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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]

• 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]

• On-chip 1.53 μm amplification in Er:LNOI micro-photonic amplifier was simulated. • The simulation was performed in comparison with Ti:Er:LiNbO 3 waveguide amplifier. • Numerical model is confirmed by consistency of numerical and experimental results. • The Er:LNOI amplifier is more efficient than the Ti:Er:LiNbO 3 waveguide. • Due to higher Er3+ population inversion extent and ultra-compact mode of Er:LNOI. On-chip 1.53 μm amplification in 980-nm-pumped Er3+-doped lithium niobate on insulator (Er:LNOI) photonic wire (PW) has been studied theoretically in comparison with that in Ti:Er:LiNbO 3 waveguide on the basis of three- and six-level models of Er3+. We show that the six-level model gives more accurate analysis and its validity is confirmed by comparing the simulated results with the previously reported experiment data. The study shows that the Er:LNOI PW has a larger Er3+ population inversion extent than the Ti:Er:LiNbO 3 because of larger overlapping factor of mode field and Er3+ population profiles, and ultra-compact mode field. As a result, the Er:LNOI PW displays better gain performance than the Ti:Er:LiNbO 3 as described below. (1) Maximum/saturation gain of Er:LNOI PW can be as much as two/three times higher than that of the Ti:Er:LiNbO 3. (2) Gain of the Er:LNOI PW increases with propagation length more strongly in the initial stage and decreases more slowly beyond optimal length. It also shows stronger dependences on both pump power (in the initial stage) and signal power. (3) Gain in the Er:LNOI PW saturates more easily, and the Er:LNOI PW has a threshold pump power one order of magnitude lower than the Ti:Er:LiNbO 3. [ABSTRACT FROM AUTHOR]

We report a mode converter based on a tapered X-cut/Y-propagation ridge-type LiNbO 3 -on-Insulator (LNOI) photonic wire (PW) with a laterally asymmetric refractive index profile realized by introducing a semi-infinite dielectric cover layer on one side of the LNOI PW. The interplay of material and waveguide birefringence enables to realize mode hybridization, which is a prerequisite for realizing mode conversion. The introduction of the semi-infinite cover layer and the use of a taper structure modify mode index and electric field distribution, enabling to realize mode conversion. Aiming at 1.55 μm wavelength, we have simulated mode hybridization and conversion characteristics of the tapered, laterally asymmetric ridge-type LNOI PW. The results show that a conversion between fundamental TM 00 and TE 00 modes can be realized. The conversion efficiency can be optimized by choosing an appropriate cover layer material and/or adjusting its thickness. An ultra-compact device with a taper length <100 μm is expected. • A mode converter based on a tapered LNOI photonic wire (PW) is proposed. • The device has a laterally asymmetric refractive index profile. • Asymmetric index profile and taper structure enable to realize mode conversion. • A conversion between fundamental TM 00 and TE 00 modes is realized. • An ultra-compact device with a taper length <100 μm is expected. [ABSTRACT FROM AUTHOR]