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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]

• Polarization beam splitter based on LNOI triple-waveguide coupler is proposed. • α-Si nanostrip is used to manipulate the coupling of TM polarization mode. • Higher polarization extinction ratio and lower insertion loss are achieved. • The device is compact and suitable for on-chip applications. • A larger feature size (∼200 nm) that facilitates fabrication is involved. Polarization beam splitter (PBS) is a fundamental component of integrated photonics to manipulate wave polarization state, and most PBS devices were designed on the popular SOI. As an alternative, a PBS may be implemented on the basis of thin-film LiNbO 3 -on-insulator (LNOI), a promising platform for developing various compact or ultra-compact photonic active and passive devices. However, smaller modal birefringence of the LNOI waveguide makes it not easy to implement a PBS of high-performance. Here, we propose a compact, high-performance PBS on the basis of a triple-waveguide coupler consisted of two LNOI ridge waveguides and an amorphous silicon (α-Si) nanostrip assisted waveguide. By making use of the α-Si nanostrip, the PBS works in the case that the TM polarization mode meets the phase matching condition while the TE polarization mode does not. Numerical results show that the PBS is compact with a length of only 31 μm. The device can achieve an ultra-high polarization extinction ratio (>29.4 dB) and an ultra-low insertion loss (<0.066 dB) in the operating wavelength range of 1500–1600 nm for the TE polarization mode, and a polarization extinction ratio of > 20 dB and an insertion loss of < 0.16 dB over a ∼ 60 nm bandwidth from 1527 nm to 1587 nm for the TM polarization mode. Moreover, the device has a larger feature size that facilitates fabrication. Overall, this device has the advantages of compact size, higher polarization extinction ratio, lower insertion loss, and larger feature size, which is suitable for on-chip applications. [ABSTRACT FROM AUTHOR]

• A TM-pass polarizer based on an air-slot embedded TFLNOI waveguide is proposed. • Designed structure supports only a TM mode while TE mode being in cut-off state. • A symmetric tetragonal dipyramid air-slot is designed to degrade insertion loss. • Both slot geometry and silica thickness take effects on polarization performance. • Optimal device shows good performance and ultra-compactness valid for on-chip use. An air-slot assisted TM-pass waveguide polarizer based on thin-film lithium niobate on insulator (TFLNOI) is proposed. It consists of a segment of ridge-type TFLNOI single-mode strip waveguide and an embedded air-slot. The air-slot has a symmetric tetragonal dipyramid structure consisted of a cuboid in the central part and two identical tetragonal pyramids in two sides, which function as polarization generation and adiabatic approximation to degrade insertion loss, respectively. Waveguide properties and transmission characteristics of the device were simulated using finite difference method. The results show that the waveguide structure with a properly designed geometry supports only a TM mode, while does not the TE mode, which is cut-off in the cuboid air-slot region. Slot length, width and silica substrate thickness all take considerable effects on the insertion loss and extinction ratio. Qualitative and comprehensible explanations are given for the effects. An optimized polarizer has been suggested that exhibits excellent polarization performance, including a larger extinction ratio of 28.4 dB, a lower insertion loss of 0.29 dB, and a broader bandwidth of ∼110 nm (1490–1600 nm) with an extinction ratio >20 dB and an insertion loss <0.5 dB. In addition, the proposed device has a length of only 13 µm satisfying on-chip use. [ABSTRACT FROM AUTHOR]

• The Y-junction and T-junction 1 × 2 power splitters based on LNOI platform are designed by an intelligent algorithm. • The combination of GA and FEM based on "Phc-like" structure is proposed. • Symmetric operations and two-dimensional models are used to improve optimization efficiency. • The optimized devices offer good performance and ultra-compact footprints suitable for on-chip use. Optical power splitters are critical components of photonic integrated circuits based on the lithium-niobate-on-insulator (LNOI) platform. However, the lithium niobate power splitters designed by traditional strategies are usually large in size, which limits their practical applications for on-chip high dense integration. Here, we focus on the efficient design of ultra-compact power splitters based on the LNOI platform. The Y-junction and T-junction 1 × 2 power splitters based on photonic-crystal-like (PhC-like) structures are designed by a genetic algorithm. The footprint of the functional region for Y-junction and T-junction power splitters are only 2.6 μm × 2.6 μm and 2.85 μm × 4.94 μm, respectively. Simulation results show that the insertion losses are about 0.1 dB and 0.7 dB at 1550 nm for these two kinds of devices. This work can as a generic approach for designing various ultra-compact functional devices based on LNOI platform. [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]