Structure Design And Transmission Characteristics Analysis Of Terahertz Microstructured Polarization Maintaining Fiber

Terahertz(THz)waves refer to electromagnetic waves with a frequency range between 0.1THz and 10 THz,located between millimeter waves and infrared radiation.In recent years,THz technology has been widely used in many scientific and engineering fields,including material characterization,broadband communication,environmental monitoring,medical imaging,and chemical and biological sensing.Although the generation and detection technologies of THz waves are becoming more and more mature,many commercial THz systems still use free space to transmit THz waves due to the lack of low-loss flexible waveguides,resulting in complex and large systems,and complicated optical path adjustment,which restricts the integration and development of THz systems.Portability makes its application limited.Therefore,THz waveguides with excellent transmission properties have become a research hotspot,especially THz microstructured optical fibers(MOFs),which have attracted much attention due to their flexible structures and tunable transmission properties.Compared with the common THz MOF with a sixfold symmetric structure and two degenerate fundamental modes,the THz polarization-maintaining MOF can have a strong polarization-maintaining ability for the input linearly polarized THz wave.It has application advantages in sensing and filtering.Polarization-maintaining fibers can be divided into high birefringent type and singlepolarization single-mode(SPSM)type according to their mechanism.Aiming at two types of polarization-maintaining fibers,this paper designs a high birefringence THz MOF,a singlepolarization single-model THz MOF,and a high birefringence THz hollow-core anti-resonant(HC-AR)MOF,its transmission characteristics are simulated and calculated,and the main research contents are as follows:(1)Highly birefringent microstructured optical fiber(MOF)with elliptical cladding air holes and small defected hole in the core area is designed for polarization-maintaining waveguide for terahertz wave.The degeneracy of the fundamental modes is broken to obtain high birefringence by selectively filling of cladding holes with epsilon-near-zero(ENZ)material and introducing the asymmetry of geometric structure and material distribution.The variation of transmission properties such as birefringence,loss and dispersion with structure parameters are investigated by finite element method(FEM).The refractive index difference between the two polarized lights perpendicular to each other is greeter than 0.01 in a wide frequency range(0.5-2 THz).The loss of the X-and Y-Polarized fundamental mode has minimum value around 0.8 THz,which are 0.903 d B/cm and 0.851 d B/cm,respectively.Dispersion can be effctively adjusted by the central defect hole.Near-zero and flattend dispersion of(0±0.054)ps/THz/cm is obtained in frequency range of 1-1.8 THz for the YPolarized fundamental mode.Transmission properties of the designed fiber are insensitive to the refractive index of ENZ material.The conclusions provides theoretical reference for the development of terahertz polarization maintaining fiber.(2)A SPSM THz MOF is designed.The THz MOF contains three circles of air holes arranged in a triangular lattice,and the ENZ material is selectively coated in the first circle of air holes adjacent to the core,which introduces the asymmetry of the refractive index distribution of the material,making the two polarized fundamental modes have different mode field distributions.Compared with the X-polarized fundamental mode,the Y-polarized fundamental mode has more energy distributed in the ENZ material,and the loss of the Ypolarized fundamental mode is much higher than that of the X-polarized fundamental mode due to the high absorption property of the ENZ material for THz waves.Therefore,after a THz wave has propagated a certain distance in this MOF,the higher loss difference will result in that only the X-polarized fundamental mode can continue to propagate,enabling single-polarization transmission.Furthermore,to achieve single-mode transmission,gain material is introduced in the core region.The mode field of the fundamental mode is approximately Gaussian,and the energy is mainly concentrated in the center of the core region,while the mode field of the higher-order mode has a lower proportion in the center of the core region.Therefore,the gain material located in the center of the core region can have a significant gain effect on the fundamental mode,thereby increasing the loss difference between the fundamental and higherorder modes,enabling single-mode transmission.The structural parameters of SPSM THz MOF were analyzed and optimized.In the frequency range of 1.261-1.604 THz(with a bandwidth of0.343 THz)for an optimized fiber,the loss of the desired X-polarization fundamental mode is at least lower than the loss of all other undesired modes(Y-polarization fundamental and higherorder modes)9.5d B/cm,that is,a loss difference greater than 9.5d B/cm is obtained.Therefore,the single-polarization and single-mode transmission can be realized after the input THz wave travels 2.1 cm in this THz MOF.(3)In order to obtain high birefringence and low loss transmission characteristics at the same time,an HC-AR THz MOF with thin struts is designed.The fiber transmits THz waves based on a hybrid conduction mechanism of anti-resonance and total internal reflection.The introduction of thin struts in the core area can significantly improve the birefringence,and the main structural parameters of the fiber are optimized.By selecting the appropriate strut thickness,high birefringence and low transmission loss can be achieved at the same time.For the optimized structural parameters,the HC-AR MOF achieves high birefringence on the order of 10-2 and maintains low-loss transmission characteristics: the loss of the X-polarized fundamental mode is 1.89×10-3 d B/cm at 1 THz.The proposed HC-AR THz MOF has high design and manufacturing tolerances.The base material of the THz polarization-maintaining MOFs designed in this paper is the polymer material Topas cyclic olefin copolymer(COC),which has excellent THz transparency and easy processing.The designed optical fiber has a simple structure,and the feasibility of preparation is considered when optimizing the structural parameters.The proposed THz polarization-maintaining MOFs can be prepared by the "extrusion-molding method","mold casting method" and 3D printing technology commonly used in polymer MOFs.The reported THz artificial metamaterial or natural material KCl can be used as the filling/coated ENZ material.In summary,the research conclusions of this paper provide theoretical guidance for the development and application of THz polarization-maintaining fibers and polarizationdependent THz functional devices.