Polarization mode dispersion measurements in an installed opticalfiber submarine cable

Highly accurate polarization mode dispersion (PMD) measurement using an interferometric detection technique with AC modulation is discussed. This technique, which searches for the interference fringe signal, was used to measure PMD in a 60 km long installed optical submarine cabled fiber 2500 m sea depth and to determine the fiber-length dependence of PMD. PMD measurement in 60-km-long installed optical submarine cabled fibers was confirmed to be less than about 0.7 ps. It was found that the fiber length dependence of the PMD measurement in the less than ~20-km-long cascaded uncabled fibers was proportional to √L; however, the PMD data in the long-length (over ~50 km long) fibers were slightly scattered     

Analysis on state of polarization of stimulated Brillouinscattering in an optical fiber ring-resonator

As a basic study of a Brillouin fiber-optic gyro, the state of polarization of stimulated Brillouin scattering in an optical fiber ring-resonator is analyzed. A high-birefringent fiber ring-resonator with 90° polarization axis rotation at the splice is considered as a candidate to have a stable lasing. It has two special states of polarization as the lasing modes, namely the polarization lateral modes. The lasing frequencies, namely the longitudinal modes for one state are fixed just in the middle point between the two neighboring lasing frequencies for the other in this resonator. This means that the stable single mode lasing may be obtained. The lasing threshold, the alternation of the lasing modes are discussed both by the simplified theory and the strict theory     

Degree of Polarization in Anisotropic Single-Mode Optical Fibers: Theory

The degree of polarization for propagation waves in anisotropic single-mode fibers is formulated in terms of light source spectrum, incident polarization condition, and fiber parameters. The polarization degree deterioration is based on the incident wave split into two eigenpolarization modes inherent in the fiber. Since the two eigenpolarization modes have different group velocities from each other, the degree of polarization is degraded when both of the modes are excited. Polarization degree is preserved when only one of the eigenpolarization modes is excited. The degradation is determined by the mutual correlation function gamma, between the two modes, which depends on the light source spectra, fiber polarization dispersion, and fiber length.     

分布反馈光纤激光器模式特性分析

根据耦合波理论,在分析分布反馈(DFB)光纤激光器纵模特性的基础上,着重阐述其偏振特性。另外给出了偏振态同耦合系数和双偏振态相移量差的关系。理论分析结果表明,当分布反馈光纤激光器输出为0阶模时,输出激光的偏振状态由耦合系数和双偏振态的相移量差共同决定,即在耦合系数一定的情况下,通过增加双偏振态相移量的差,或在双偏振态相移量差一定的情况下,通过减小耦合系数,可以实现单偏振输出。实验中在经载氢处理的掺铒光纤上制作分布反馈光纤激光器,由于耦合系数较大和双折射效应过小,输出为双偏振态。     

熔融拉锥过程中的偏振调制效应及应用

对光纤熔锥耦合器拉制过程中的偏振调制效应作了定性的理论分析,耦合区内的几何双折射和应力双折射导致的对称模和反对称模在x偏振方向和y偏振方向上的传播常数差不一致,从而引起两个方向的偏振光耦合的空间周期不同,存在相位差别,当相差正好达到π时实现偏振分离.在此基础上制作了熔锥型偏振分束器,消光比达到了16.5dB.     

Polarization dispersion measurement on twisted single-mode optical fibers

A new method for measuring polarization dispersion between two eigen polarization modes is presented. It utilizes the measured phase shift corresponding to birefringence difference between two adjacent wavelengths. This method makes it possible to measure small polarization dispersion by using a short test piece even when eigen polarization modes are elliptically polarized. A polarization dispersion of 0.4 ps/km in twisted single-mode fibers can be measured in an approximately 4-m-long fiber by using 1.152- and 1.161-μm lines in a He-Ne laser. Experimental data that polarization dispersion decreases with an increase in fiber twist ate in very close agreement with theoretical results.     

双偏振微结构光纤光栅的折射率传感特性分析

研究了微结构光纤(MOF)布拉格光栅(FBG)的双偏振折射率传感特性。利用有限元方法对保偏微结构光纤(PM-MOF)的传输特性进行了分析,计算了传输特性曲线,分析了该种光纤单模传输的范围。同时结合耦合模理论,仿真了该种光纤均匀布拉格光栅在充入不同介质时的反射谱,得到两个偏振模LP0x1和LP0y1的中心波长差与介质折射率的关系曲线,通过特性曲线拟合出相应公式。同时,仿真了该光纤光栅的温度特性,研究结果表明,两个偏振模中心波长对温度响应相似,因此利用双偏振差分的方式来探测,具有较强的抗干扰能力,这一特性为光纤生物传感器的应用提供了理论依据。     

Design of a fiber polarizer based on an asymmetric dual-core photonic crystal fiber

We propose a novel optical polarizer based on an asymmetric dual-core photonic crystal fiber(PCF) with triangular lattice air-holes.The fiber is designed as that the effective indices of modes in the two cores are matched at one polarized state but mismatched at another polarized state.As a result,one of the polarization states is coupled to the other core and transferred into a high-order mode.The transmission properties of the polarizer are investigated by the semi-vectorial beam propagation method(SV-BPM...     

Full Strain Tensor Treatment of Fiber Bragg Grating Sensors

Embedded fiber Bragg gratings can be subjected to arbitrary states of strain including shear strain. Such perturbations can cause coupling between polarization modes. Coupled-mode theory in Bragg gratings so far neglected this effect and only considered forward-backward coupling. Polarization mode coupling within a Bragg grating leads to interdependencies between Bragg reflection peaks which have so far been unaddressed. We formulate a full strain tensor treatment of fiber Bragg gratings, considering the coupling of the polarization modes within the grating. We give an approximation for the coupling coefficients affecting the polarization mode coupling and numerically solve the coupled-mode equations for representative states of strain. We show in which way shear strain affects the optical response of a grating and demonstrate how the fiber's beat length influences this characteristic.     

Design of optical strip-loaded waveguides with zero modalbirefringence

Integrated optic devices are usually sensitive to the polarization state of light, because the two polarized modes of the waveguides that form the devices in general have different propagation constants and may also suffer from different losses. The performance of such devices becomes unstable when they are connected to single-mode fibers, as the polarization state of the output light from a practical fiber link usually fluctuates in an unpredictable manner. A simple solution could be provided by using waveguides in which the two polarized modes are degenerate, i.e., have equal propagation constants. In this paper, it is shown theoretically with the spectral index method that the quasi-transverse electric (TE) and quasi-transverse magnetic (TM) modes of a properly designed optical strip-loaded waveguide can have equal propagation constants. The conditions for achieving mode degeneracy, or zero modal birefringence, are presented and discussed. Strip-loaded waveguides with degenerate polarized modes can be used potentially for forming polarization-insensitive optoelectronic devices     

Nondegenerate solution of TE and TM modes in step-index fibers using power-series expansion method

The power-series expansion method has been used to compute the exact cutoff frequencies of TM modes in step-index fibers for various values of relative refractive index difference (Δ), ranging from 0.005 to 0.1. The difference between the propagation constants of TM and TE modes, intermodal delay between TM and TE modes, and their variation with respect to the relative refractive-index difference has also been calculated. Numerical results show that for a fiber withDelta = 0.005the intermodal delay between TM01and TE01modes is of the order of 1.5 ps/km at frequencies far away from cutoff, whereas it is of the order of 0.62 ns/km forDelta = 0.1.     

对邦加球测量偏振模色散算法的研究

对基于邦加球的偏振模色散 (PMD)测量方法进行了充分的研究 ,利用输出偏振态的随机性 ,提出了一种比较简单而实用的算法 ,利用这种算法 ,对 11km单模光纤和 1 6 7m长保偏光纤进行了测量 ,实验证明这种算法具有很好的实用价值。     

Large polarization mode dispersion in fiber optic cables

The recently observed difference in polarization mode dispersion (PMD) between spooled and cabled single-mode optical fibers is investigated experimentally. To discriminate between a large polarization mode-coupling length or a large birefringence as a cause of high PMD, both parameters are measured. The results show a factor of 50 difference between the mode-coupling lengths of spooled and cabled fibers, which accounts for the larger PMD's of cabled fibers. It is suggested that cables with a loose tube design especially suffer from large polarization mode-coupling lengths. Internal stress is shown to be the dominant source of birefringence in standard telecommunication fibers by observing the shift of the PMD spectrum over a wide wavelength interval as a function of temperature     

Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

采用双折射光纤设计分布式应力传感器

采用扫描式迈克尔逊白光干涉仪对应力场引起的高双折射保偏光纤(PMF)内部的分布式偏振模式耦合进行定量测试和理论分析。由偏振模耦合的强度计算应力的大小,通过由双折射引起的2个正交的偏振模的光程差计算应力的位置。提出了分布式光纤应力传感器的结构设计。该传感器可实现1km的测量范围,对应力作用点的空间位置测试精度可达到mm量级,对偏振模耦合强度测试可达到-80dB的灵敏度。     

Characterization of single-mode coated fibers at low temperatures using periodic distortion of the fiber axis

Microbending effects on cabled single-mode optical fibers are analyzed both theoretically and experimentally over the temperature range -30°C-25°C. The loss in single-mode fiber is attributed to the coupling between the guided and radiative modes caused by micro-bending effects. The primary coupling between the guided LP01and the radiative LP11modes is studied by imposing periodically controlled perturbations on the fiber axis at different temperatures.     

基于保偏光纤光栅的双波长掺铒光纤激光器

提出了一种基于保偏光纤(PMF)中布拉格光栅的波长间隔可调的可开关双波长掺铒光纤激光器(EDFL)。由于和光纤布拉格光栅(FBG)两个反射峰对应的不同波长的两纵模在偏振态上是止交的．从而在均匀展宽的掺铒光纤中增强了偏振烧孔(PHB)效应。这种偏振烧孔效应大大减小了不同模式之间的竞争，因此可在室温下得到稳定的双波长振荡。另一方面。通过调整偏振控制器的状态．即改变腔内的双折射状念，光纤光栅的两个反射峰强度会发生变化。基于以上原理。便形成了对激光振荡模式的选择．即通过调整偏振控制器的状态可使激光器工作在稳定的双波长状态或在两波长之间转换。通过改变加在光纤光栅上侧向应力的大小和方向．可有效控制双波长激射的波长间隔．实验中得到了0．2～1．1nm的可调间隔。     

Eigenstates of polarization of elliptical single-mode fibers

The degree of polarization of radiation in an elliptical polarization-maintaining fiber and the polarization states of eigenpolarization modes of such fibers have been experimentally investigated in a wide spectral range. It has been shown that the elliptical fibers are uniform, homogeneously twisted fibers with elliptically polarized eigenpolarization modes. The ellipticity of the eigenpolarization mode is independent of fiber length and increases with wavelength, while the azimuth of the eigenpolarization mode is spectrally independent     

Design parameters for fluoride multimode fibers

Numerical calculations of losses due to polymer coatings and macrobending have been made for step-index multimode fluoride fibers. To minimize such losses, fiber parameters must be chosen to give a large value for V, the normalized frequency. Due to the long propagation wavelength (2.5 μm) for fluoride fiber, the parameters needed are very different from those of silica fiber. Using the criterion that ⩾90% of the modes have losses ⩽0.01 dB/km, it was found that the cladding diameters need to be rather large compared to step-index silica fiber parameters. Although the mode distributions are unknown for fluoride fiber, using a theoretical Gaussian distribution, losses were calculated for the various fiber parameters     

Far- and near-field investigations on the lasing modes intwo-dimensional photonic crystal slab lasers

We present experimental investigations on the lasing modes in two-dimensional (2-D) hexagonal cavities defined by photonic crystals on slab waveguide structures. The far-field emission patterns and near-field intensity distributions of the lasing modes are analyzed in polarization-resolved 2-D angular distribution measurements and spectrally resolved near-field scanning optical microscopy. The far- and near-field analyses result in identification of the various lasing modes and their subsequent classification into one- and two-dimensional modes. In the one-dimensional modes oscillating between two parallel boundaries of the cavities, longitudinal and transverse modes are identified and found to have transverse-electric polarization. In the two-dimensional modes oscillating two-dimensionally in the cavities, various modes including whispering-gallery-like modes are observed and found to exhibit various polarization states