Characterization of specially designed polarization maintaining photonic crystal fiber from far field radiation patterns

We report the development of theory and experiment for the characterization of polarization maintaining photonic crystal fiber (PM PCF) from far field intensity measurements. In this specially designed photonic crystal fiber, the air hole diameters along orthogonal axes adjacent to the core region are different, and hence create an effective index difference between the two orthogonal polarization modes. From the experimental measurements of the far field radiation pattern, we obtain transmission characteristics of PM PCF in terms of mode field diameters (MFD), V-values along major and minor axes, mode field area, birefringence, core radius, effective refractive index of cladding and numerical apertures along major and minor axes of PM PCFs. It is shown that the experimentally obtained geometrical and wave guiding parameters of the PM PCF match with the manufacturer/simulation/scanning electron microscope (SEM) data within the experimental limits.     

Coupling between elliptic screw polarization modes in single-mode optical waveguides with linear birefringence and regular twist of anisotropy axes in the presence of random axis twist

An analysis is made of single-mode fiber optical waveguides with linear birefringence and regular twist of anisotropy axes in the presence of random twist, which shows that, in this case, the coupling of orthogonal polarization modes having elliptic polarization in the screw coordinate system comoving with twist cannot be characterized by one coupling parameter, as differentiated from the case where constant twist is absent. In this case, the coupling between polarization modes is characterized by three independent parameters, which are different in nature and have different effects on the coupling of polarization modes. An estimate is made of the domain of applicability of the classical formula for the h parameter in single-mode fiber optical waveguides without regular twist of anisotropy axes.     

Method for improving the resolution and accuracy against birefringence dispersion in distributed polarization cross-talk measurements

We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.     

Method for improving the resolution and accuracy against birefringence dispersion in distributed polarization cross-talk measurements

We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.     

Influence of the irregularity of twisting of the birefringence axes of single-mode optical fibers on the polarization nonreciprocity of fiber ring interferometers

A particular example of the so-called minimum-configuration fiber ring interferometer whose loop comprises two different-length segments of a single-mode optical fiber with the same linear birefringence but different twisting is used to show that even in the case where the birefringence axes at the loop input coincide with the polarizer transmission direction, the phenomenon of polarization nonreciprocity of the fiber ring interferometer can emerge. It is shown that polarization nonreciprocity can be eliminated by the proper adjustment of the axes of a single-mode optical fiber, but the above adjustment should be changed if the waveguide temperature is changed. It is also shown that polarization nonreciprocity is a function of the light wavelength and the polarization nonreciprocity value in a fiber ring interferometer varies quasiperiodically in response to the temperature change in an optical fiber with random irregularities.     

Theoretical analysis of feedback high birefringence fiber loop mirror for application in single-frequency fiber lasers

A feedback high birefringence fiber loop mirror with a piece of erbium-doped fiber and a polarization controller is newly proposed. When the erbium-doped fiber is properly pumped to offset the loss of the feedback ring in the loop mirror, narrowband transmission peaks with large effective free spectral range can be achieved by the intrinsic vernier effect between orthogonally polarized lights respectively traveling along the two primary axes of the high birefringence fiber. In addition, the side transmission suppression ratio can reach about 6 dB. These special properties are independent of the polarization state of the input signal, which makes it feasible for the proposed feedback high birefringence fiber loop mirror to be employed as a narrowband filter in single-frequency fiber lasers.     

Accurate method for measuring the thermal coefficient of group birefringence of polarization-maintaining fibers

We present a method to accurately measure the group birefringence variation with temperature in high-birefringence polarization-maintaining (PM) fibers using a distributed polarization analyzer. By analyzing polarization cross-talk peaks purposely induced at both ends of a PM fiber, the temperature coefficient of group birefringence can be accurately obtained. We confirm the theoretical prediction that the group birefringence of PANDA and TIGER PM fibers decrease linearly with temperature from -40 °C to 80 °C, and find that the temperature coefficients are -5.93 × 10(-7) °C(-1) and -5.29 × 10(-7) °C(-1) for two types of PANDA fibers, and -5.36 × 10(-7) °C(-1) for a TIGER fiber.     

椭圆六角分布微结构光纤中的双折射

利用全矢量有限元方法,对椭圆六角对称分布微结构光纤(MF)的双折射特性进行了理论分析,得到了双折射大小与结构参量、入射波长间的依赖关系。分析表明:合理设计结构参量可得到10-3量级大小的双折射;微结构光纤的双折射对光波波长极其敏感,并出现随波长变化快慢轴交换的现象。采用统计的方法对双折射与微结构光纤空气孔直径随机变化的统计相关性进行了理论分析,假定空气孔直径为高斯分布,获得了双折射大小的平均值和标准偏差与随机变化孔径的统计关系。结果表明,由孔径分布不规则所产生的双折射的大小主要取决于空气孔直径的平均值。     

Analysis of a Sagnac interferometer with low-birefringence twisted fiber

The fiber Sagnac interferometer of low birefringence and twist is analyzed numerically in the linear region. A novel method for measurement of the birefringence of the fiber and the angle of rotation of the axes inside the fiber loop of the interferometer is also presented.     

外应力对Y波导器件尾纤消光比的影响

本文研究了外应力对Y波导器件尾纤消光比及其温度稳定性的影响机理.在对保偏光纤固定部分进行应力分析的基础上,研究了外应力的大小、方向对于光纤应力双折射及尾纤消光比的影响,并分析了消光比的全温稳定性.结果表明,外应力是影响消光比常温性能及全温稳定性的主要因素.当外应力的方向与光纤固有应力双折射的主轴不重合时,尾纤最大消光比会低于保偏光纤的固有消光比,并且耦合后波导芯片与光纤慢轴之间对轴失准.当温度变化时,外应力的变化将导致消光比随温度的波动.当外应力沿着保偏光纤固有双折射主轴方向时,能够确保尾纤在耦合过程中实现最大的消光比,并保证波导芯片偏振方向与光纤慢轴的严格对准,此时尾纤消光比具有最佳的温度稳定性.对V型槽和U型槽光纤固定块进行了尾纤消光比性能及其温度稳定性的比较分析,试验验证表明,V型槽方案相对于难以保证外应力方向与光纤慢轴重合的U型槽固定块设计方案而言,可以有效改善消光比性能及其全温温度稳定性.     

对轴误差对光纤陀螺输出的影响

理论分析并试验研究了Y波导保偏尾纤与保偏光纤线圈之间熔接点的对轴误差对光纤陀螺输出的影响.根据简化的光纤陀螺光路误差模型,对光纤陀螺的输出相位误差与光路中主要耦合点的对轴误差的关系进行了理论推导和仿真分析,并利用一个实际的闭环光纤陀螺试验研究了主要熔接点的对轴误差变化对光纤陀螺零偏和零偏稳定性的影响.结果表明,Y波导保偏尾纤与保偏光纤线圈之间的对轴误差是引起光纤陀螺输出误差的重要因素,必须尽量减小.     

Distributed high-temperature pressure sensing using air-hole microstructural fibers

We present spatially resolved Rayleigh scattering measurements in different polarization-maintaining (PM) fibers for high-temperature pressure sensing. The pressure-induced birefringence in the fiber cores is interrogated using polarization-resolved frequency-swept interferometry. The pressure responses of a PM photonic crystal fiber and a twin-air-hole PM fiber are investigated for a pressure range of 0 to 13.8 MPa (0-2000 psi) at room temperature and at temperatures as high as 800 °C. The proposed sensing system provides, for the first time to our knowledge, a truly distributed pressure-sensing solution for high-temperature applications.     

Generation of cavity-birefringence-dependent multi-wavelength bright–dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright–dark pulse pair in a mode-locked thulium-doped fiber laser(TDFL). The nonlinear polarization rotation(NPR) and nonlinear optical loop mirror(NOLM) were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation. By incorporating different lengths of high birefringence polarization-maintaining fiber(PMF), the fiber laser could operate stably in a multi-wavelength emission state. Compared with the absence of the PMF, the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright–dark pulse pair.     

拉曼增益对双折射光纤中孤子传输特性的影响

本文采用考虑拉曼增益的耦合非线性薛定谔方程,利用分步傅里叶方法求解并仿真模拟了光孤子脉冲在不同性质的双折射光纤中传输时的演化过程.结果表明,拉曼增益可以有效抑制非线性耦合导致的孤子漂移,同时会导致光孤子脉冲峰值在传输时不断增大,产生拉曼放大效应.拉曼增益也可以有效抑制双折射光纤中传输的相邻光孤子之间的相互作用.     

Polarization mode dispersion probability distribution for arbitrary distances

The probability distribution of the differential group delay (DGD) at any fiber length is determined by use of a physically reasonable model of the fiber birefringence. We show that if the fiber correlation length is of the same order as or larger than the beat length, the DGD distribution approaches a Maxwellian in roughly 30 fiber correlation lengths, corresponding to a couple of kilometers in realistic cases. We also find that the probability distribution function of the polarization dispersion vector at the output of the fiber depends on the angle between it and the local birefringence vector on the Poincaré sphere, showing that the DGD remains correlated with the orientation of the local birefringence axes over arbitrarily long distances.     

磁光非线性光纤中光参量增益的研究

将光纤中磁光效应和非线性效应作为微扰,推导了磁光四波混频的耦合模方程.通过解析解研究了各向同性磁光非线性光纤的参量过程,并指出利用磁光耦合系数的色散特性可以实现四波混频的相位匹配.同时,采用龙格-库塔法分析了在线双折射磁光非线性光纤中,忽略费尔德常量的波长依赖性时,左旋圆偏振光参量增益的磁控特性,指出了实验中采用较高费尔德常量的非线性光纤的必要性.研究表明:1)对于低线双折射磁光非线性光纤,优化双折射大小可以获得最大的参量增益;2)根据参量增益对磁光耦合系数的单调依赖特性,适当选择光纤长度、泵浦功率以及输入导波光的偏振态,可使参量增益的磁可调范围大大提高.     

Highly Birefringent Microstructure Fiber with Zero Dispersion Wavelength at 0.64 Micrometer

Abstract

In this article, we have designed a microstructure fiber, which consists of elliptical air holes at the core region. We have investigated its optical properties using finite difference time domain method. The fundamental mode of the proposed microstructure fiber can induce very high birefringence. It has been realized that the value of birefringence is mainly decided by the shape of the air holes present in the first and second rings. The zero dispersion wavelengths of both fast and slow axes have been shifted to 0.64 micrometer. The proposed birefringent microstructure fiber may be useful in optical communication and sensors.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VI. Evolution in the degree of polarization of nonmonochromatic radiation traveling in a twisted optical waveguide

An analysis is made of the evolution of polarization of nonmonochromatic radiation travelling through single-mode optical fibers in the presence of random coupling between orthogonally polarized modes, which is caused by the random twist of axes of natural linear birefringence of an SMF, and in the presence of a regular axis twist. It is shown that the depolarization length of nonmonochromatic radiation in an SMF increases with increasing regular SMF twist, regardless of the presence of a random axis twist. Using the Monte Carlo method, the dependences of the mathematical expectation and the mean-square deviation of the degree of polarization of nonmonochromatic radiation on the fiber length for SMFs with different linear birefringences and regular axis twists are calculated. It is shown that the fiber length on which the degree of polarization of nonmonochromatic radiation reaches its limiting value increases and the limiting value itself decreases with an increasing regular twist. It is also shown that an induced SMF twist is able to improve parameters of a fiber interferometer, in particular, to decrease random phase changes and intensity fadings of the interference signal, which are caused by random coupling between polarization modes.     

Polarization locked vector solitons and axis instability in optical fiber

We experimentally observe polarization-locked vector solitons in optical fiber. Polarization locked-vector solitons use nonlinearity to preserve their polarization state despite the presence of birefringence. To achieve conditions where the delicate balance between nonlinearity and birefringence can survive, we studied the polarization evolution of the pulses circulating in a laser constructed entirely of optical fiber. We observe two distinct states with fixed polarization. This first state occurs for very small values birefringence and is elliptically polarized. We measure the relative phase between orthogonal components along the two principal axes to be +/-pi/2. The relative amplitude varies linearly with the magnitude of the birefringence. This state is a polarization locked vector soliton. The second, linearly polarized, state occurs for larger values of birefringence. The second state is due to the fast axis instability. We provide complete characterization of these states, and present a physical explanation of both of these states and the stability of the polarization locked vector solitons. (c) 2000 American Institute of Physics.     

Complete determination of the stress tensor of a polarization-maintaining fiber by photoelastic tomography

The use of photoelastic tomography to obtain the two-dimensional axial stress profile of a polarization-maintaining (PM) fiber with high resolution and accuracy is described. We illustrate, for what is believed to be the first time, the two-dimensional distribution of the local principal axes of the fiber's cross section, which is directly related to the fiber's PM ability. We demonstrate that the stress-induced anisotropy as well as all the stress tensor components of the fiber can be fully determined.