Analysis of nonlinear PCF for birefringence application using FDTD method

The effect of ellipticity on birefringence in a hexagonal photonic crystal fiber having elliptical air holes with Kerr nonlinearity is investigated, with and without defect using finite difference time domain (FDTD) simulations. It is found that the birefringence increases with the increase of ellipticity. Further this analysis is extended to a double defect structure, where two adjacent air holes are omitted horizontally from the hexagonal structure. This double defect structure is found to have more birefringence than the structure without defect. It is raveled that birefringence due to no defect is more for lower value of ellipticity; however at higher value of ellipticity, birefringence due to double defect is more than the one that could be due to no defect.     

拉曼效应对低双折射光纤的偏振态演化分析

在准连续情况下, 通过求解低双折射光纤中含有拉曼效应的右旋与左旋圆偏振光所满足的耦合非线性薛定谔方程, 得到了归一化功率以及相位差的解析解. 利用解析解,推导出了椭圆率和方位角的表达式, 研究了拉曼效应和传输距离对偏振态演化的影响, 并用相平面法对椭圆率和方位角随着输入功率的变化进行了直观描述. 结果表明: 低双折射光纤在传输的过程中, 拉曼效应和传输距离都是影响其偏振态稳定性的因素, 均改变了椭圆率和方位角的演变周期以及振荡幅度.     

Polarization characteristic calculation of three-layer elliptical single-mode fibres

A vector solution is obtained for fields of fundamental modes of an elliptical three-layer optical fibre, which is valid for small ellipticity of boundaries between the layers and for arbitrary differences of core and cladding indices. The modal birefringence is investigated as a function of fibre parameters on the basis of this solution. It is shown that the two-step fibre birefringence is larger than the W-fibre birefringence for equal differences of the core and inner cladding indices. The relationship is inverse for equal differences of the core and outer cladding indices. The correlation between the birefringence value and the effective diameter of the single-mode fibre is discussed.     

Stress distribution and birefringence measurement in double-clad fiber

Modal birefringence and stress distribution measurements in a single-mode double-clad Nd⁺³-doped fiber are reported. The contribution of different mechanisms, such as stress-induced birefringence, bending and core ellipticity, to the total birefringence is determined for a fiber wound onto a drum. A stress distribution pattern in the cross-section of the fiber is presented. The stresses are shown to be applied to a fiber core along the cladding minor axis.     

Linear Transformation of the Polarization Modes in Coiled Optical Spun-Fibers with Strong Unperturbed Linear Birefringence. I. Nonresonant Transformation

A linear transformation of orthogonal polarization modes in coiled optical spun-fibers with strong unperturbed linear birefringence, which causes the emergence of the dependences of the integrated elliptical birefringence and the ellipticity and azimuth of the major axis of the ellipse, as well as the polarization state of radiation (PSR), on the length of optical fiber has been considered. Optical spun-fibers are subjected to a strong mechanical twisting, which is frozen into the structure of the optical fiber upon cooling, in the process of being drawn out from the workpiece. Since the values of the local polarization parameters of coiled spunwaveguides vary according to a rather complex law, the calculations were carried out by numerical modeling of the parameters of the Jones matrices. Since the rotation speed of the axes of the birefringence is constant on a relatively short segment of a coiled optical spun-fiber in the accompanying torsion (helical) coordinate system, the so-called “Ginzburg helical polarization modes” (GHPMs)—two mutually orthogonal ellipses with the opposite directions of traversal, the axis of which rotate relative to the fixed coordinate system uniformly and unidirectionally—are approximately the local normal polarization modes of such optical fiber. It has been shown that, despite the fact that the unperturbed linear birefringence of the spun-fibers significantly exceeds the linear birefringence, which is caused by the winding on a coil, the integral birefringence of an extended segment of such a fiber coincides in order of magnitude with the linear birefringence, which is caused by the winding on the coil, and the integral polarization modes tend asymptotically to circular ones. It has been also shown that the values of the circular birefringence of twisted single-mode fibers, which were calculated in a nonrotating and torsion helical coordinate systems, differ significantly. It has been shown that the polarization phenomena occur in the process of linear transformation of local polarization modes, which lead to small quasi-harmonic oscillations of the birefringence integral parameters of the optical spun-fibers, which depend on their length, and the period of these oscillations is approximately equal to half of the effective period of polarization beating.     

激光陀螺本征模偏振态与磁敏感特性的理论研究

环境磁场的干扰是影响激光陀螺精度的重要因素之一. 为了减小二频机械抖动激光陀螺的磁敏感性, 推导了环形腔的琼斯矩阵, 其中考虑了非共面角、增益介质、腔损耗、腔镜反射各向异性和应力双折射, 采用矩阵本征问题求解的摄动理论分析了环形腔的偏振态和磁敏感特性. 研究表明, 为了减小激光陀螺的磁敏感性, 应减小非共面角和腔损耗, 增大腔镜的反射各向异性. 腔镜应力双折射究竟会增大还是减小磁敏感性与应力作用主轴、受应力腔镜的位置有关, 此外它还会引起顺时针和逆时针模式之间的偏振非互易性. 磁敏感性与腔失谐近似成线性关系, 导致激光陀螺工作于增益峰值时磁敏感性并非最小. 这些结果对减小二频机械抖动激光陀螺的磁敏感性具有较好的指导意义. 关键词： 激光陀螺 磁敏感性 椭圆度 琼斯矩阵     

压缩六角点阵椭圆孔光子晶体光纤的低色散高双折射效应

设计了一种新型结构低色散高双折射光子晶体光纤,该光纤纤芯缺失一根空气柱,包层沿光纤长度方向排布压缩六角点阵椭圆空气孔.采用全矢量平面波法,对其色散和双折射特性进行了数值模拟.研究发现,通过改变光纤结构参数,即改变六角点阵压缩比ξ、相对孔间隔f和空气孔椭圆率η,可以调节该光纤低色散高双折射工作波长.若调整光纤结构参数ξ=048, f=035, η=055, x方向孔间隔Λ_x=12　μm时,该光纤呈现低色散高双折射效应,在1360—167 关键词： 导波与光纤光学 全矢量平面波法 低色散高双折射 六角点阵压缩比     

Fabrication and study of microstructured optical fibers with elliptical holes

We report the fabrication of what are believed to be the first microstructured optical fibers with uniformly oriented elliptical holes. A high degree of hole ellipticity is achieved with a simple technique that relies on hole deformation during fiber draw. Both form and stress-optic birefringence are characterized over a broad wavelength range. These measurements are in excellent agreement with numerical modeling and demonstrate a birefringence as high as 1.0 x 10(-4) at a wavelength of 850 nm.     

椭圆形高折射率芯Bragg光纤的偏振和色散特性

应用全矢量有限元方法,分析了椭圆形高折射率芯Bragg光纤的偏振和色散特性,详细讨论了光纤结构参量对模式双折射的影响,结果发现：椭圆形高折射率芯Bragg光纤的模式双折射在10－3量级,比传统保偏光纤高出一个量级,且随波长的增加而单调递增;模式双折射随包层低折射率材料填充比的增加而减小,但与包层周期的变化几乎无关.此外,提高椭圆率或材料间的折射率比可显著增强模式双折射.研究结果有助于设计高性能的一维微结构保偏光纤.     

A comparison of the effect of the ellipticity on the small and large solid-core PCF’s characteristics

In this paper we study small and large solid-core PCFs with elliptical shaped air-holes for the hexagonal lattice, which were constructed by omitting one air-hole (small core) and seven air-holes (large core), respectively, and we compare their characteristics with those with circular holes. We use two types of ellipticity: vertical and horizontal. Birefringence, dispersion, and the numerical apertures of the fundamental modes are analyzed using commercial simulation software. Also, the confinement loss is obtained. The effect of ellipticity on these characteristics is investigated in detail. It is found that the type of ellipticity of the air holes affects the numerical aperture and hence confinement loss significantly, but it has no crucial effect on either the dispersion or the birefringence for the small and large core PCFs.     

Properties of index-guided PCF with air-core

In the paper, index-guided triangular PCF with air-core is introduced which guides light by total internal reflection (TIR) when the air-core is smaller than the air holes in cladding. Properties of dispersion and leakage loss are investigated using finite-element method. The total dispersion curve gets more flattened and leakage loss increases significantly as the air-core diameter increases. High birefringence PCF with elliptical air-core is put forward and the birefringence increases with the ellipticity of the air-core.     

Highly birefringent soft glass rectangular photonic crystal fibers with elliptical holes

In this paper we report on the fabrication of highly birefringent photonic crystal fiber with a photonic cladding composed of elliptical holes ordered in a rectangular lattice. The fiber features a group birefringence G of 0.82×10^?4 at 725 nm. We discuss the influence of structural parameters including the ellipticity of the air holes and the aspect ratio of the rectangular lattice on the birefringence and on the modal properties of the fiber.     

基于椭圆孔微结构纤芯的高双折射光子晶体光纤

提出了一种在纤芯引入微小椭圆孔实现高双折射光子晶体光纤的方法.采用全矢量有限元方法和完美匹配层条件,依次研究了光子晶体光纤在纤芯中引入单个、双个、三个微小椭圆孔情况下的双折射特性.模拟结果表明,纤芯微小椭圆孔主导了光子晶体光纤的双折射特性,其参量及个数对双折射有着重要的影响,增大椭圆面积、椭圆率可以进一步提高光子晶体光纤的双折射.最后设计和研究了一种采用三环阵列椭圆孔微结构纤芯的光子晶体光纤,其双折射可以达到2.7×10－3.     

偏振保持光纤的模式双折射

偏振保持光纤是单模光纤的一种特殊类型,其模式双折射与温度、芯和皮折射率差及椭圆度、热膨胀系数、光纤直径、弹性模量、泊松比、频率、波长、拉丝张力等诸多因素有关,对与偏振保持光纤模式双折射相关的上述因素进行分析,指出光纤模式双折射越高,光纤的偏振态保持就越好,为更好地设计新型保偏光纤或合理选择保偏光纤类型奠定了基础.     

Mode Coupling in Single-Mode Helical Fibres Under Perturbations

The mode coupling in a helical fibre with a circular cross-section undergoing perturbations is investigated in the Serret-Frenet frame. The coupled-mode equations have been derived and are compared with the conventional ones. Applying the coupled-mode equations to a helical fibre with an elliptical core deformation, we obtained, for the first time, a rigorous analysis of the transmission characteristics of the fibre. The numerical examples show that the mode ellipticity and elliptical birefringence increase as the core ellipticity increases and also vary as the pitch or the core offset change.     

Index of refraction, optical activity and electro-optic coefficient of bismuth titanium oxide (Bi12TiO20)

12 TiO₂₀. The first two can be measured by simple and classical methods, but the coexistence of optical activity, electrogyration, field-induced linear birefringence, and piezoelectric and photoelastic effects in photorefractive materials such as Bi₁₂TiO₂₀ complicate the measurement of the electro-optic coefficient. For normal incidence of linearly polarized light we derive analytic expressions for the polarization of light that has passed through the crystal. The ellipticity of the polarization is a function of the electric-field-induced linear birefringence and hence of the electro-optic coefficient of the crystal. Therefore measurement of the ellipticity as a function of an electric field externally applied to the crystal leads to an electro-optic coefficient r₄₁ of 5.3±0.1 pm/V. Received: 22 December 1996/Revised version: 21 March 1997     

Highly birefringent extruded elliptical-hole photonic crystal fibers with single defect and double defects

Highly birefringent elliptical-hole photonic crystal fibers （PCFs） with single defect and double defects are proposed, which are supposed to be achieved by extruding normal circular-hole PCFs based on a triangular-lattice photonic crystal structure. Comparative research on the birefringence and the confinement loss of the proposed PCFs with single defect and double defects is presented. Simulation results show that the proposed PCFs with single defect and double defects can be with high birefringence （even up to the order of 10^-2）. The confinement loss increases when the ellipticity of the air hole of the PCFs increases, which nevertheless can be overcome bv increasing the ring number or the air holes in the fiber cladding.     

拉曼效应对低双折射光纤偏振特性的影响

在低双折射光纤中,利用线偏振光满足的包含拉曼效应的非线性耦合模传输方程,通过引入斯托克斯参量,导出了斯托克斯参量所满足的耦合模传输方程.利用庞加莱球图示法,描述了拉曼增益效应作用下光波偏振态的演化,研究分析了拉曼效应对低双折射光纤中光波偏振态演化规律的影响.结果表明,当输入功率与运动常量满足一定关系时,拉曼增益效应改变了光波传输时其偏振态演化周期和偏振态的椭圆率.     

Design of temperature-compensated elliptical core birefringent fibres

A new type of temperature compensated birefringent optical fibre is reported. The proposed fibre is designed aiming to minimising the stress birefringence by matching the thermal expansion coefficients between the core and the cladding and maximising the geometric birefringence by increasing the refractive index difference between the core and the cladding and increasing the core ellipticity. The optimum dopant concentrations satisfying the requirements is determined by solving a set of nonlinear algebraic equations. The fibre aimed at satisfying the requirements is fabricated and characterised. The experimental results show that the temperature dependence of the fibre birefringence is 6.97 × 10⁻⁴/°C. Methods of further improvement of the compensation are discussed and an alternative dopant material, titanium, for the improvement is suggested.     

光学电流互感器中线性双折射与法拉第效应的分离检测

分析了线性双折射对光学电流互感器(OCT)电压输出的影响,指出了差除和信号处理方案检测输出光信息的不完备性问题,用琼斯矩阵理论证明了差除和方案不能独立检测块状玻璃传感头中线性双折射的结论。提出了光学电流互感器的椭偏检测方案,从椭圆偏振光参量方程出发,推导了温致线性双折射、法拉第旋转角与输出椭圆偏振光椭率、倾角之间的解析式,证明了椭偏检测法对输出光信息检测的完备性,实现了温致线性双折射和待测电流的同时测量。为保证实时性,提出了输出椭圆光的三态检测方案,采用水平、垂直和45°三个偏振态的实时检偏,既克服了线性双折射的影响又实现了待测电流的实时测量,从检测原理上解决了温致线性双折射对光学电流互感器的影响。