Ultra-broadband thin-film lithium niobate TM-pass waveguide polarizer using subwavelength grating metamaterial

An ultra-broadband TM-pass waveguide polarizer based on thin-film LiNbO3-on-insulator waveguide and sub-wavelength grating (SWG) metamaterials is proposed and modeled. The device consists of an SWG in the central part and two coupling regions in the input and output sides. The SWG, formed by a series of bending nano-stripes, functions as a filter of polarization, and the two coupling regions serve to degrade mode mismatch loss. By tailoring the duty cycle of the SWG metamaterial, the SWG structure supports only the TM guided mode, while the TE guided mode is cutoff and thus filtered out. Simulation shows that, with a SWG meta-material formed by a series of 90 degrees -bending nano-stripes, the polarizer can achieve an ultra-large bandwidth of-415 nm (1.26-1.675 mu m) with an insertion loss < 1.1 dB and an extinction ratio > 29.4 dB. Moreover, the proposed polarizer tolerates a 5 nm deviation in both width and thickness of the nano-stripes. Such an ultra-broadband device may find its use in high-capacity optical communication systems, and is proposed firstly on the basis of the lithium-niobate-on-insulator platform, to the best of our knowledge.