Simple model for frozen-in viscoelastic stress in optical fibers

We have measured residual stress in a commercial single mode optical fiber and observed its relaxation in an annealing process with a polariscopic stress measurement method. After annealing the fiber for an hour at various high temperatures in a specially made image furnace we have measured the temperature dependent relaxation of frozen-in viscoelastic stress in the fiber. We have proposed a simple physical model to explain recently observed relaxation of frozen-in viscoelastic stress in optical fibers based on the Kelvin-Voigt model of viscoelastic materials. The experimental results are explained with our model of frozen-in viscoelastic stress by introducing simplified four-step procedures for fiber drawing.     

拉丝张力对光纤性能的影响

分析拉丝张力对光纤轴向应力、拉丝诱导缺陷、光损耗、强度、折射率分布、截止波长、瑞利散射系数等性能的影响。指出拉制光纤必须选择合理的拉丝张力，以确保光纤质量。     

Drawing parameters optimization for birefringence reduction in optical fibers

The impact of the drawing process on the stress-induced birefringence in optical fiber is experimentally analyzed. By means of the photoelastic tomographic technique, residual internal stresses are measured in fiber samples drawn under different conditions of temperature, drawing tension and speed. From measured residual stress distributions, the stress-induced birefringence and beatlength are mathematically derived and compared, proving that an effective birefringence reduction can be obtained by a proper choice of drawing parameters.     

CO(2) laser writing of long-period fiber gratings in optical fibers under tension

We demonstrate experimentally that the efficiency of writing a long-period fiber grating in a single-mode fiber by CO(2) laser pulses increases significantly with the axial stress applied along the fiber. We attribute the enhancement in writing efficiency to the generation of nonuniform inelastic frozen-in strains across the fiber under tension by CO(2) laser heating. Controlling the axial stress distribution along a fiber during the CO(2) laser writing process thus provides an additional degree of freedom for control of the grating characteristics.     

光纤折射率的依赖因素

叙述光纤折射率与组成、温度、波长、模场直径和拉丝张力（拉丝温度）以及衰减等因素的相互依赖关系。指出拉制光纤时要严格控制拉丝张力,以防止因拉丝张力不当而引起的光纤折射率变化和衰减增加。     

Residual stress relaxation in the core of optical fiber by CO(2) laser irradiation

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.     

光纤耦合LD输出光场特性研究

根据光纤传输的模式理论,分析了单芯光纤输出端面的光场分布.利用单模光纤测量了纤端光场的强度分布,拟合了光纤输出端面光强分布的近似表达式.根据此数值计算和实验测量了多芯光纤输出端面的强度分布,结果基本吻合.表明光纤出射端面的光强分布应是介于高斯分布和几何分布之间的一种准高斯分布.     

Viscoelasticity and plasticity mechanisms of human dentin

Theoretical models of viscoelastic behavior and plastic deformation mechanisms of human dentin are considered. Using the linear viscoelasticity theory in which creep and relaxation kernels have the form of fraction-exponential functions, numerical values of instantaneous and long-time Young’s moduli and other characteristics of dentin viscoelasticity under uniaxial compression are found. As dentin plastic deformation mechanisms, mutual collagen fiber sliding in the region of contact of their side surfaces, separation of these fibers from each other, and irreversible tension of some collagen fibers, are proposed. It is shown that the second mechanism activation requires a smaller stress than that for activating others. The models of plastic zones at the mode I crack tip, which correspond to these mechanisms, are studied. It is shown that the plastic zone size can increase from a few hundreds of nanometers to hundreds of micrometers with increasing applied stress.     

Design of double-coated optical fibers to minimize long-term axial-strain-induced microbending losses

The design of double-coated optical fibers to minimize long-term axial-strain-induced microbending losses is investigated. The microbending loss is dominated by the compressive radial stress at the interface between the glass fiber and primary coating. To know the long-term axial-strain-induced microbending losses in double-coated optical fibers, the stresses in fibers are analyzed by the viscoelastic theory. To minimize these long-term microbending losses, the thickness and Young's modulus of the secondary coating should be decreased if the strength of coating is satisfied. Meanwhile, the Poisson's ratio of the primary coating should be increased, but the Young's modulus of the primary coating and relaxation time of the secondary coating should be decreased. Alternatively, the radius and relaxation time of the primary coating exist their optimum values. The long-term axial-strain-induced microbending losses in single-coated optical fibers are also discussed.     

Observation and analysis of residual stress development resulting from OH impurity in optical fibers

We report experimental results on the development of residual stress due to OH impurity in optical fibers. The effect of OH impurity on residual stress is demonstrated by direct residual stress measurement. The residual stress at the outer-cladding/jacketing-tube boundary of the fiber drawn at 3.48 N was found to be -61 MPa . The residual compressive stress is attributed to the viscosity decrease induced by a significant OH impurity at the boundary, as measured by a Fourier transform infrared microscope.     

Effect of asymmetric stress relaxation on the polarization-dependent transmission characteristics of a CO2 laser-written long-period fiber grating

The effect of stress-induced birefringence on polarization-dependent transmission characteristics was thoroughly investigated for a long-period fiber grating (LPFG) that was fabricated by use of a stress relaxation method with a CO2 laser. A series of two-dimensional axial stress profiles for one complete period of a LPFG was measured with an optical tomographic measurement technique. We found that the asymmetry in the stress distribution of the cladding was much larger than in the core of the LPFG. The splitting of polarization-dependent loss peaks in the optical transmission spectrum was calculated based on the measured asymmetric stress profiles and was compared with an experimental result.     

Characterization of fiber Bragg grating for maximum reflectivity based on modulation depth of refractive index

This study proposed the modeling of fiber Bragg grating as a sensor for the optimum reflectivity with minimum discontinuities at the end of center wavelength. Recent advancement in optical sensor technology, make it necessary to generate the optimum results with minimum error. In this analysis, dependence of grating output spectra on various parameters has been studied for minimum bandwidth, side lobes and maximum reflectivity. In previous studies effect of length and other parameters has been studied. In this paper author has included the effect of modulation depth also. Study shows that grating length and modulation depth of refractive index of core are very critical factors which significantly alter the grating characteristics. Here low absorption third window of optical communication is used for the sake of low attenuation. All the simulation has done using GratingMOd software using transfer matrix method.     

Strain-insensitive and high-temperature long-period gratings inscribed in photonic crystal fiber

We fabricate and demonstrate strain-insensitive and high-temperature long-period gratings in endlessly single-mode photonic crystal fiber by use of focused pulses of a CO2 laser and a periodic stress relaxation technique without geometrical deformation and elongation of the fiber. The thermal dependence of mode coupling at 1299.59 nm is 10.9 pm/degrees C from 24 to 992 degrees C, whereas the coefficient of strain sensitivity is -0.192 pm/muepsilon up to the maximum strain of 2.74%epsilon. It is found for what is believed to be the first time that, in contrast with the traditional fiber case, the coupling resonance shifts toward shorter wavelengths under applied strain, indicating that the refractive index of the core is decreased as a result of the rebuilding of tension attributed to the stress-elastic effect, and the cladding modes is highly dispersive because of airholes arranged in the fiber cladding.     

一种胶体光子晶体修饰的光纤

设计了一种胶体光子晶体修饰的光纤. 采用恒温快速蒸发法直接在经切割刀处理后的光纤端面生长胶体晶体,再与另一根切割后的光纤在毛细玻璃管中完成对接,制备成胶体光子晶体修饰的光纤. 用扫描电子显微镜和光谱分析仪对样品的形貌、结构以及光学特性进行分析. 实验结果表明,粒径为640 nm、体积分数约为0.5%的SiO₂胶体微球溶液在60 ℃的情况下沉积,大约12 h后可得到质量较高的胶体光子晶体. 在SEM下,观察到端面的胶体晶体为面心立方(fcc)结构. 透射光谱证明,该结构在(111)面上     

Initial stage of stress relaxation in oriented polymers

The initial stage of stress relaxation, i.e., a decrease of the stress with time in an extended solid at a fixed length of the specimen, has been studied in oriented fibers of a linear polymer, polypropylene, chosen as the object of investigation. Experimental data have been obtained, and the stress in the object from the beginning of tensile loading of the specimen has been analyzed. It has been found that the stress relaxation caused by thermal fluctuation conformational transitions in stretched chain polymer molecules occurs from the beginning of the stretching of the polymer fiber. The time dependence of the stress relaxation of polypropylene fibers in all initial stages of deformation of the fibers has been elucidated.     

Nonlinear viscoelastic dynamics of nanoconfined wetting liquids

The viscoelastic dynamics of nanoconfined wetting liquids is studied by means of atomic force microscopy. We observe a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids. We show that the origin of the measured nonlinear viscoelasticity in nanoconfined water and silicon oil is a strain rate dependent relaxation time and slow dynamics. By measuring the viscoelastic modulus at different frequencies and strains, we find that the intrinsic relaxation time of nanoconfined water is in the range 0.1-0.0001 s, orders of magnitude longer than that of bulk water, and comparable to the dielectric relaxation time measured in supercooled water at 170-210 K.     

光纤涂料对光纤衰减的影响

分析光纤涂料的厚度、应力松驰、热处理及辐射等对光纤衰减的影响。提出应根据光纤技术要求选择既符合工艺条件又与光纤材料相匹配的涂料,并严格控制涂料厚度和同心度,以保证光纤质量。     

Frequency resolved analysis of thermally induced refractive index changes in fiber amplifiers

Temperature induced refractive index changes are an important aspect in today's fiber amplifiers with high average power. For many processes, their time dependence is critical. Here, we analyze the impact of radial heat diffusion on the optical phase. We modulated the pump power in a 10?W amplifier and measured the frequency response of the optical phase. We compared the result with the calculated frequency response of the temperature in the fiber core, which shows the same characteristics. Additionally, we analyzed the influence of fiber parameters on the temperature dynamics.     

Synthesis of MFI zeolite films on optical fibers for detection of chemical vapors

We report the development of a novel zeolite-incorporated optical fiber sensor and demonstrate its capability for in situ detection of chemical vapors. The sensor comprises a polycrystalline silicalite thin film grown upon the cleaved end face of a standard single-mode optical fiber. The sensor device operates by measuring the optical reflectivity of the zeolite crystals, which changes reversibly in response to the amount of chemical vapor adsorbed in its crystalline microporous structure. The sensor has been successfully demonstrated for measuring the concentration of isopropanol vapor in mixtures with nitrogen gas.