Thermal effects of Brillouin gain spectra in single-mode fibers

Brillouin gain spectra in two 250-m-long single-mode fibers with GeO₂-doped core/pure-silica cladding (fiber A) and pure-silica core/F-doped cladding (fiber B) were measured at temperatures ranging from -40 to +60°C at a wavelength of 1.32 μm. The temperature coefficients of Brillouin frequency shift were found to be 1.17 and 1.33 MHz/°C for fibers A and B, respectively. Temperature coefficients of Brillouin gain bandwidth were found to be -0.12 and -0.10 MHz/°C. These measurements provide useful information for applications of stimulated Brillouin scattering     

Characteristics of a doubly clad optical fiber with a low-index inner cladding

This paper investigates a novel type of optical fiber which is composed of three portions: the core, inner cladding, and outer cladding. It is assumed that the core has the largest refractive index of the three, and the outer cladding index is the next largest. When the index difference between the core and the inner cladding is much larger than that between the outer cladding and the inner, the new fiber has a stronger confinement property than a usual singly clad fiber in the single-mode region. It is also assumed that the inner-cladding thickness is larger than or comparable with the core radius. This confinement is the origin of the following three properties of potential importance in single-mode optical communication. 1) As compared with a singly clad fiber, the largest core area for single-mode operation is roughly twice. 2) The group delay arising from waveguide characteristics has an opposite sign against that of typical glass dispersion. (Singly clad fiber has a dispersion of the same sign.) 3) The field is much more tightly confined within the core as compared with a singly clad fiber. This minimizes extra attenuation due to absorption in the cladding.     

Perturbation theory of a doubly clad optical fiber with a low-index inner cladding

A doubly clad optical fiber which is composed of the core, inner cladding, and outer cladding is called aW-type fiber when the core has the largest index of refraction of the three and the inner cladding has the lowest. AW-type fiber is known to have several advantages over a conventional singly clad (SC) fiber. This paper presents simple, closed form approximations of aW-type fiber with respect to cutoff, the attenuation constant in the leaky wave region and a rapid change of the group velocity near cutoff, etc. In the approach to be described, aW-type fiber is thought of as a combined system of an SC fiber and an outer perturbing medium.     

Propagation in Doubly Clad Single-Mode Fibers

General propagation properties and universal curves are given for doubly clad single-mode fibers with inner cladding index higher or lower than outer cladding index, using the two parameters inner cladding/core radii ratio and inner cladding/core index differences ratio. LP01, LP11, and LP02 cutoff conditions are examined. It is shown that dispersion properties largely differ from the singly clad single-mode fiber case, leading to large new possibilities for low-loss dispersion-free fibers at any wavelength between 1.3 and 1.7 µm.     

双包层手征光纤的色散特性

在对双包层手征光纤(W型)进行解析求解的基础上，用数值计算方法研究了纤芯和内外包层的手征参数和内包层厚度对模式色散特性的影响，给出了双包层手征光纤中基模的归一化传播常数b、群延迟d(Vb)／dV和波导色散Vd2(V6)／dV2随纤芯和内外包层的手征参数和内包层厚度的变化关系曲线。结果表明在双包层光纤的纤芯和内外包层中引入手征，可以极大地改变光纤中基模的色散特性，特别是内包层厚度不同时，色散特性也极为不同．     

Rayleigh scattering reduction method for silica-based optical fiber

The effect of the thermal treatment of silica-based glasses and glass fibers on their Rayleigh scattering is investigated experimentally. The Rayleigh scattering coefficients of bulk glasses are found to be increased 5-10% by heating them to 1800 °C because the density fluctuation is in proportion to their fictive temperature. Based on these results, we propose a method for reducing the Rayleigh scattering losses of silica-based optical fibers by drawing them slowly at low temperatures. We used this method to obtain a GeO₂ doped silica core single-mode fiber with a minimum loss of 0.16 dB/km at 1.55 μm. As a result, we confirmed that the reduction in the fictive temperature of silica-based glasses and glass fibers reduces their Rayleigh scattering     

Achievement of uniform attenuation from 1.25 to 1.6 μm using anew three-layered Co2+ doped optical fiber

We report a new high attenuation fiber, whose cross section is a three-layered structure-the cobalt-doped core, the depressed inner cladding, and the outer cladding doped with cobalt in a ring. The proposed doping profile of cobalt ions both in the core and the cladding effectively attenuated the cladding modes to result in reduction of the modal interference. Enhancement of attenuation uniformity over a factor of three compared with prior techniques was achieved with the attenuation deviation less than 0.03 dB from 1250 to 1600 nm     

高双折射光子晶体光纤特性分析

建立了基于透明边界条件(TBC)的全矢量迦辽金有限元法(FEM)分析二维光子晶体光纤(PCF)的模型,并对椭圆芯等5种高双折射光子晶体光纤基模的模式双折射、限制损耗及色散特性进行了数值分析和比较.通过减小内包层中沿x方向的空气孔,增大沿y方向的空气孔构成的一种光子晶体光纤的模式双折射在波长1550 nm处高达5.96×10-3,而椭圆芯光子晶体光纤为1.52×10-3.研究表明,可通过增加内包层中两个正交方向上空气孔的尺寸差来获得高双折射;同时还得出内包层中放大的空气孔减小限制损耗,增加色散,而减小空气孔尺寸带来的影响则刚好相反;内包层上空气孔数量越少,色散越平坦.     

Residual stresses in a doubly clad fiber with depressed innercladding (DIC)

Thermal and mechanical stresses developed in concentric three-layered optical fiber-core, and inner and outer cladding, have been thoroughly studied for various concentrations of dopants and geometric structures. In order to examine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoretical calculations. The thermal stresses were calculated for three temperature ranges in which the glass in each layer has a different thermal expansion coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In order to include the time dependence of the stress below softening point, Maxwell's one dimensional viscoelasticity was applied. In a parametric study, the analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index difference. With an increase of core index above the silica, the thermal stresses in core increased linearly, but the depressed inner clad does not affect the stresses in core. From the parametric studies and modeling it was found that when the depressed inner cladding (DIC) layer has a large cross-section or high dopant concentration, the mechanical stress in core change from compression to tension     

A novel strain-induced thermally tuned long-period fiber gratingfabricated on a periodic corrugated silicon fixture

We demonstrate a new method for fabricating tunable long-period fiber grating by bonding a cladding etched fiber to a corrugated silicon fixture. The long-period grating is formed by a built-in periodic sheer strain on the fiber/fixture interface. Loss tuning is achieved by varying the temperature of the fixture. Dynamic tuning range as large as 20 dB is obtained at the resonance wavelength over a temperature range from 25°C to 245°C     

Fiber Lasers:Emerging in Major Markets

Since its inception,the fiber laser has attracted Since its inception, the fiber laser has attracted users because of its large gain and the ability to produce continuous lasing. The modern fiber laser is pumped by high-power multimode single-emitter diodes or diode bars, typically through a cladding surrounding a single-mode core.This single-mode core is typically 5～12 μm in diameter. The double-clad fiber consists of an inner singlemode core doped with the appropriate rare-earth ions such as neodymium, erbium, ytterbium and thulium.The cladding is made of undoped glass that has a lower index of refraction. The pump light is injected into the cladding and then propagates along the structure, passing through the active core and producing a population inversion.     

Optical spot-size converters for low-loss coupling between fibersand optoelectronic semiconductor devices

Structures of spot size converters that allow low loss and easy coupling between an optical semiconductor device and a fiber are proposed and designed theoretically. These spot-size converters have a tapered small core for expanding the mode field. They also have a double cladding region which consists of an n⁺-doped InP substrate as the outer cladding and a p-doped or nondoped InP layer as the inner cladding with a ridge structure. This double cladding utilizes the plasma effect of a carrier which makes the refractive index of highly doped n-InP lower than that of p-doped or nondoped InP. The double-cladding structure can tightly confine an expanded mode field in the inner cladding, and results in low radiation loss at the tapered waveguide, in addition, this structure reforms the mode field shape into a Gaussian-like shape and achieves a low loss coupling of less than 1 dB with a large misalignment tolerance for fiber coupling. These spot-size converters are easily fabricated and applicable to all types of optical semiconductor devices     

太赫兹波段反谐振光纤偏振分束器设计

偏振分束器是在光传感和集成光学等领域中非常重要的光学元器件,太赫兹波段被认为是未来大容量无线通信的载体.而双芯空芯反谐振光纤结构设计更加多变,对包层结构没有严格要求,能实现较高的性能以满足人们更多样化的需求,以双芯空芯反谐振光纤为基础设计适用于太赫兹波段的偏振分束器越来越值得深入研究.提出一种以环烯烃聚合物为基底材料的...     

双包层手征光纤中的半漏模

研究了纤芯和内外外层都有手征的双包层手征光纤中的半漏模，给出了单包层和双包层手征光纤中几个低阶模的有效折射率和功率损耗系数随手征参数的变化关系曲线。结果表明，双包层手征光纤中的半漏模特性与半单包层手征光纤中的相比有很大的不同；当只有包层中有手征时，单包层手征光纤中也存在半漏模。     

Analysis of Brillouin frequency shift and longitudinal acoustic wave in a silica optical fiber with a triple-layered structure

We report a thorough analysis on the Brillouin frequency shift as a function of geometrical parameters in a silica optical fiber consisting of triple-layered structure, GeO/sub 2/-doped core, P/sub 2/O/sub 5/, and F co-doped inner cladding, and pure silica outer cladding. General characteristic equations for the Brillouin frequency shift were analytically derived and analyzed for various fiber parameters. In experiments, three-layered optical fibers were fabricated and their Brillouin frequency shifts were measured in the wavelength region of 1.55 /spl mu/m by a pump-probe technique. The longitudinal acoustic velocity in each layer was found significantly affected by the thermal stress as well as the dopant concentrations. We confirmed both in theory and experiment that the inner cladding of a three-layered optical fiber does provide a new degree of freedom in precise control of the Brillouin frequency shift.     

Propagation of lower-order modes in a radially anisotropiccylindrical waveguide with liquid crystal cladding

The propagation behavior of the four lower-order modes, HE₁₁ , TE₀₁, TM₀₁, and HE₂₁, in a radially anisotropic cylindrical waveguide with liquid crystal cladding is studied both theoretically and experimentally. The cylindrical waveguide is a doubly-clad fiber with an isotropic core and inner cladding and a radially anisotropic outer cladding made of nematic liquid crystal. Theoretically, the propagation and decay constants for the TE₀₁ and TM₀₁ modes are obtained by solving the wave equations exactly, while those for the HE₁₁ and HE ₂₁ modes are derived using perturbation techniques under the weakly guiding approximation. It is predicted that in such a structure the guided TE₀₁ mode can be separated from the leaky HE₁₁, TM₀₁, and HE₂₁ modes. The theoretical results show good agreement with the experimental observations for a 3 cm long fiber cell with a 5 μm inner cladding radius     

Double Clad Er-doped Fiber Amplifier

Presented is a theoretical study of double-clad Er-doped fiber power amplifier（EDFA）. Two kinds of double clad fibers（DCF） with rectangular and ＂flower＂ inner clad shapes are studied, and these fibers have different coupling constants and propagation losses. We calculate the effective pump power absorption ratio along the fiber with different coupling constants from the first cladding to the doped core and with different propagation losses for the power in the inner cladding. Then the gains of the double clad Er-doped fiber amplifiers versus fiber lengths are calculated using the EDFA model based on propagation and rate equations of a homogeneous, two-level medium.     

掺镱光纤激光器粒子数空间分布研究

针对基于重叠因子模型的速率方程不能分析粒子数空间分布的情况,依据双包层掺Yb3＋光纤激光器中,内包层光强近似于均匀分布、而纤芯中光场近似于高斯分布的特点,建立了基于光强分布的速率方程。依据该方程,分析了正向、反向和双向泵浦方式下,Yb3＋上能级粒子数的空间分布。结果表明：沿光纤轴向,不同的泵浦方式导致上能级粒子数呈现不同的分布特征;而沿光纤径向,无论采用何种泵浦方式,上能级粒子数都呈现中间低、两端高的抛物线分布结构。     

Cladding-mode resonances in hybrid polymer-silica microstructuredoptical fiber gratings

We examine the cladding-mode resonances of fiber gratings UV written into the core of a novel microstructured fiber whose air regions are subsequently infused with polymer. The cladding spectrum changes with temperature because of the strong temperature dependence of the polymer refractive index and we show that these changes can be understood qualitatively using a simple model of the polymer-silica waveguide. Our results imply wavelength and amplitude tuning regimes for long period gratings written into this fiber with tunability enhanced over conventional long period gratings because of the small effective inner cladding diameter of our fiber     

Dispersion minimisation in single-mode fibres over a wide spectral range

The waveguide and material dispersion are investigated for various silica-based single-mode optical fibres with arbitrary refractive-index profiles. It is shown that in the case of the doubly clad optical fibre with an index difference between the core and the outer cladding ?1=1% and that between the inner cladding and the outer ?2=?1%, the diameter of the core 2a=7.2 ?m, and the thickness of the inner cladding t=0.3a, the total dispersion is reduced within ± 1 ps/km/nm over a wide spectral range of ?=1.35?1.67 ?m.