| Photonic Crystal Fiber?PCF?has been a research hotspot in the field of optical fiber technology because it exhibits many advantages including infinite single mode,high birefringence,ultra-flat dispersion,and adjustable transmission performance.At the same time,many studies have shown that PCF has great research value and developmental potential,because it is very suitable for applications in optical communications,nonlinear optics,fiber-optic sensing and other fields.However,a lot of problems caused by material defects,especially the problems of non-linearity and light damage,have seriously restricted the further development of the traditional PCF with solid core.Therefore,exploring and developing hollow core fiber?HCF?can effectively avoid the intrinsic defects caused by optical fiber materials.Negative curvature hollow core fiber?NC-HCF?as a new type of HCF has gradually begun to lead the development of HCF,because it exhibits many excellent advantages including wide transmission passband,low loss,high damage threshold,and single-mode transmission.By optimizing the structure of PCF and NC-HCF and investigating the variations of their optical transmission characteristics,four new optical fiber structures with high birefringence and low loss transmission characteristics are proposed.The main contents of this thesis are as follows:?1?The definition,working principle,advantages and disadvantages of PCF and HCF are introduced.It focuses on summarizing the basic theory and usage of the full vector finite element method,and the definition and calculation metod of birefringence and confinement loss.The realization method and application value of PCF and HCF with high birefringence are described.?2?A new compressed fluoride PCF is proposed.Its cladding consists of four elliptical holes,where two elliptical holes with larger ellipticity are placed in the inner cladding.And the finite element method is used to simulate the optical transmission characteristics,in which the material of the fiber is fluoride glass.Based on the optimizing the structural parameters,a fiber with high birefringence and low confinement loss is achieved.The performance is listed as follows:the confinement loss of x-and y-polarization modes at 1.064?m are under4.74×10-8 dB/m,and are respectively 9.06×10-6 and 5.70×10-7 dB/m at 1.310?m.The calculated confinement loss of x-and y-polarization modes at 1.55?m are respectively2.3×10-3 and 1.14×10-4 dB/m,and the corresponding birefringence is as high as 3.89×10-2.Importantly,this fiber achieves high-performance transmission at all three windows of 1064,1310,and 1550 nm.When the wavelength is 2?m,this fiber still maintains high birefringence,reaching 4.85×10-2.?3?Four types of NC-HCF structures?A1,A2,B1,B2?are designed,and the influence of structural parameters on fiber performance is investigated.For the structures A1 and A2 with the same capillary size,they only show the performance of confinement loss,and the changing trend of confinemen loss with wavelength fit the theory of anti-resonance reflection very well.In contrast,for B1 and B2 with the incompletely same internal capillary,they both exhibit excellent confinement loss and birefringence,especially birefringence can reach the order of 10-5.It is also found that the confinement loss of B1 and B2 can be affected by the structure of A1 and A2.In addition,we further investigate the optical properties of B1 and B2in the mid-infrared band after enlarging their core radius,capillary radius,and capillary thickness by 3 times at the same time.It can be found that they also exhibit satisfactory optical performance in a wide transmission band of 3.8-4.6?m,where the confinement loss is less than1 dB/m and the birefringence is higher than 10-5.?4?A high birefringence NC-HCF with low confinement loss is designed.In the horizontal diameter direction,two large air holes with resonant thickness are placed,thereby greatly improving the birefringence performance.After analyzing the effects of capillary radius,thickness and adjacent gap on birefringence and confinement loss,the best fiber structure is obtained.In both of the 2.90-2.91?m and 3.00-3.44?m bands,this fiber exhibits outstanding performance,especially at 2.90?m with the maximum birefringence(1.56×10-4)and the minimum confinement loss?0.024 d B/m?.The birefringence is greater than 10-44 in 3.00-3.44?m band,and the confinement loss is less than 0.2 dB/m in 3.00-3.24?m band.We find that changing the thickness of big capillary can effectively adjust the band in which there is high birefringence,and the increased radius of big capillary within an appropriate range can obviously enhance birefringence.?5?We designed two NC-HCF structures and discussed their confinement loss,where one has a single-layer capillary and the other has a double-layer capillary.The calculated results show that the confinement loss of NC-HCF with a single-layer capillary structure can be optimized to 1.70×10-3 dB/m,and its bandwidth range is 1.2-1.6?m,which can cover two windows of 1.31 and 1.55?m.For the NC-HCF with a double-layer capillary structure,the optimal structure is to place 8 capillaries in each layer.And the confinement loss of this double-layer capillary structure fiber is extremely low?less than 10-44 dB/m?at the wavelength of 2.1-2.35?m.Besides,the angle ratio can be adjusted to achieve ultra-low confinement loss transmission of light waves in different wavebands. |
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