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ween ear mpr t de elliptical polarizations. By making the convertor mode in a single waveguide (for example, a vertically June 15, 2007 / Vol. 32, No. 12 / OPTICS LETTERS 1683from a microring we gain all the usual advantages these structures provide, including economy of size, the ability to use current fabrication techniques, and frequency dependence due to resonance. By using simple geometries, the analysis of such microrings is quite simple, and their behavior should be very pre-dictable. Finally, we show that, owing to the nonlin-ear Kerr effect in Si, the polarization of the transmit-ted signal exhibits a bistable behavior [11,12]. This effect allows us to use the microring as a switch that can exist in two different states determined by, for example, the header of a packet. Each of these states corresponds to a different polarization of transmitted signal. For a dielectric waveguide horizontally coupled to a microring, the two polarizations that are parallel and perpendicular to the direction of coupling (TE, hori-zontal electric, and TM, vertical electric, respectively) polarized mode) passes close to a microring, then it is no longer an eigenmode of this coupled waveguide system. It can instead be thought of as a superposi-tion of bonding and antibonding modes of the coupled waveguide system [14]: In the same way, a signal in the microring can be thought of as superposition: It was confirmed numerically that these simple ex-pansions are accurate approximations of the actualNonlinear polarization c reson Chris Fietz1, *