Reconfigurable and Multifunctional Polarizer Based on Lithium-Niobate-On-Insulator Platform Assisted by Phase-Change Material
Objective Silicon-on-insulator(SOI),as a mature on-chip platform,has been widely implemented in photonic integrated circuits(PICs)due to its large relative refractive index difference(2.0)and compatibility with CMOS processes.However,it has noticeable drawbacks.Generating a light source is challenging,and its linear photoelectric effect is absent,making direct modulation of the light field difficult due to the centrosymmetric crystal structure of silicon.In contrast,thin film lithium niobate-on-insulator(TFLN)emerges as an ideal optical platform candidate.It possesses various excellent properties,such as large electro-optic,thermo-optic,and acoustic-optic coefficients,as well as nonlinear properties,and stable chemical and physical characteristics inherent to its material properties.Moreover,it offers a wide transparent window(from 400 nm to 5 μm)and relatively high refractive index contrast(0.7)for light field confinement.The TFLN can be obtained from the lithium-niobate-on-insulator(LNOI)wafer utilizing the smart cut fabrication process,readily available from commercial companies.By combining the unique properties of TFLN with the LNOI wafer,numerous high-speed modulation devices have been proposed,positioning TFLN as a promising alternative to SOI in both classical and quantum domains.However,lithium niobate is an anisotropic crystal,and its intrinsic birefringence leads to strong polarization dependence.In practical systems,the purity of polarization significantly affects device performance.Therefore,polarization management devices play a crucial role in this optical integrated platform.Polarizers act as filters,efficiently removing interference modes while preserving the target mode,typically within a single waveguide.Owing to the increasing demand for reconfigurable and multifunctional polarization devices for photonic integrated circuits,we implement a reconfigurable and multifunctional polarizer based on an LNOI platform assisted by phase-change material.Methods The structure of the proposed device is divided into two parts:the triple-waveguide coupler and the polarization control regions.The triple-waveguide coupler contains an α-Si-assisted waveguide in the center and two LNOI ridge waveguides on the sides.Although the involvement of the α-Si nanostrip complicates the fabrication of the device,it provides a new degree of freedom to manipulate the polarization state,overcoming the drawbacks of low mode birefringence in the LNOI waveguide.With the aid of the α-Si nanostrip,the TM polarization mode can achieve phase matching,while the TE polarization mode exhibits significant phase mismatch,enabling the separation or combination of TE and TM modes.The mode polarizer section consists of the lithium niobate(LN)waveguide and the Ge2Sb2Se4Te1(GSST)loaded on the top center of the LNOI waveguide sidewards.By controlling the state of the GSST,this structure can control whether TE and TM modes pass through the waveguide or not.Due to the large imaginary part of GSST in the crystalline state,efficient absorption loss for the polarization state can be realized by rationally optimizing structure parameters.The working principle is analyzed as follows:when the GSST layer is triggered into the crystalline state,its high refractive index will exhibit a nonuniform distribution in the vertical direction.Consequently,a significant portion of power distribution from the TE and TM modes,whose electric field is polarized in the vertical direction,will be directed into the GSST layer rather than the LN waveguide.Meanwhile,this concentrated power will be effectively attenuated by the GSST block within a certain length due to its high material imaginary part.On the other hand,when the GSST is in the amorphous state with a lower refractive index,the main light field of the TE and TM modes will be distributed in the LN waveguide.Due to the low extinction coefficient of GSST,only a small fraction of the light power conveyed in the LN waveguide will practically not be lost and will continue to flow into the triple-waveguide coupler.Results and Discussions By rationally designing the geometric parameters of the a-Si,the TM polarization mode can achieve phase matching,while the TE polarization mode exhibits significant phase mismatch,enabling the separation or combination of TE and TM modes(Fig.3).We optimize the geometric parameters of the GSST layer so that the TE and TM modes can be filtered out when GSST is crystalline while minimizing their influence when GSST is amorphous.Consequently,the polarizer exhibits a higher extinction ratio and lower insertion loss at a wavelength of 1550 nm(Fig.4).By controlling the state of the two pieces of GSST,the proposed device offers four output modes(TE mode,TM mode,TE+TM mode,and None).We simulate the propagation of the electric field at a wavelength of 1550 nm using the 3D finite difference time domain(3D-FDTD)method to study the performance and optimize the structure of the polarizer.With a wavelength of 1550 nm,the extinction ratio of the TM mode is 34.3 dB,and the insertion loss is 0.42 dB.Similarly,the extinction ratio of the TE mode is 34.2 dB,with an insertion loss of 0.27 dB.The device's performance parameters are analyzed across the wavelength range of 1530 to 1580 nm,covering the communication C band(Fig.6).As the TE mode is wavelength-insensitive,the extinction ratio remains stable above 33 dB,with an insertion loss of less than 0.3 dB.The extinction ratio of the TM mode is consistently above 32 dB,with an insertion loss of less than 0.5 dB.Additionally,we analyze the manufacturing error tolerance of the device(Fig.7).The results indicate that the multifunctional polarizer has a high tolerance for manufacturing errors.Conclusions We propose a reconfigurable and multifunctional polarizer based on the LNOI platform,assisted by phase-change material.The status of the TE and TM modes can be controlled to achieve four output modes by manipulating the phase state of the phase-change materials.The 3D-FDTD method is employed to study the performance and optimize the structure of the polarizer.The results demonstrate that the device offers four output modes(TE mode,TM mode,TE+TM mode,and None).The length of this polarizer is 69 μm,and the polarization extinction ratio of the TE mode is 34.2 dB,with an insertion loss of 0.27 dB.For the TM mode,the polarization extinction ratio is 34.3 dB,with an insertion loss of 0.42 dB at a wavelength of 1550 nm.In the wavelength range of 1530 to 1580 nm,the polarization splitting ratio is higher than 28 dB.Furthermore,we analyze the manufacturing error tolerance of the device.The results indicate that the multifunctional polarizer has a high tolerance for manufacturing errors.This high-performance polarizer with reconfigurability and multifunctionality has wide potential in integrated optical systems on LNOI platforms.Compared with traditional polarizers,the proposed polarizer offers the advantages of multifunctionality,high performance,and reconfigurability,with broad application potential in reconfigurable intelligent optical interconnection systems on LNOI platforms.
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