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• A thin-film lithium niobate TM-pass waveguide polarizer is proposed. • Subwavelength grating metamaterial for polarization generation. • Simulation shows that the device can operate with a bandwidth ∼415 nm. • The device shows reasonable extinction ratio and insertion loss. An ultra-broadband TM-pass waveguide polarizer based on thin-film LiNbO 3 -on-insulator waveguide and subwavelength 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°-bending nano-stripes, the polarizer can achieve an ultra-large bandwidth of ∼415 nm (1.26–1.675 μm) 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. [ABSTRACT FROM AUTHOR]

Multi-mode Ti:LiNbO 3 planar waveguide was fabricated by in-diffusion of 100-nm-thick Ti-metal film coated onto a near-stoichiometric LiNbO 3 substrate prepared by vapor transport equilibration. The crystalline phase and composition in the waveguide were characterized. The guided modes in the planar waveguide were characterized by prism coupling technique. The refractive index profile is constructed from the measured mode indices using the inverse Wentzel–Krames–Brillouin method, and correlated with the Ti 4+ - concentration profile, which was obtained from secondary-ion-mass-spectroscopy analysis. The results show that the waveguide still retains the LiNbO 3 phase, its composition is near-stoichiometric. The index change and Ti 4+ -concentration follow an exponential relationship with a power index 0.4 for an ordinary ray and 1.07 for an extraordinary ray. A comparison of various congruent and near-stoichiometric bulk materials or waveguides allows to conclude that the relationship is different from congruent to near-stoichiometric composition, from one fabrication method to another, and from bulk material to waveguide configuration. The difference is explained qualitatively. [ABSTRACT FROM AUTHOR]

• Polarization beam splitter(PBS) based on LNOI triple-waveguide coupler is proposed. • A plasmonic waveguide is used to exalt modal birefringence and control TM mode coupling. • Good performance of high polarization extinction ratio and low insertion loss is expected. • The device has compact size for on-chip application and definite fabrication tolerance. • Present work offers a promising strategy for compact, efficient PBS based on LNOI. A compact polarization beam splitter based on a triple-waveguide plasmonic directional coupler is proposed and modeled. The directional coupler consists of two usual lithium-niobate-on-insulator (LNOI) ridge waveguides and a metal-capped plasmonic LNOI waveguide located between them. It utilizes stronger polarization dependence of modal birefringence of a plasmonic waveguide. By appropriately designing the plasmonic waveguide geometry, the TM component wave meets the resonant coupling condition and outputs at cross port. Instead, the TE component wave does not meet the resonant coupling condition, cannot be resonant in the triple-waveguide directional coupler and outputs at through port directly. Excellent performance is expected for an optimal device. First, the device shows an ultra-high polarization extinction ratio(PER) of 28.3 dB and an ultra-low insertion loss(IL) of 0.06 dB for the TE-polarized wave, and of 31.7 dB and of 0.31 dB in order for the TM-polarized wave. Second, the working bandwidth with PER > 25 dB and IL < 0.08 dB is 100 nm (1500–1600 nm) for the TE-polarized wave, and that with PER > 20 dB and IL < 0.4 dB is 30 nm (1534–1564 nm) for the TM-polarized wave. Third, the device has a compact structure of 33 μm in length and exhibits definite fabrication tolerance. Overall, the proposed device exhibits ultra-high PER and ultra-low IL, and has a compact size suitable for on-chip use. A comparison shows that present device exhibits advantage in IL and/or size over that based on other LNOI structures or the SOI platform. [ABSTRACT FROM AUTHOR]

Effects of finite hole depth and non-cylindrical hole shape on stop-band characteristics of photonic crystal formed by air-hole square lattice in Ti-diffused LiNbO 3 strip waveguide were studied theoretically. The study shows that hole depth determines the contrast of stop-band, and the hole radius and conical angle determine the bandgap and location. Cylindrical holes must be sufficiently deep so as to overlap most of waveguide mode and hence obtain a stop-band with high contrast, sharp edge and broad bandgap. Non-cylindrical holes seriously affect the stop-band features. Conical holes cause low contrast and narrow bandgap, and the stop-band shifts with the conical angle. For the cylindrical-conical hybrid holes, the cylindrical portion determines the desired features. Given the difficulty in fabricating high aspect-ratio cylindrical holes, we propose to fabricate the holes at the bottom of a shallow trench, which is introduced into waveguide surface prior to the hole milling. [ABSTRACT FROM AUTHOR]

Electro-optic (EO) property of titanium-diffused LiNbO 3 optical waveguide was studied using prism coupling technique. The EO-effect-induced increment ΔN of effective refractive index (at the wavelength 1311 nm) was measured as a function of externally applied electric field E for TM 0 and TM 1 modes guided in the titanium-diffused LiNbO 3 planar waveguide. By using the linearity between ΔN and E, the EO coefficient r 33 is determined. For reference, the measurement was also performed on a bulk material of congruent LiNbO 3 , yielding r 33 = 24.9 ± 0.8 p.m./V, which is in qualitative agreement with that obtained by extrapolating previously reported result to the 1311 nm wavelength, ~24 p.m./V, confirming the feasibility of the measurement method. Measurements on the titanium-diffused planar waveguide give r 33 = 25.7 ± 0.6 p.m./V for the TM 0 mode and 25.7 ± 0.4 p.m./V for the TM 1 mode. Both values can be considered as identical within the error, and are the same as that of the bulk material, indicating that the EO property is well preserved in the titanium-diffused LiNbO 3 waveguide. The main reason is that the LiNbO 3 crystalline phase is well preserved in the titanium-diffused waveguide layer. • Electro-optic (EO) property of Ti:LiNbO 3 planar waveguide is studied. • The study uses the linearity between refractive index change and electric field. • The study shows that EO property is well preserved in Ti:LiNbO 3 waveguide. • Little effect of Ti-diffusion doping on EO property is qualitatively explained. [ABSTRACT FROM AUTHOR]