Increase of photosensitivity in Ge-doped fibers under strain

Straining a Ge-doped optical fiber increases its UV photosensitivity. The speed of grating fabrication in a standard AT&T telecommunication fiber increases by 18 times, while the final refractive-index change is enhanced by a factor of 5. Measurements reveal opposite changes in the stress of the fiber core induced by UV light in strained and unstrained fibers.     

Bragg grating fabrication in germanosilicate fibers by use of near-UV light: a new pathway for refractive-index changes

Using 334-nm light, we demonstrate side writing of Bragg gratings with an index change of ~10(-4) in germanium-doped fibers. No hydrogen loading of the fibers was required. These gratings have the same temperature stability as gratings fabricated with 240-nm light. Our results suggest that photoionization is not needed for formation of gratings in Ge-doped glass. We observe an enhancement of the 334-nm photosensitivity in boron-codoped fibers and suggest that B facilitates a structural transformation of the glass.     

Photoinscription of Bragg gratings within a germanosilicate fiber subjected to a high static electric field

The kinetics of Bragg grating growth in germanosilicate fibers subjected to a high static electric field are compared with those obtained without any electric field. The gratings were written by exposure of the fiber core to laser light at 244 or 193 nm. These experiments gave some clues about the mechanisms responsible for both the photosensitivity in germanosilicate fibers and the nonlinear second-order UV-induced susceptibility in silica glasses. The refractive-index modulation proved to be significantly higher in the fibers subjected to an electric field. Furthermore, the change in the fiber's mean effective refractive index as a function of exposure time was not monotonic. This evolution can be explained by the assumption that some electric-field-induced diffusion of electron trapped centers [Ge(1) and Ge(2)] from the fiber core is involved.     

Higher-Order Structure of Poly(Ethylene 2,6-Naphthalene Dicarboxylate) Fibers Produced by Direct High-Speed Spin-Drawing

In order to efficiently produce poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) fibers comprising α-phase crystallites with a high melting temperature, a direct high-speed spin-drawing (HSSD) system was implemented. The higher-order structure of the HSSD PEN fibers was investigated by focusing on the radial anisotropy of the fibers. A nucleating agent was added to the PEN fibers, and its effect on the formation of the higher-order structure was also studied. Highly crystalline PEN fibers predominantly composed of α-phase crystallites were produced by operating the HSSD system at 4?km/min. The PEN fibers exhibited a melting temperature of 288.8?°C and a storage modulus of 31.4?GPa. The excellent thermal stability of the HSSD PEN fibers was presumably brought about by the formation of a shish-kebab-like structure. The generation of the shish-kebab-like structure and radial anisotropy seemed to be more dependent on the effect of tensile stress during the drawing process rather than from the nucleating additive. The nucleating agent used in this study effectively increased both the α- and β-phase crystallite sizes. Such structural modification, however, did not appear to contribute to the thermal stability enhancement of the PEN fibers.     

评估环圈光纤机械可靠性的模型

为评估应用于光纤陀螺和光纤电流传感器等传感领域的光纤环圈的机械可靠性,排除光纤涂覆和环圈灌胶等因素,必须评估所用光纤在弯曲状态下的机械可靠性.在已被广泛接受的均匀拉伸应力状态下通信光纤的机械可靠性模型基础上,根据这种环圈光纤在弯曲状态下的一般应力分布,对于绕环张力和弯曲应力同时存在的情形,建立了评估这种环圈光纤的机械可靠性的一种模型.该模型通过采用弯曲状态下光纤表面的最大弯曲应力值,和由于一般情况下弯曲中轴内侧的压缩应力远小于外侧拉伸应力的这一事实而忽略内侧的压缩应力,从而简化了弯曲应力的不均匀分布;同时通过该模型中所含的多个参量的选择给出了最为保守的结果.利用该模型进行的数值计算预测了采用不同直径的光纤分别绕制的不同环圈直径的环圈光纤在不同服役条件下的机械可靠性.结果表明,除了光纤半径和疲劳因子等这些光纤自身的固有因素外,绕环张力、环圈半径和筛选张力等工艺参量对环圈光纤的机械可靠性也是有重要影响.     

评估环圈光纤机械可靠性的模型

为评估应用于光纤陀螺和光纤电流传感器等传感领域的光纤环圈的机械可靠性,排除光纤涂覆和环圈灌胶等因素,必须评估所用光纤在弯曲状态下的机械可靠性.在已被广泛接受的均匀拉伸应力状态下通信光纤的机械可靠性模型基础上,根据这种环圈光纤在弯曲状态下的一般应力分布,对于绕环张力和弯曲应力同时存在的情形,建立了评估这种环圈光纤的机械可靠性的一种模型.该模型通过采用弯曲状态下光纤表面的最大弯曲应力值,和由于一般情况下弯曲中轴内侧的压缩应力远小于外侧拉伸应力的这一事实而忽略内侧的压缩应力,从而简化了弯曲应力的不均匀分布;同时通过该模型中所含的多个参量的选择给出了最为保守的结果.利用该模型进行的数值计算预测了采用不同直径的光纤分别绕制的不同环圈直径的环圈光纤在不同服役条件下的机械可靠性.结果表明,除了光纤半径和疲劳因子等这些光纤自身的固有因素外,绕环张力、环圈半径和筛选张力等工艺参量对环圈光纤的机械可靠性也是有重要影响.     

Solar hypersensitization of optical fibers

Hydrogen-loaded fibers were exposed to solar radiation for 20 days, in which time the hydrogen was allowed to out-diffuse. Gratings written in these fibers show an increased photosensitivity compared with pristine fibers. Results show the solar radiation has a similar effect on both stripped and unstripped fibers. This work agrees with the fundamental process of using low fluence, long UV sources to photosensitize hydrogen-loaded, germanium-doped fibers.     

Grating formation in pure silica-core fibers

Strong grating formation in pure silica-core fibers by use of 193-nm ArF-laser radiation is reported. Unsaturated refractive-index changes of Dn~0.3x10(-3) were observed in nontreated fiber, and changes of Dn~0.5x10(-3) were observed in fibers with a high hydroxyl concentration. Possible mechanisms of photosensitivity in pure silica-core fibers are discussed.     

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.     

Trans-4-stilbenemethanol-doped photosensitive polymer fibers and gratings

We report the development of photosensitive polymeric optical fibers for gratings. The fiber is made of a copolymer of methyl methacrylate and butyl methacrylate, and its core is made of a copolymer of methyl methacrylate, ethyl methacrylate, and benzyl methacrylate (90/4/6 molar ratio) doped with trans-4-stilbenemethanol. Ultraviolet absorption and photoinduced refractive-index change of the doped core system are presented, and its photosensitivity is demonstrated by fabrication of Bragg gratings in the core of the fiber by use of a pulse laser at 325 nm.     

Photosensitivity of As2S3 chalcogenide thin films at 1.5 microm

Chalcogenide glasses are promising candidates for all-optical switching and various nonlinear applications. However, we show that As2S3 thin films are photosensitive at wavelengths in the 1.5-microm telecommunication window. This sensitivity is evidenced by the formation of self-written waveguides in slabs, where channels as narrow as 1 microm are created. We also show the detrimental effects of such photosensitivity in ridge waveguides. This photosensitivity seems to occur only in thin-film form and not in bulk samples or fibers.     

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.     

熊猫光纤的结构与双折射特性关系研究

本文用微元算法研究了熊猫光纤的结构参量对熊猫光纤双折射特性(拍长)的影响.由实验数据拟合出了计算拍长的熊猫光纤掺杂区的有效热膨胀系数.计算结果表明,熊猫光纤的猫眼半径、猫眼与纤芯的距离、猫眼的不一致等都是影响拍长的因素.所得结果对熊猫光纤的制作提供了理论指导.     

Large increase in photosensitivity through massive hydroxyl formation

We report a large increase in photosensitivity of germanium-doped silicate fibers by rapid heat treatment of hydrogen-loaded fibers at 1000 degrees C before exposure of the fibers to 242-nm radiation. The increase in photosensitivity is compared with thermally induced absorption caused by introduction of massive amounts of hydroxyl species. The absorption loss was measured to be 0.02 dB/cm mol.% OH at 1.55 mum. Strong gratings (Dn > 1 x 10(-4)) in germanium-free phosphorous-doped fibers in the presence of 242-nm radiation have also been manufactured by this technique.     

A novel measurement method for the photosensitivity of fibers

An online measurement method for the photosensitivity of fibers is proposed. This method is simple and useful. Theoretical and experimental studies have been carried out. This method is based on the Mach-Zehnder interferometer (MZI). One arm of the MZI is externally exposed to UV light at 248 nm originating from a KrF excimer laser. Then the output optical spectrum is measured, by some relevant analysis, and the photosensitivity of the fiber can be obtained. The relationship between the refractive index increment of fibers and the exposure time is in accordance with the absorbing model that is brought forward by H. Patrick, which shows that this method has high precision.     

Radiation sensitivity of thick core all-glass fibers in reactor radiation fields

Radiation induced losses and the recovery behavior of several optical fibers have been determined by in situ steady state and by pulse measurements in a nuclear reactor. Agreement with the literature was found for Suprasil and Fibropsil core fibers in ionizing radiation, but our results on Suprasil-W core fibers show an appreciably higher radiation hardness of this material than reported elsewhere. Additionally, increased long term losses were observed, when the mixed radiation flux contained a noticeable content of neutrons.     

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's.     

Polymer Optical Fiber Photosensitivities and Highly Tunable Fiber Gratings

In this paper, recent progress in the investigation of photosensitivities of polymer optical fibers and the development of polymer optical fiber grating is reported. Photosensitivities in various polymer fibers, including doped and undoped, multimode and single-mode polymer fibers, have been experimentally characterized and evaluated for fiber grating application. In particular, the wavelength dependence of material absorption and photosensitivity has been found essential to the fabrication of polymer optical fiber grating. Based on the results of photosensitivity research, polymer optical fiber gratings have been successfully fabricated. More importantly, polymer optical fiber grating has been demonstrated highly tunable, with a tuning range more than 70 nm. The unique feature of high tunability in polymer optical fiber grating has great potential in various applications including optical fiber WDM systems and fiber sensor systems. Several important issues that remain to be investigated will also be discussed.     

Polymer Optical Fiber Photosensitivities and Highly Tunable Fiber Gratings

In this paper, recent progress in the investigation of photosensitivities of polymer optical fibers and the development of polymer optical fiber grating is reported. Photosensitivities in various polymer fibers, including doped and undoped, multimode and single-mode polymer fibers, have been experimentally characterized and evaluated for fiber grating application. In particular, the wavelength dependence of material absorption and photosensitivity has been found essential to the fabrication of polymer optical fiber grating. Based on the results of photosensitivity research, polymer optical fiber gratings have been successfully fabricated. More importantly, polymer optical fiber grating has been demonstrated highly tunable, with a tuning range more than 70 nm. The unique feature of high tunability in polymer optical fiber grating has great potential in various applications including optical fiber WDM systems and fiber sensor systems. Several important issues that remain to be investigated will also be discussed.