Resonant mode conversion in the waveguides with unbroken and broken PT symmetry

We study resonant mode conversion in parity time (PT)-symmetric multimode waveguides, where symmetry breaking manifests itself in the sequential destabilization (i.e., the appearance of complex eigenvalues) of the pairs of adjacent guided modes. We show that efficient mode conversion is possible even in the presence of the resonant longitudinal modulation of the complex refractive index. The distinguishing feature of the resonant mode conversion in the PT-symmetric structure is a drastic growth of the width of the resonance curve when the gain/losses coefficient approaches a critical value, at which symmetry breaking occurs. We found that in the system with broken symmetry, the resonant coupling between the exponentially growing mode and the stable higher-order one effectively stabilizes dynamically coupled pairs of modes and remarkably diminishes the average rate of the total power growth. (C) 2014 Optical Society of America