow. We investigated various deviations from ideal conditions including detuned schemes (gratings period not matched with the difference between propagation constants), unequal real and imaginary coupling coefficients, self-coupling coefficients, saturation effects and spatial separation of the real and imaginary perturbations. Finally the SSP is exemplified in the design of a novel broad-band "add" component for optical add/drop multiplexing system.