Chirped rectilinear core longitudinally-graded optical fibers

Reported here is a straight-forward and flexible method to fabricate silica optical fibers of circular cladding cross-section and rectilinear cores whose aspect ratio and refractive index profile changes with position along the fiber in a deterministic way. Specifically, a modification to the process recently developed to produce longitudinally-graded optical fibers, LGFs [Opt. Express 20 (2012) 17394–17402], was employed. Herein reported are MCVD-derived germanosilicate fibers with rectangular cores where the aspect ratio changes by nearly 200% and the average refractive index changed by about 5%. Fiber losses were measured to be about 50 dB/km. Such rectangular core fibers are useful for a variety of telecommunication and biomedical applications and the dimensional and optical chirp provides a deterministic way to control further the modal properties of the fiber.     

Circular core polarization-maintaining optical fibers withelliptical stress-induced cladding

The reproducible technology for producing high-birefringence fibers with stress-induced elliptical cladding and circular core is described. The authors have obtained fibers that have a birefringence of about (1-3) 10^-4, a mode coupling parameter of about (2-7) 10 ^-5 m^-1, and loss of less than 0.5 dB/km at 1.6 μm. The authors have found effects restricting the capability of test fibers to maintain the state of linear polarization     

Mode conversion in optical fibers with monotonically increasing core radius

In this paper we discuss mode conversion in a straight fiber which maintains its circular cross section but whose radiusa(z)increases monotonically with the length coordinatezso that it passes from the single-mode to the multimode regime. Our principal concern is with step-index fibers which are treated in the scalar approximation. We study conversion of power from the incident dominant mode to other core and cladding modes for several functionsa(z). The results of our calculations show clearly that the dominant mode is able to adapt itself to the changing fiber radius provided the change is gradual. For comparison, we also consider a fiber with an infinitely extended radial parabolic-index profile as an approximation to graded index fibers. We show that it too exhibits mode conversion similar to that of the step-index fiber, provided that the comparison is made on the basis of corresponding mode spot sizes instead of actual core radii.     

Birefringence caused by thermal stress in elliptically deformed core optical fibers

Birefringence and polarization dispersion introduced by thermal stress in single-mode fibers with elliptical core are formulated here in terms of fiber structure parameters, thermal expansion coefficients, and thermoelastic constants. Thermal stress distribution and birefringence characteristics are estimated within first-order perturbation with respect to fiber core ellipticity. Normalized frequency dependence of birefringence and polarization dispersion is also derived in this paper. Birefringence and polarization dispersion in elliptical core fiber are seen to depend on thermal stress rather than geometrical anisotropy for relatively small relative index differences. Numerical examples are presented for germanosilicate, borosilicate, and phosphosilicate glass fibers.     

Drawing optical fibers with an elliptical core from cylindrical tube preforms

An original technique for manufacturing quartz optical fibers with an elliptical light-guiding core is proposed and tested. The dependences of the ellipticity of the light-guiding core on parameters of the manufacturing process—the temperature of the heating element of the high-temperature furnace and rarefaction inside the cylindrical tube preform—are studied. Test fiber samples are drawn and their optical properties are investigated.     

Single-polarization single-mode optical fibers

Strictly speaking, an ordinary axially symmetrical single-mode fiber is a "two-mode" fiber because two orthogonally polarized HE11modes can be propagated in it. This fact results in the instability of the polarization state of the propagated mode when geometrical perturbation exists in the fiber, and also the so-called polarization mode dispersion. These are harmful in some applications of single-mode fibers to communication and measurement. To prevent these adverse effects, single-polarization single-mode (SPSM) optical fibers have been developed. Three basic types of the SPSM fiber are elliptical-core fiber, stress-induced birefringent fiber, and side-pit fiber. This paper describes the principles of these three types, performance obtained experimentally, theoretical approaches, and measurement techniques related to the SPSM fibers. Finally, relevant technical tasks in the future are mentioned.     

Ultrahigh birefringent optical fibers

The modes of weakly guiding fibers, constructed from anisotropic material which exhibits ultrahigh birefringence, are investigated. These modes are plane polarized unless the fiber is virtually uniaxial, withn_{x} = n_{y} neq n_{z}. The modal intensity patterns are, in general, noncircular, even for fibers of circular cross section.     

Polarization in optical fibers

Recent research on fibers with very small or very large birefringence for polarization-dependent applications is reviewed. The nature of random coupling between normal modes of polarization is analyzed and discussed in connection with various applications.     

Low-loss fluoride optical fibers for midinfrared optical communication

Attempts to improve transmission loss characteristics in fluoride fibers are described. Optical loss in current fibers is dominated by two major extrinsic loss factors, defect scatterers and impurities. Scatterer analysis using a Raman microprobe has revealed that the majority of them are ZrO2crystallites. These crystallites dominate the fiber scattering characteristics, having both wavelength independent and Rayleigh wavelength^-4dependencies according to their size. Excess loss due to OH groups which causes absorption at around 2.9 μm is quantified as 2000-5000 dB/km/ppm, depending on glass composition. These results suggest that further efforts in glass synthesis should concentrate on eliminating oxide and hydroxide impurities, and on the further purification of the raw materials. The key for realizing high-quality, low-loss, and long-length fluoride fibers is currently related to whether or not oxide scatterers can be completely eliminated.     

Microbending loss evaluation in arbitrary-index single-mode optical fibers. Part II: Effects of core index profiles

Microbending losses in single-mode fibers with several types of core refractive index profiles are compared. Numerical calculations were carried out to characterize fibers with step, power-law,Wand ring-shaped index profiles. Step-index andWfibers exhibit a small excess loss near the single-mode operation upper limit. However, permissible offset misalignment in fiber splice at constant microbending loss is nearly identical for step, power-law, andWfibers. An index dip at the core center has an undesirable influence on the required splicing accuracy. The effects of fiber curvature statistics and index profile parameters are investigated in detail.     

Sensing with suspended-core optical fibers

The development of optical fibers with suspended cores has enabled the demonstration of a range of powerful new techniques for chemical and biological sensing. Here the fabrication, design and application of this new class of fibers are reviewed. The performance and potential of sensors based on these fibers is evaluated, including dip sensors for sensing small sample volumes, exposed-core fibers for real-time and distributed measurements, and surface functionalized fibers for the specific detection of chemicals and biomolecules.     

如何选用光纤

文中以光纤最新标准为依据 ,介绍光纤的技术发展 ;同时 ,阐述了光纤的分类、结构和性能 ;最后 ,按光纤的特点提出了选用方案建议。     

Practical LEOS applications-handling optical fibers

The preparation of optical fibers for efficient light coupling is discussed. Key parts and techniques commonly used in fiber optics laboratories are described. The techniques are fiber termination, splicing fibers, and coupling lasers to fibers     

High-modulus low-linear-expansion-coefficient loose-jacket optical fibers

Optical fibers loosely jacketed with high-modulus low-linear-expansion-coefficient polymers have been proposed, and the mechanical, thermal, and optical properties have been investigated. The loose-jacket tube is made of highly oriented polyoxymethylene (POM), which is produced by tensile-drawing the isotropic POM tube with dielectric heating. The oriented POM exhibits Young's moduli 20-40 GPa and linear expansion coefficients of the order of10^{-5}-10^{-6}degC^-1. Owing to the low linear expansion coefficients of the material, the oriented POM loose-jacket optical fiber has no low-temperature excess losses, which is due to fiber bends caused by thermal contraction of the loose tube, in the-60-80degC temperature range.     

开放式微结构光纤研究

与传统微结构光纤相比,开放式微结构光纤不仅保持了可通过灵活的几何结构设计,从而获得优化光学传感性能的优点,利用其开放式无需预填充的特点,还可较大幅度地提高传感实时工作效率。本文对近年来出现的几种具有代表性的开放式微结构光纤进行了讨论。比较了利用开放全内反射型、WagonV％甜型及嵌入导流孔型开放式微结构光纤作为实时气液传感器时各自的优缺点。研究表明,这几种开放式微结构光纤都在气液实时传感领域显示了较为独特的优点。借助于开放式微结构光纤,可为未来高精度实时光纤传感器领域的设计提供一种较有竞争力的技术支撑。     

Bending effects in optical fibers

Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed.