In this paper, polarization rotation due to optical waveguide bending is analytically and numerically investigated. It is shown that the process of polarization conversion in the bend strictly depends on the characteristics of the straight waveguide. The strength of polarization coupling linearly increases with the waveguide refractive index contrast, whereas birefringence is responsible for the phase mismatch that limits the conversion efficiency. Waveguides with refractive index profile symmetrical in the direction orthogonal to the plane of the bend do not exhibit polarization rotation. Inside optical ring resonators, polarization rotation induced by waveguide bending is found to be highly enhanced by the cavity effect, which gives rise to strongly frequency dependent polarization rotation and dramatically alters the ring's spectral response. In the case of single-ring phase shifters, polarization rotation is effectively described by means of an approach based on the theory of poles-zeros loci, which straightforwardly provides conditions for complete polarization conversion, also in the presence of birefringence and losses. The detrimental effects caused by undesired polarization coupling on the spectral response of widespread optical devices based on ring resonators are also discussed. From this point of view, birefringence is demonstrated to be beneficial to make optical components less sensitive to the effects of polarization rotation.
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