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A polarization-independent racetrack type micro-ring resonator formed by silicon-on-insulator slot waveguides with a phase compensation section included was investigated and proposed. By tuning the ratio of lengths of the slot waveguide and the channel waveguide the cumulative phase difference between quasi-TE and quasi-TM modes can be well eliminated which allows for a polarization independent oper ation over a wide spectral range. The finesses are 226 and 225 for the quasi-TE and quasi-TM modes respectively, with a free spectral range of 9 nm achieved as well as a compact device size of 30 m, while delivering a good polarization-independent performance with the resonance mode mismatch less than 0.5 nm. Keywords: micro-ring resonator, racetrack resonator, polarization-independent, slot waveguide, directional coupler, phase compensation, silicon-on-insulator. 1. INTRODUCTION The micro-ring resonator (MRR) is one of the most important optical components of photonic integrated circuits (PICs) and versatile to realize many functions such as filters [1], switches [2] , modulators [3] and sensors [4]. MRRs must be capable of polarization-independent (PI) operation while maintaining single-mode (SM) and low round-trip losses for commercial viability. Without employing an extrinsic polarization management scheme such as polarization diversity [5][6], the MRR must be designed to be intrinsically PI in order to obtain identical response for input optical signals of random polarizations. As bend-based devices, MRRS inherently have higher polarization sensitivity in contrast to straight waveguide based devices. In high index contrast platforms such as the silicon-o n-insulator (SOI) platform, most MRRs realized thus far by using ridge, or photonic wire waveguides [7][8] with cross-sectional dimensions reduced to submicron scale. The bend base feature and the extremely compact size of MRRs make these narrow ridge waveguides strongly polarization dependent, consequently result in unpredictable and non-overlapping reso nance wavelengths and filter transfer functions for quasi-TE- and quasi-TM-polarized input signals [9][10]. In this letter, an racetrack type optical MRR based on slot waveguides is proposed and analyzed by using rigorous full-vectorial analysis methods based on finite-difference-time-domain (FDTD) and beam-propagation-method (BPM) schemes [11] . The employing of slot waveguides allows us to optimize the polarization-dependent properties with an extra degree of freedom (i.e. the width of slot) and therefor e realize a PI MRR as well as a compensation operation. The resonance wavelength mismatch between the two orthogonal polarization modes is less than 0.5 nm over 1530 1570 nm , with the finesses of 226 and 225 achieved for the quasi-TE and quasi-TM modes resp ectively. The free spectral range of ? nm is obtained as well as a compact device size of less than 30 m. |
