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Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings

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Abstract

An ultrasmall polarization splitter and rotator (PSR) on a silicon-on-insulator (SOI) platform is proposed based on an asymmetric directional coupler (ADC) consisting of a bridged subwavelength grating (BSWG) and a strip silicon waveguide with another SWG embedded in the middle slot. The inputting TE mode is tightly confined in BSWG and directly passes through a bar port with little coupling to the adjacent waveguide because of the phase mismatch, while the phase-matching condition allows the injected TM mode to be evanescently decoupled from BSWG and simultaneously converted into the TE mode supported by a SWG slot waveguide before propagating out through the cross port. Both polarization-dependent behaviors within the proposed PSR can be readily realized and efficiently manipulated by independently controlling the attributes of the ADC waveguides, which are distinctly engineered by different SWG segments. We have verified what we believe, to the best of our knowledge, is the first combination of BSWG and a SWG slot waveguide that provides an additional degree of freedom in the design and a stronger modal hybridization due to the doubled effects from jointly used SWGs. Further, the increasing number of inserted SWGs in the present device drastically reduces the device footprint, leading to a coupling length of only 5.5 µm. From the results, the conversion loss of $-0.28\;{\rm dB}$ (polarization conversion ratio of 93.7%) has been obtained for a TM-to-TE conversion at 1.55 µm. The TM mode of the resulting PSR shows its cross talk is less than $-14.3\;{\rm dB}$ and the conversion loss is better than $-0.63\;{\rm dB}$ in a 95 nm bandwidth that has covered the entire C and L bands. The cross talk and insertion loss of the TE mode are lower than $-29\;{\rm dB}$ and $-0.42\;{\rm dB}$, respectively, as the operating wavelength varies.

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