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.     

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.     

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.     

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。     

Ultra-flattened negative dispersion for residual dispersion compensation using soft glass equiangular spiral photonic crystal fiber

Abstract

We present a numerical investigation of an equiangular spiral photonic crystal fibre (ES-PCF) in soft glass for negative flattened dispersion and ultra-high birefringence. An accurate numerical approach based on finite element method is used for the simulation of the proposed structure. It is demonstrated that it is possible to obtain average negative dispersion of –526.99 ps/nm/km over 1.05–1.70 μm wavelength range with dispersion variation of 3.7 ps/nm/km. The proposed ES-PCF also offers high birefringence of 0.0226 at the excitation wavelength of 1.55 μm. The results here show that the idea of using the proposed fibre can be potential means of effectively directing for residual dispersion compensation, fibre sensor design, long distance data transmission system and so forth.     

Dispersion-compensating photonic crystal fiber with wavelength tunability based on a modified dual concentric core structure

We present a 5-layer air-hole dispersion-compensating photonic crystal fiber (PCF) with a modified dual concentric core structure, based on central rod doping. The finite element method (FEM) was used to investigate the structure numerically. If the structural parameters remain unchanged, a high degree of linear correlation between the central rod refractive index and the operating wavelength can be achieved in the wavelength range of 1.5457–1.5857 μm, which suggests that the operating wavelength can be determined by the refractive index of the centre rod. A negative dispersion coefficient between –5765.2 ps/km/nm and –6115.8 ps/km/nm was obtained by calculation and within the bandwidth of 108 nm (1.515–1.623 μm) around 1.55 μm, a dispersion coefficient of –3000 ps/km/nm can be ensured for compensation. In addition, this proposed PCF also has the advantage of low confinement loss, between 0.00011 and 0.00012 dB/m, and ease of fabrication with existing technology. The proposed PCF has good prospects in dispersion-compensating applications.     

Three octave spanning supercontinuum by red-shifted dispersive wave in photonic crystal fibers

This article presents a three-layer index guided lead silicate (SF57) photonic crystal fiber which simultaneously promises to yield large effective optical nonlinear coefficient and low anomalous dispersion that makes it suitable for supercontinuum (SC) generation. At an operating wavelength 1550 nm, the typical optimized value of anomalous dispersion and effective nonlinear coefficient turns out to be ~4 ps/km/nm and ~1078 W^?1km^?1, respectively. Through numerical simulation, it is realized that the designed fiber promises to exhibit three octave spanning SC from 900 to 7200 nm using 50 fs ‘sech’ optical pulses of 5 kW peak power. Due to the cross-phase modulation and four-wave mixing processes, a long range of red-shifted dispersive wave generated, which assists to achieve such large broadening. In addition, we have investigated the compatibility of SC generation with input pulse peak power increment and briefly discussed the impact of nonlinear processes on SC generation.     

Modeling of dispersion and nonlinear characteristics of tapered photonic crystal fibers for applications in nonlinear optics

In this article, we investigate for the first time the dispersion and the nonlinear characteristics of the tapered photonic crystal fibers (PCFs) as a function of length z, via solving the eigenvalue equation of the guided mode using the finite-difference frequency-domain method. Since the structural parameters such as the air-hole diameter and the pitch of the microstructured cladding change along the tapered PCFs, dispersion and nonlinear properties change with the length as well. Therefore, it is important to know the exact behavior of such fiber parameters along z which is necessary for nonlinear optics applications. We simulate the z dependency of the zero-dispersion wavelength, dispersion slope, effective mode area, nonlinear parameter, and the confinement loss along the tapered PCFs and propose useful relations for describing dispersion and nonlinear parameters. The results of this article, which are in a very good agreement with the available experimental data, are important for simulating pulse propagation as well as investigating nonlinear effects such as supercontinuum generation and parametric amplification in tapered PCFs.     

Advanced design and optimization of single mode photonic crystal fibers

This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the properties of PCFs such as dispersion, dispersion slope and loss, an artificial intelligence method, the Nero-Fuzzy system, is applied. In addition, a special cost function which simultaneously includes the confinement loss, dispersion and its slope is used in the proposed design approach. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits an ultra low confinement loss and low dispersion at 1.55 µm wavelength with a nearly zero dispersion slope over the C communication band.     

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.     

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.     

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 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.     

正方形空气孔光子晶体光纤特性分析

为了同时获得高双折射和色散平坦特性的光子晶体光纤,本文提出了一种包层以椭圆空气孔为纤芯,四周环绕正方形空气孔的光子晶体光纤结构。基于不同纤芯椭圆率、不同纤芯填充材料,对所提光子晶体光纤结构的双折射、色散、非线性等性能进行了讨论。结果表明,在波长1.55 μm处,当纤芯椭圆率不同,填充材料相同时,最大双折射值为0.37,最大非线性系数值277.76 W-1·km-1;当纤芯填充材料不同,椭圆率相同时,最大双折射值为0.34,最大非线性系数值为307 W-1·km-1。在波段1.26 μm~1.8 μm范围,色散呈现出近零色散平坦特性,变化范围不超过±12.5 ps/(nm·km),带宽0.6 μm。     

Design of ultra-low and ultra-flattened dispersion single mode photonic crystal fiber by DE/EDA algorithm

This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the effective index of propagation of the mode and then, the other properties of structure, a finite difference frequency domain (FDFD) solver is applied. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits a dispersion of 0.22 ps nm^?1 km^?1 at 1.55 µm wavelength with a variance of ±0.4 ps nm^?1 km^?1 over the C communication band and a nearly zero dispersion slope.     

Multi-channel terahertz wavelength division demultiplexer with defects-coupled photonic crystal waveguide

Abstract

Terahertz (THz) wavelength division demultiplexer based on a compact defects-coupled photonic crystal waveguide is proposed and demonstrated numerically. This device consists of an input waveguide that perpendicularly coupled with a series of defects cavities, each of which captures the resonance frequency from the input waveguide. Coupled-mode theory and finite element method are used to analyze the transmission properties of the structure. It is found that the transmission wavelength centered around 1 THz can be adjusted by changing the geometrical parameters of defects cavities, which equals to THz waves generated by optical methods such as difference frequency generation and optical rectification. Applications in this frequency range are urgently needed. Furthermore, the highest transmission efficiency of 0.94 can be achieved when a perfect wavelength-selective mirror is set in the output waveguide.     

Highly nonlinear all-solid photonic crystal fibers with low dispersion slope

A fluorine-doped trench-assisted structure is proposed to improve the nonlinearity of photonic crystal fibers (PCFs). Three all-solid highly nonlinear PCFs with low dispersion slope and low confinement loss are designed. They exhibit all normal dispersion, two zero dispersion wavelengths (ZDWs) and one ZDW just at 1.55 μm, respectively. The lowest dispersion slope is 5.12×10(-4) ps/(km·nm(2)), which is 2 orders of magnitude lower than that of conventional highly nonlinear fibers. A nonlinear coefficient of 31.5 W(-1)·km(-1) and low loss of 9.62×10(-5) dB/km at 1.55 μm has been achieved for this PCF.