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.     

双包层和三包层椭圆光纤保偏特性比较

本文利用高斯近似法分析了三包层椭圆光纤的保偏特性，用曲线图的方式给出了保偏特性随长短轴比值、包层与纤芯长轴比值和折射率差变化的趋势，并且与双包层的保偏特性进行了比较     

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。     

Parametric study of depressed inner cladding single-mode fibers

A parametric study on the depressed inner cladding (DIC) single-mode fibers shows the influence of the inner-cladding diameter on the effective cutoff wavelengths of the first two modes, determining a practical transmission window. The set of curves that is obtained can be considered universal, at least for a value of the inner-cladding/core index differences ratio close to 0.5. The behavior of the effective cutoff wavelengths is explained by studying the effective index of the modes and the fundamental mode diameter, this explanation being confirmed by a bending and microbending sensitivity test. It appears that DIC fibers optimized for long-range transmission should have an inner cladding to core diameter ratio greater than 3 and a relative depth of the inner cladding smaller than 0.5.     

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.     

双包层单模光纤光学参数对传输特性的影响

对特征方程直接求导解得了基模群时延和群速色散,讨论了双包层单模光纤不同折射率分布对传播常数、群时延、群速色散以及零色散点波长的影响.研究结果表明:光学参数,即纤芯和外包层的相对折射率(P)以及内包层和外包层的相对折射率(R)对该类光纤的传播常数、群速度、群速色散和零色散点波长均有比较明显的影响,影响的大小及规律随参数的不同又有所不同.这些规律为设计满足不同色散要求的双包层单模光纤提供了理论依据.     

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.     

Design Consideration on Broad-Band W-Type Two-Mode Optical Fibers

Structural design for broad-band W-type two-mode optical fibers is investigated. The optimum parameters are numerically determined as follows: the operating V-value with zero group delay time difference Delta lambda between the LP/sub 01/ and LP/sub 11/ modes is 6.7, the ratio of core radius to inner cladding radius is 0.6, and the index profile parameter is 2.02. then, the core radius is 12.3 µm for Delta=0.3 percent at the operating wavelength of 1.3 µm. The V-value deviation tolerance from the optimum to maintain Delta lambda less than +-20 ps/km is 21 percent, which is 20 times larger than that of the earlier design made on two-layer index profile.     

Narrow-depressed cladding fiber design for minimization of claddingmode losses in azimuthally asymmetric fiber Bragg gratings

The cladding and radiation mode loss characteristics of photo-induced gratings written in fibers with a narrow-depressed inner cladding are compared to the properties of gratings written in matched clad, photosensitive clad, and wide-depressed cladding fibers under varying grating azimuthal asymmetry conditions. Fibers with narrow-depressed claddings are found to have lower cladding mode losses for small degrees of asymmetry than previously proposed fiber designs for cladding-mode reduction     

双包层椭圆光纤的保偏特性

本文对双包层椭圆光纤进行了解析求解，得到了模式特征方程。研究了基模的保偏特性，给出了不同椭圆比下的归一化双折射和模间色散随归一化频率，纤芯内包层相对折射率差，内包层和纤芯半长轴比等参数的变化关系曲线。     

Depressed index substrate tubes to eliminate leaky-mode losses in single-mode fibers

Silica tubing to be used as substrate tubes for MCVD has was demonstrated for a fiber having only one-fourth the deposited clad-merical tubing in that it is both dehydrated and fluorinated during processing to yield a silica which is dry and whose refractive index is lower than that of silica. These tubes are advantageously used to prepare single-mode fibers having fluorinated-silica cladding which controls dispersion and reduces scattering losses. In this case, the refractive index of the deposited cladding can be matched to that of the outer tube thus avoiding leaky-mode losses commonly encountered in depressed-index clad fibers. Using such an approach, low loss (0.28 dB/km at 1.55 μm) was demonstrated for a fiber having only one-fourth the deposited cladding required when a commercial silica tube is used. Such tubes could provide ecomomies to the processing of and the performance of complex cladding single-mode structures and impact multimode fiber designs.     

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．     

Simple approximations for chromatic dispersion in single- mode fibers with various index profiles

Simple approximate formulas of chromatic dispersion and zero-chromatic dispersion wavelength are empirically derived for single-mode fibers with various index profiles having the deformations such as a central dip, hump, and/or tailing at a core cladding interface. These profiles are typical which are commonly obtained in current single-mode fibers. The approximation is based on the exact solution of vector-wave equation. These accuracies are better than 0.3 and 0.03 percent for the chromatic dispersion and zero-chromatic dispersion wavelength, respectively. With the aid of the simple approximations, the properties of chromaric dispersion and zero-chromatic dispersion wavelength for practical single-mode fibers are discussed. The relation between the chromatic dispersion and basic fiber parameters of single-mode fibers, effective cutoff wavelength of the LP11mode, and the spot-size of the LP01mode, are clarified.     

The performance of depressed-cladding single-mode fibers withdifferent b/a ratios

Depressed-cladding single-mode fibers with a first cladding-to-core ratio (b/a) ranging from seven to one and relative refractive-index differences of the core from 0.25 to 0.35% and of the cladding from 0 to -0.2% are investigated experimentally with respect to the LP₁₁ mode cutoff properties, the mode-field diameter, the dispersion, and the bending attenuation. Numerical calculations based on actual profile parameters of the bend loss and the mode-field diameter agree well with experiment. It is demonstrated that low attenuation and bend losses independent of b/a can be obtained if the cutoff wavelength is kept constant by adjusting the core radius, the core index or the index of the first 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.     

Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings

Cutoff characteristics are calculated for the fundamental mode in a single-mode double-clad lightguide structure whose refractive index in the inner cladding is less than the index of the outer cladding. Results of this study indicate how to choose the proper depressed cladding width, depth in order to reduce long-wavelength losses that have been observed in experimental MCVD fibers with fluorine-doped phosphosilicate claddings.     

Analytical expression for the coupling between two optical fibercores with α-power refractive-index distribution

The authors derive closed-form expression for the coupling coefficient and the butt coupling overlap integral for the coupling between two linearly polarized modes of any order, each propagating an α-power refractive-index optical fiber with uniform cladding. To show the usefulness of these formulas, they present numerical results for the coupling between two identical single-mode fibers and clearly demonstrate the variations of the coupling properties as the fiber structure changes. These results are useful when performing coupled-mode analysis of couplers formed between two fibers with the assumed index profiles. The authors also apply the obtained general formulas to evaluating the coupling characteristics between two identical single-mode fibers for various core index distributions     

Radiating leaky-mode losses in single-mode lightguides with depressed-index claddings

Cutoff characteristics are calculated for the fundamental mode in a single-mode double-clad lightguide structure whose refractive index in the inner cladding is less than the index of the outer cladding. Results of this study indicate how to choose the proper depressed cladding width and depth in order to reduce long-wavelength losses that have been observed in experimental MCVD fibers with fluorine-doped phosphosilicate claddings.     

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     

Automatic geometric dimension measurements of single-mode fiberpreforms using a novel shadow method

A method is described in which a test preform is immersed in an index-matching liquid so as to detect optical signals from the core/cladding and cladding/index-matching liquid interfaces simultaneously. A light-beam sweeper with a mirror is used to sweep a He-Ne laser beam across a fiber preform, and the time intervals during which the preform intercepts the beam and casts a composite shadow on a photodetector are measured. The method is simple and accurate and has sufficient spatial resolution (better than 10 μm) to obtain detailed measurements of core eccentricity, core and cladding diameters and their ratio, and ellipticity in a step-index single-mode fiber preform for a subscriber line. On the basis of this principle, an automated geometry measurement system has been developed. The system is attractive for quality control applications in mass-production lines of single-mode fibers