Investigation Of Novel Polarization-Maintaining Few-Mode Fiber And Terahertz Polarization-Maintaining Waveguide

Polarization-maintaining waveguide,as an important branch of waveguide,is essential in polarization-sensitive and polarization-dependent applications,such as communication systems and sensing systems.Combining the concept of"few-mode"with"polarization-multiplexing",polariztion-maintaining few-mode fiber can be used as an implementation of space-division-multiplexing technology and sparkles novel applications for further expansion of optical communication transmission capacity.At the same time,in polarization-sensitive Terahertz(THz)systems,such as THz sensing,spectral imaging and wireless communication,THz polarization-maintaining waveguide not only provides a medium for the trsansmission of polarization information,but also improves the stabilitity and reduces the size of THz systems.THz polarization-maintaining waveguides have currently inspired great interests.In this context,the thesis is devoted to investigate polarization-maintainig few-mode fibers and THz polarization-maintaining waveguides,including strong-coupling multi-core and hole-assisted polarization-maintainig few-mode fibers,and photonic bandgap and anti-resonant THz polarization-maintaining hollow-core waveguides.The main innovative results of the thesis are summarized as follows.1.We have proposed and investigated the polarization-maintaining supermode fiber based on strong-coupling multi-core structure.The influences of structural and optical parameters in quasi-elliptically arranged eight-core and sixteen-core fibers are discussed via numerical simulations.With optimized parameters,the strong-coupling eight-core and sixteen-core fibers could support up to 14 and 20 vector modes with different polarization states and spatial orientations,independently.At 1550 nm,all the 20 modes in sixteen-core fiber are separated from their adjacent modes and the effective areas of the fundamental modes are compatible with standard single-mode fiber.Wavelength-dependent performance is analyzed.With wavelength ranging from 1520 nm to 1580 nm,the mode number of sixteen-core fiber maintains 20 meanwhile satisfing the target of the effective index difference between arbitrary modes higher than 10-4.2.By introducing four non-circularly symmetrically distributed air-holes in elliptical ring core fiber,we have proposed and analyzed a polarization-maintaining few-mode fiber.With optimized parameters,such fiber could support 10 vector modes and the effective index differences between all modes are higher than 1.65×10-4 at 1550 nm.The minimum value of effecitve index differences is 1.52x 10-4 over a bandwidth ranging from 1510 nm to 1630 n.Numerical simulation results indicate that the polarization-maintaining performance of such hole-assisted fiber is effectively improved in comparison with corresponding elliptical ring core fiber.Mode number of the proposed fiber could be further expanded with modified structural parameters.3.We have studied a central-hole-assisted elliptical core fiber.With optimized parameters of fiber core and central hole,such fiber could support 10 vector modes separating from each other.The effective index differences between arbitrary modes are beyond 1.8×10-4 over wavelength range of 1510-1630 nm.We have investigated the scalability of mode number in central-hole-assisted elliptical core fiber.The guided polarization-maintaining modes could be expanded to 14 with modified structural parameters and the minimum effective index difference among all modes is 1.38×10-4 at 1550 nm and the value varies slightly with operating wavelength in range of 1510-1630 nm.4.We have discussed the viability of exploiting hollow-core photonic bandgap waveguide to transmit polarization-maintaining modes.The properties of vector modes in hollow-core photonic bandgap waveguide based on triangularly and squarely arranged lattice are investigated using numerical simulations.For 4-cell photonic bandgap waveguide with triangularly arranged hexagonal holes,a broad transmission window with high birefringence(above 10-4),low loss(below 0.01 cm-1)covering 0.23 THz(0.82 THz to 1.05 THz)is observed.The effective index difference between fundamental modes is 9.4x10-4 at 0.9 THz.We have analyzed the influences of structural parameters on modal polarization-maintaining properties,and the results indicate that the guided vector modes are less sensitive for parameters surrounding core region.Moreover,we have investigated 21-cell,24-cell and 32-cell hollow-core photonic bandgap waveguides based on square lattice.Numerical simulations reveal that a narrower defect core could increase the effective index difference between the same mode groups,particularly for higher-order modes.Larger air-core could greatly increase the number of guided modes while the modal coupling is enhanced.5.We have proposed and studied a THz polarization-maintaining hollow-core anti-resonant waveguide base on four half-elliptical cladding polymer membranes.Modification of the thickness for a pair of polymer membranes could introduce modal birefringence in order of 10-4 but reduce the available low-loss transmission bandwidth.Numerical simulations indicate that large core is preferred toward low-loss transmission but provides poor polarization-maintaining performance.The dimension of cladding membranes,rather than the shape,holds great influences on loss profile.The proposed hollow-core anti-resonate waveguide exhibits low-loss(lower than 10 dB/m),high birefringence(no less than 10-4)and low group velocity dispersion(below 0.6 psTHz-1cm-1)within the frequency bandwidth of 0.46-0.6 THz and 0.74-1.1 THz.The bend-induced loss are below 4 dB/m with bend radius larger than 2 cm,along x-axis or y-axis.