Evanescent field coupling between a single-mode fiber and a planar waveguide with absorption or gain

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Abstract
A coupled-mode model is presented, which is capable of describing the light transfer between a single-mode fiber and a planar waveguide (PWG), taking into account the TE or the TM polarization of the PWG modes and an absorption or a gain into the PWG or the cover medium. Applying this model to the practical case of a coupler of limited length (both with distributed and fixed coupling strength) I show that the initially strongly asymmetric resonance curve of coupling broadens, becoming more and more symmetric as the absorption increases. When the PWG and/or the cover medium possesses a gain, a reverse resonance (with an amplified fiber power) appears next to the resonance minimum of the fiber power.