Study On Transmission Characteristics And Orbital Angular Momentum Of Negative Curvature Terahertz Fiber

With the rapid development of terahertz transmission technology,many kinds of fiber devices that can be used for terahertz wave transmission have been reported.Especially,negative curvature terahertz fiber has gradually attracted extensive attention of researchers because of its simple structure design,which can realize the transmission of terahertz wave with low loss.In addition,generating orbital angular momentum modes in fibers is an effective method to expand communication capacity and improve communication speed.In order to guide terahertz wave with low confinement loss and low flat dispersion,and support a variety of orbital angular momentum modes in negative curvature fibers,in this thesis,we have focused on the study of negative curvature terahertz fiber with nested structure,double negative curvature cladding structure and double elliptic tube cladding structure based on numerical analysis method.The fiber samples are fabricated by 3D printing technology,and preliminary experimental measurments are performed based on the terahertz time-domain spectroscopy system.The main contents and innovations are as follows:（1）The negative curvature terahertz fiber with nested triangular cladding structure is designed.The confinement loss and waveguide dispersion of the fiber are analyzed by changing the thickness of cladding tube wall and the nested structure in2.0-2.8 THz band.Then,the effects of different bending states of the nested triangle structure edges on transmission characteristics,such as confinement loss and waveguide dispersion,are investigated and analyzed.The results show that the fiber achieves best transmission performances when the thickness is 90μm.The confinement loss of the fiber is 0.003 d B/cm when the triangular structure edge is in the external bending state,which is 40%lower than that of the straight edge.The minimum confinement loss of the fiber is 0.002 d B/cm in the inner bending state,which is 60%lower than that in the straight edge.In the specific operating frequency ranges,the fluctuation range of waveguide dispersion is-0.02-0.20ps/（THz×cm）for the inner bending state,and the range is-0.19-0.19ps/（THz×cm）for the straight edge and outer bending state.（2）A double negative curvature cladding structure is designed,which is connected by elliptic tube and circular tube.The transmission performances of fiber such as confinement loss and bending loss are analyzed by changing the ratio of semi-major axis and semi-minor axis of elliptic tube in the frequency of 1.0-1.3THz and 1.84-2.5 THz band.The results show that when the ratio of semi-major axis and semi-minor axis of the elliptical tube is 1.9,the confinement loss is as low as10^-6（d B/cm）at 2.44 THz.In the 2.04-2.4 THz band,the waveguide dispersion remains between-0.18-0.05ps/（THz×cm）.When the bending radius is 25 cm,the bending loss can reach the order of 10^-5（d B/cm）at 2.5 THz.When the elliptic tube is arranged horizontally,the fiber core with large aperture can be obtained,and the performance of confinement loss and waveguide dispersion can be further improved.（3）A negative curvature terahertz fiber with double-layer elliptic cladding tube is designed.The refractive index difference between HE_l+1,1 and EH_l-1,1 is calculated by introducing annular core structure between the double-layer elliptic tube.The transmission characteristics of orbital angular momentum modes are studied in the frequency of 0.4-0.8THz.Numerical results show that the effective refractive index differences of the modes can reach the order of 10^-2,and it can effectively generate 50-52 orbital angular momentum modes in the 0.4-0.8 THz band.The minimum confinement loss can reach the order of 10^-15（d B/cm）,mode purity is stable between85%and 95%.For the low order modes（l<10）,the absolute value of waveguide dispersion is between 0.33-0.8ps/（THz×cm）in 0.45-0.75 THz band.（4）The fiber samples are prepared by photocuring 3D printing technology,and the fabrication process of fiber samples is introduced in detail,including the establishment of fiber model,slicing and printing process.The samples are preliminary measured based on the terahertz time-domain spectroscopy.