Numerical analysis of birefringence and coupling length on dual-core photonics crystal fiber with complex air holes

A type of high birefringence dual-core photonic crystal fibers (DC-PCFs) with a central row of elliptical air holes have been proposed. The transverse electric field vector distributions of the two modes are evaluated, the birefringence or coupling length with the different parameters is numerically analyzed based on finite-element method. The numerical results show values for the birefringence of 8.247 × 10⁻³ (for wavelength, λ = 1.5 μm and lattice length, Λ = 1.3 μm), and for the coupling lengths about 3.1 mm and 2.6 mm (λ = 1.5 μm and Λ = 1.5 μm) to modes of x and y polarized, respectively. With the increasing of the air-filling fraction in proposed DC-PCF, the coupling length becomes longer and the birefringence becomes higher.     

A novel polarization splitter based on dual-core hybrid photonic crystal fibers

Highly birefringent dual-core photonic crystal fibers (PCFs) can be used as a polarization splitter because the orthogonal polarization modes with dissimilar coupling lengths are easily separated from each other. Different from the traditional methods achieving high birefringence, a new highly birefringent hybrid PCF that guides light by both index guiding and bandgap guiding is proposed. Firstly, a novel polarization splitter based on this kind of dual-core hybrid PCF is designed. The transmission modes, coupling lengths for the two orthogonal polarizations and performance of the proposed polarization splitter are investigated and numerically analyzed. The results demonstrate that it is possible to obtain a 4.72-mm-long polarization splitter. The splitting ratio is better than −20 dB in a large wavelength range of 1.53-1.72 μm. Its bandwidth is about 190 nm.     

Achieving both high birefringence and low leakage loss in double-clad photonic crystal fibers

In this paper, a novel double-clad photonic crystal fiber (DC-PCF) is proposed for achieving both high birefringence and low leakage loss. According to numerical simulation of the proposed PCF, the extraordinarily high birefringence (over 2×10⁻²) and low leakage loss of the order of 0.0001 dB/km over a large wavelength range are achieved simultaneously. Single-polarization single-mode (SPSM) operation with low leakage loss is also discussed and can be realized and optimized in the PCF by adopting suitable structure parameters.     

双芯高双折射光子晶体光纤耦合特性研究

设计了一种双芯高双折射光子晶体光纤,采用多极法（multipole method）和光纤的模式耦合理论研究了光纤的双折射、耦合长度以及色散特性.数值研究发现,对于空气孔节距 Λ=1.2　μm,空气孔直径d=1.0　μm的光纤,在1.55　μm处双折射度为1.24×10^-2;对应x偏振模的耦合长度为21.6　μm,对应y偏振模的耦合长度为24.3　μm. 这种具有高保偏度和极短耦合长度的双芯光子晶体光纤对于微型光子器件的研制具有重 关键词： 光子晶体光纤 双芯 双折射 耦合长度     

Design and analysis of a photonic crystal fiber with four-hole unit

A novel photonic crystal fiber (PCF) based on a four-hole unit is proposed in order to meet the requirements of high birefringence, negative dispersion and confinement loss in fiber-optic communication. The proposed design has been simulated based on the full vector finite element method (FVFEM) and anisotropic perfectly matched layers (APML). Analysis results show that the proposed PCF can achieve a high birefringence to the order of 10⁻² at the wavelength of 1.55 μm, a large negative dispersion over a wide wavelength range and confinement losses lower than 10⁻⁹ dB/m simultaneously, which has important applications in polarization-maintaining (PM) fibers, single-polarization single-mode (SPSM) fibers, dispersion compensation fibers and so on.     

Highly birefrigent photonic crystal fiber with high negative dispersion for broadband dispersion compensation

In this paper, we proposed a dual-enhanced core photonic crystal fiber (PCF) with high birefringence and ultra-high negative dispersion for dispersion compensation in a polarization maintained optical system. Using finite difference time domain (FDTD) method, we presented dispersion compensating PCF (DC-PCF) with negative dispersion between −1650 ps nm⁻¹ km⁻¹ and −2305 ps nm⁻¹ km⁻¹ in C-band and particularly −2108 ps nm⁻¹ km⁻¹ in λ = 1.55 μm wavelength. By this method, we can compensate dispersion in 124 km long span of a conventional single mode fiber (SMF) by 1 km-long of the DC-PCF at λ = 1.55 μm wavelength. Moreover, fundamental mode of the proposed PCF can induce birefringence about 3.5 × 10⁻³ at 1.55 μm wavelength.     

Photonic crystal fiber with high birefringence and low confinement loss

In this paper, a highly birefringent index-guiding photonic crystal fiber with low confinement loss is proposed by enlarging the central row of air holes in the structure. By employing the multipole method, properties of this structure, including the effective index, birefringence and confinement loss, are investigated. Simulation results indicate that high birefringence of 1.65 × 10⁻³ can be reached at the wavelength of 1.55 μm, and a low confinement loss on the order of 10⁻⁶ dB/km can be achieved at the same wavelength. Moreover, the impacts of air hole sizes on birefringence and confinement loss are also analyzed in detail.     

Highly birefringent photonic crystal fiber polarization splitter made of soft glass

A soft glass dual core polarization splitter based on highly birefringent photonic crystal fiber (PCF) is proposed and the full vector finite element method (FEM) is employed to analyze the impacts of structural parameters on birefringence and the coupling length, and simulation results show that high birefringence on the order of 10⁻² can be obtained at 1.55 μm, moreover, hole size, hole pitch and elliptic ratio all affect birefringence and the coupling length. Based on these results, the PCF's structure is optimized to realize a polarization splitter of 282 μm whose largest extinction ratio is around −45.42 dB at 1.55 μm. Meanwhile, the bandwidth at the extinction ratio of −10 dB is about 90 nm, and around 32 nm at −20 dB.     

Research on terahertz photonic crystal fiber characteristics with high birefringence

A new high birefringence photonic crystal fiber is proposed within the terahertz frequency region. It has two types of claddings, the inner is composed of six ellipse air holes arranged in a honeycomb array and the outer surrounded by circle holes. By using the full vector finite element method with anisotropic perfectly matched layers absorption boundary condition, the birefringence, chromatic dispersion and confinement loss of the fundamental mode are evaluated. The results show that the birefringence can achieve 10⁻³ when the wavelength increases from 600 μm to 900 μm. This structure will provide some reference value for the designing of high birefringence terahertz photonic crystal fiber.     

High birefringence and low loss circular air-holes photonic crystal fiber using complex unit cells in cladding

We propose a high birefringence and low loss index-guiding photonic crystal fiber (PCF) using the complex unit cells in cladding by the finite-element method. Results show that the birefringence and confinement loss in such PCF fiber is determined not only by the whole cladding asymmetry but also the shape of the PCF core. The maximal modal birefringence and lowest confinement loss of our proposed structures at the excitation wavelength of λ = 1550 nm can be achieved at 8.7 × 10⁻³ and 5.27 × 10⁻⁵ dB/km, respectively.     

Loss-reduced highly birefringent selectively liquid-filled photonic crystal fibers

We have theoretically investigated the birefringence and loss properties of the selectively liquid-filled photonic crystal fibers with the liquid asymmetrically infiltrated into one-line air holes along x-axis. A high birefringence value B = 1.74 × 10⁻³ can be achieved at λ = 1.55 μm. By varying the index of the infiltrating liquid, the birefringence values are shown to be well tuned. In addition, the confinement losses can be efficiently reduced by diminishing the number of liquid holes, which is quite useful for optical devices.     

中红外高保偏硫系玻璃双芯光子晶体光纤耦合特性研究

设计了一种中红外硫系玻璃双芯光子晶体光纤,利用多极法和模式耦合基本理论研究了这种光纤的耦合特性.数值模拟发现,空气孔间距Λ = 5.4 μm,空气孔半径为r = 1.35 μm,空气填充率d/Λ = 0.5的光纤,在归一化波长λ/Λ = 2.04 μm处双折射可以达到0.551×10^-2,在归一化波长λ/Λ = 0.93 μm处x偏振方向的耦合长度为145.32 μm,y偏振方向的耦合长 关键词： 双芯光子晶体光纤 硫系玻璃 双折射 耦合长度     

Polarization maintaining large nonlinear coefficient photonic crystal fibers using rotational hybrid cladding

In this paper, we present and explore a new hybrid cladding design for improved birefringence and highly nonlinear photonic crystal fibers (PCFs) in a broad range of wavelength bands. The birefringence of the fundamental mode in such a PCF is numerically analyzed using the finite element method (FEM). It is demonstrated that it is possible to design a simple highly nonlinear hybrid PCF (HyPCF) with a nonlinear coefficient of the about 46 W⁻¹ km⁻¹ at a 1.55 μm wavelength. According to simulation, the highest modal birefringence and lowest confinement loss of our proposed structure at the excitation wavelength of λ = 1.55 μm can be achieved at a magnitude of 1.77 × 10⁻² and of the order less than 10² dB/km with only five rings of air-holes in the fiber cladding.     

Analysis of a highly birefringent photonic crystal fiber with ellipse–rhombus air core

A novel highly birefringent photonic crystal fiber with small effective mode area is proposed. Four elliptical air-holes are designed in the internal layer of the fiber to cause the anisotropy, and the fiber has the property of highly birefringence and small effective mode area. The influences of elliptical air-holes on effective index, birefringence, effective mode area and nonlinear coefficient are analyzed by using full-vector finite element method (FEM). Simulation results show the birefringence can achieve the magnitude of 10⁻³ under the condition of d > 2.3 μm and a > 1.4 μm, which d and a are the distance and semimajor axis of elliptical air holes. By adjusting the parameters d and a, different effective mode area and nonlinear coefficient can be obtained, which demonstrates the flexibility of the proposed photonic crystal fiber.     

Squeezed hexagonal highly birefringent photonic crystal fiber with low effective modal area

A modified hexagonal index guiding photonic crystal fiber made of pure silica with high birefringence and a low effective modal area is proposed, and properties, including birefringence and effective modal area, are numerically analyzed using the multipole method. Numerical results show that high birefringence of 1.362 × 10⁻² and a low effective modal area of 3.435 μm² are achieved at 1.55 μm, simultaneously. Moreover, impacts of hole spacing and hole size on birefringence and effective modal area are also investigated in detail.     

Characterization of structural irregularities in highly birefringent photonic crystal fiber using torsional acoustic polarization coupling

We demonstrate a novel method to characterize structural irregularities in a highly birefringent photonic crystal fiber (HB PCF) with high accuracy based on polarization coupling by torsional acoustic wave. Birefringence variation induced by the irregularities of air-hole structure in the fiber cross-section is analyzed via the transmission spectra of the acousto-optic coupling between two orthogonal polarization modes propagating along the fiber. The estimated maximum birefringence variation of two sections in the same batch of the HB PCF is 3 × 10^− 6 and 10^− 5, respectively, from nominal birefringence value of 4.86 × 10^− 4. The comparison between the experimental results and the numerical simulation resulted in the estimation of few tens of nm variation in the air-hole structure in the PCF.     

Characteristics analysis of extruded elliptical hole photonic crystal fibers with square air-core

An extruded elliptical hole photonic crystal fibers PCF with square air-core is proposed. By using a full vector finite-element method FV-FEM and anisotropic perfectly matched layers APML, the structure and optical properties of the proposed PCF are analyzed. Simulation results show that the birefringence of the proposed photonic crystal fiber can be up to the order of 10⁻², and has a flattened dispersion from 1.20 μm to 1.80 μm. The proposed PCF may have important application in super-continuum SC generation, dispersion compensation, fiber-optic sensing systems and other aspects.     

Highly birefringent photonic crystal fibers with flattened dispersion and low effective mode area

A highly birefringent index-guiding photonic crystal fibers with flattened dispersion and low effective area is proposed by introducing elliptical air holes in the cladding and small holes both in the core area and in the cladding. With the plane wave expansion (PWE) method, the birefringent, dispersion and effective area of the fundamental modes in such photonic crystal fibers are analyzed in detail. The simulation result shows that high birefringence with a magnitude of the order of 10⁻³, flattened chromatic dispersion from 1100 nm to 1800 nm and low effective area (which mean high nonlinearity) are obtained. Furthermore, the influences on the birefringence and dispersion by geometrical parameters have also been discussed and a modest number of design parameters are given.     

Design of single polarization single mode dispersion compensating photonic crystal fiber

In this paper, we present a photonic crystal fiber based on hexagonal structure for improved negative dispersion as well as high birefringence in the telecom wavelength bands. It is demonstrated that it is possible to obtain negative dispersion coefficient of −712 ps/(nm km) and relative dispersion slope (RDS) perfectly match to that of single mode fiber (SMF) of about 0.0036 nm⁻¹ at the operating wavelength 1550 nm. The proposed fiber exhibits high birefringence of the order 2.11 × 10⁻² with nonlinear coefficient about 57.57 W⁻¹ km⁻¹ at 1550 nm. Moreover, it is confirmed that the designed fiber successfully operates as a single mode in the entire band of interest.     

Coupling characteristics of high birefringence dual-core As2S3 rectangular lattice photonic crystal fiber

A type of As₂S₃ chalcogenide glass mid-infrared dual-core photonic crystal fiber has been proposed. The dual-core photonic crystal fiber (PCF) consists of two asymmetric cores. The high polarization property and the coupling characteristics have been studied by using the finite element method and mode coupling theory. Numerical results show that the birefringence at wavelength λ = 10 μm is up to 0.01386 and the coupling length can reach wavelength λ = 5 μm, 261 μm and 271.44 μm for x-polarized mode and y-polarized mode, respectively. It demonstrates that a 6.786-mm-long fiber can exhibit an extinction ratio of better than -10 dB and a bandwidth of 180 nm.