一种高双折射低限制性损耗光子晶体光纤设计

本文设计了一种适用于长距离光纤通信的新型光子晶体光纤。该光纤包层内椭圆形和圆形空气孔呈交错排列,纤芯两侧为两个小椭圆空气孔。利用有限元分析方法对所设计光纤的传输特性进行分析并对其结构进行了优化,确定了最佳结构。结果表明,波长为1550 nm时,此新型光子晶体光纤在最佳结构下可提供高达3.51×10^-2的高双折射和低至1.5×10^-9 dB/m的限制性损耗。与现存的引入椭圆形空气孔的光子晶体光纤相比,本文中的光子晶体光纤的双折射系数有较大提高,限制性损耗系数降低了5个数量级。另外,本文还详细研究了光子晶体光纤的色散随光子晶体光纤结构的变化以及其布里渊增益特性,并分析了其可制造性。基于其高双折射和低限制性损耗特性,此种光纤可应用于长距离光纤通信系统。     

High birefringence photonic crystal fiber with a complex unit cell of asymmetric elliptical air hole cladding

High birefringence induced by elliptical air hole photonic crystal fibers (EHPCFs) is analyzed numerically using the finite-element method. Statistical correlations between the birefringence and the various parameters are obtained. We found that the complex elliptical air hole is better than that of a circular one to obtain high birefringence in photonic crystal fibers. Our suggested structures can considerably enhance the birefringence in EHPCFs and show that the birefringence can be as high as 1.1294 x 10(-2), which is higher than the birefringence obtained from conventional step-index fiber (5 x 10(-4)), circular air holes PCF (3.7 x 10(-3)), and elliptical hollow PCF (2.35 x 10(-3)).     

Low chromatic dispersion and high birefringence investigated in elliptical air hole photonic crystal fiber

An index-guided, two-dimensional photonic crystal fiber (PCF) with triangular lattice and solid air core was designed and investigated using the effective index method. A modal analysis of three-ring elliptical air hole photonic crystal fiber is presented: first the innermost ring was made elliptical and then the cells of the inner two rings were made elliptical and finally all three rings contained elliptical cells. The above configuration was then compared with three-ring circular air hole photonic crystal fiber keeping the area of air holes and simulation parameters of the wafer the same to obtain the fundamental space-filling mode (FSM). The analysis was performed using full vector FDTD for TE and TM polarizations. The complex effective refractive index generated was then used to calculate the chromatic dispersion and birefringence properties of the PCF.     

Numerical investigation of birefringence and confinement loss formed by rectangular/elliptical/circular air holes photonic crystal fibers

In this paper, high birefringence and low confinement loss of rectangular air holes photonic crystal fibers (PCFs) are numerically investigated and compared with elliptical and circular patterns using the finite element method. The mode birefringence of the proposed PCFs with rectangular air holes at λ?=?1.55?µm reaches 8.1?×?10^?2 and the confinement loss is less than 5?×?10^?3?dB/km. Besides, a high birefringence up to 2.76?×?10^?2 is also achieved from the proposed circular air holes PCF, which is the highest value compared to conventional circular air holes PCF.     

Improved single mode property in elliptical air hole photonic crystal fiber

The single mode property of a holey, index-guided two-dimensional photonic crystal fiber (PCF) is investigated using the alternate direction implicit method. The modal analysis of a three-ring elliptical air hole photonic crystal fiber is performed and compared with the equivalent three-ring circular air hole photonic crystal fiber keeping the air-fill fraction the same for both cases. The fiber is investigated for single mode operation. The effect of the rotation of the ellipse axes on these characteristics is also considered. It is observed that a PCF with elliptical air holes exhibits a better single mode property over a wider wavelength range in the optical domain. Also when the diameter of the air hole is properly catered for, they can show an endlessly single mode property.     

Improving of high birefringence photonic crystal fiber with low confinement loss using small elliptical air holes in the core region

In this paper, a novel design double lattice photonic crystal fiber is proposed for achieving both high birefringence and low confinement loss. In this structure, circular air holes are arranged as octagonal lattice in the cladding and elliptical as rectangular lattice in the core region. Numerical results illustrate that the birefringence in such fibers is determined not only by the double lattice but also the changing of the shape and the arrangement of the air hole in the first inner rings of the cladding. The birefringence property and confinement loss are studied by employing the finite difference time domain method with transparent boundary condition. The numerical results demonstrate that the maximal birefringence and lowest confinement loss of our optimized structure PCF at the excitation wavelength of λ = 1550 nm can be achieved at 5.16 × 10^?2 and 0.003 dB/km, respectively.     

Design and optimization of broadband and polarization-insensitive dual-core photonic crystal fiber coupler

In this paper, we propose a novel (to our knowledge) broadband and polarization-insensitive dual-core photonic crystal fiber (PCF) coupler through the introduction of an elliptical-shaped central air hole to offset the slight birefringence arising from the dual core. With a full vectorial finite element method and anisotropic perfectly matched layers as the external boundaries, the impact of several fiber parameters on the coupling characteristics of dual-core PCF is investigated in detail. Through optimizing the main fiber parameters, including core diameter, size and ellipticity of the central air hole, and refractive index difference, broadband and polarization-insensitive characteristics are achieved in the wavelength range from 0.8 to 1.7 μm. The variation of the coupling ratio is stabilized at 50±1%, and the coupling ratio difference between x polarization and y polarization is less than 2% over the wavelength range. This dual-core PCF makes it easier to develop a 3 dB coupler over a wide wavelength for passive optical networks and large optical systems.     

Study of the sensitivity of gas sensing by use of index-guiding photonic crystal fibers

We demonstrate an absorption transmission spectrum of CH(4) in a 16.9 cm long index-guiding photonic crystal fiber (PCF) fabricated in our laboratory. One of the main factors to improve the sensitivity is to increase the fraction of power in PCF cladding air holes. We study the fraction of power in PCF cladding air holes as a function of the index-guiding PCF parameters. We found that a PCF with small spacing and a large air-filling ratio has a higher fraction of power in its cladding air holes. At the same time the mode area in this PCF is small and would generate strong nonlinear effects in the fiber. If we use a PCF in which the core is formed by missing seven air holes, it is immediately obvious that the PCF used as a sensor has higher sensitivity and a larger mode area.     

Dispersion engineering of photonic crystal fibers by means of fluidic infiltration

We present a technique based on the optofluidic method to design a photonic crystal fiber (PCF) experiencing small dispersion over a broad range of wavelengths. Without nano-scale variation in the air-hole diameter or the lattice constant of Λ, or even changing the shape of the air holes, this approach allows us to control the dispersion of the fundamental mode in a PCF simply by choosing a suitable refractive index of the liquid to infiltrate into the air holes of the PCF. Moreover, one can design a different PCF such as a dispersion flattened fiber (DFF), dispersion shifted fiber (DSF), by utilizing fluids of various refractive indices.     

A novel structure of photonic crystal fibre for dispersion compensation over broadband range

This paper studies a novel structure of photonic crystal fibre (PCF) for dispersion compensation at broadband wavelengths. The application of broadband is investigated using a design model based on combination of modal properties and dispersion compensation. The newly designed PCF with defect introduced is recorded over transmission spectrum range 146.7–256.98 THz, i.e., 1.16–2.04 µm. The modal characteristics and dispersion compensation of 2D PCF with circular air holes defect introduced are investigated and compared to those of conventional hexagonal 2D PCF. Changes in bandwidth behaviour are also observed by changing refractive index and geometric parameter of PCF.     

Enhancement of evanescent field exposure in a photonic crystal fibre with interstitial holes

In this work, a modified photonic crystal fibre (PCF) that we refer to as Sunny PCF with enhanced evanescent field exposure structure is proposed. The Sunny PCF with triangular interstitial air holes surrounding the core region increases the interaction of the guided mode with the air. Full-vectorial finite element method with perfectly matched layer boundary condition is used to design and simulate the sensitivity and confinement loss characteristics of the proposed Sunny PCF. By adding sunny structure to a conventional PCF with air-filling ratio of 0.9, the highest achievable sensitivity with negligible confinement loss can be boosted up to 21.23% from 15.83% at the operating wavelength of 1550 nm. Sunny PCF can achieve the same sensitivity as suspended-core holey fibre with lower confinement loss. A preliminary Sunny PCF has been fabricated to prove the feasibility of the proposed structure.     

A single-mode highly birefringent dispersion-compensating photonic crystal fiber using hybrid cladding

Based on the hybrid cladding design, a single-mode photonic crystal fibre (PCF) is proposed to achieve an ultra-high birefringence and large negative dispersion coefficient using finite-element method. Simulation results reveal that with optimal design parameters, it is possible to achieve an ultra-high birefringence of 2.64 × 10^?2 at the excitation wavelength of 1.55 μm. The designed structure also shows large dispersion coefficient about ?242.22 to ?762.6 ps/nm/km over the wavelength ranging from 1.30 to 1.65 μm. Moreover, residual dispersion, effective dispersion, effective area, confinement loss and nonlinear coefficient of the proposed PCF are discussed thoroughly.     

Band Gap Characteristic and Transmission Spectrum of Ring-shaped Holes Photonic Crystal in THz Range

采用平面波展开法研究了2D三角晶格空气孔型光子晶体的带隙结构和空气环型光子晶体的完全带隙。结果发现空气孔半径R从0.41a到0.49a之间变化时,空气孔型的完全带隙会越大;在同一空气孔半径(R=0.41a~0.47a)下空气环型结构都能得到比空气孔结构更宽的完全带隙,得到最优参数R=0.47a,r=0.133a下的完全带隙为0.1698THz,是空气孔结构的1.15倍。使用时域有限差分法研究了空气环型结构的透射谱,根据计算结果设计了(0.7198~0.8655)THz波段的滤波器。     

Highly birefringent large negative dispersion-flattened photonic crystal fibre for broadband residual dispersion compensation

A triangular lattice photonic crystal fibre is presented in this paper for residual dispersion compensation. The fibre exhibits a flattened negative dispersion of ?992.01 ± 6.93 ps/(nm-km) over S+C+L wavelength bands and ?995.83 ± 0.42 ps/(nm-km) over C-band. The birefringence is about 4.4 × 10^?2 at the excitation wavelength of 1550 nm which is also very high. Full vector finite element method (FEM) with a perfectly matched absorbing layer (PML) boundary condition is applied to numerically investigate the guiding properties of this PCF. The fibre operates at fundamental mode only. All these properties endorse this fibre as a suitable candidate for compensating residual dispersion and polarization maintaining applications.     

Line shapes of photon-gated persistent spectral hole burning in polarization spectroscopy

We introduce a method of polarization spectroscopy into photon-gated persistent spectral hole burning, which results in considerable improvement in the signal-to-background ratio. Fifty multiple spectral holes burnt into meso-phenyl-tetrabenzoporphyrinato-zinc/aromatic cyanide/poly(methyl methacrylate) were clearly detected by this measurement technique. We developed a simplified model to account for the hole shapes. Background birefringence combined with refractive dispersion is thought to play a key role in the generation of asymmetric line shapes.     

Simultaneous measurement of multiparameters using a Sagnac interferometer with polarization maintaining side-hole fiber

A Sagnac interferometer with a section of a polarization maintaining side-hole fiber for multiparameter measurement is proposed. The sensor was experimentally demonstrated to be sensitive to torsion, temperature, and longitudinal strain, simultaneously. The birefringence in the investigated side-hole fiber is induced simultaneously by the elliptical shape of a germanium-doped core and by field overlap with the air holes surrounding the core. The latter effect is purely geometrical and causes high chromatic dispersion of the group birefringence in the long wavelength range, which results in a different period of spectral interference fringes. A different wavelength response is obtained for each interference fringe peak when the fiber is subjected to torsion, temperature, or longitudinal strain. A matrix equation for simultaneous measurement of the three parameters--torsion, temperature, and longitudinal strain--is also proposed.     

A kind of single-polarization single-mode photonic crystal fiber

A kind of single-polarization and single-mode totally internal reflection photonic crystal fiber (SPSM TIR-PCF) is proposed in this paper. It is a PCF structure with elliptical air holes in the cladding and four large holes in the first ring. A full-vector plane wave expansion method is employed to analyze this PCF structure. The numerical results show that this PCF structure can realize an ultra-broad SPSM bandwidth of 540?nm with a confinement loss less than 0.1?dB?km^?1, the broadest bandwidth to the best of our knowledge. Moreover, the structure that we proposed can realize a high nonlinear coefficient.     

Fatigue strength of woven fabric composites with drilled and moulded-in holes

The fatigue strength of glass-fiber woven-roving composites with a circular hole has been examined. Circular holes of two types, drilled and moulded-in, were considered. Experiments on the fatigue life and damage processes in (0, 90)_s and (45, −45)_s woven-roving composites with hole diameters of 5, 10, 14 and 19 mm were conducted. Experimental results show that laminates with moulded-in circular holes exhibit higher fatigue strength than those with drilled holes as a consequence of the altered mode of failure. Photographs of fatigue failure processes and specimen elongation with life are presented. The failure phenomena of laminates with drilled and moulded-in holes are compared and discussed.     

Nonlinear pulse propagation in chalcogenide As2Se3 glass photonic crystal fiber using RK4IP method

Pulse propagation through chalcogenide As(2)Se(3) glass photonic crystal fiber (PCF) is numerically investigated using fourth-order Runge-Kutta in the interaction picture (RK4IP) method. The fully vectorial effective index method (FVEIM) is used to calculate the variation of effective refractive index, effective area, dispersion, and nonlinear coefficient (γ) in As(2)Se(3) PCF with wavelength for different values of pitch and air hole size. The RK4IP method is used to demonstrate the soliton propagation, self-phase modulation (SPM), soliton collision and cross phase modulation (XPM) in the designed As(2)Se(3) PCF. The numerically obtained value of soliton collision length (L(col)=51.3L(D)) using the RK4IP method is found to be in good agreement with the theoretical value of soliton collision length (L(col)=51.408L(D)) obtained from inverse scattering transform method, thus providing a verification of the RK4IP accuracy in solving generalized nonlinear schr?dinger equation (GLNSE). We also evaluate and apply the value of wavelength for distortionless (L(NL)=L(D)) propagation of the soliton pulse.