MacRobends of bow-tie fibres with constant curvature

The letter treats macrobends of bow-tie fibres with constant curvature, using our previously published work as the theoretical building block. In the practical case of unequal core and cladding anisotropy, there exists a single-polarisation window which shifts towards the shorter-wavelength range as the bend radius decreases. Also treated theoretically is the extinction ratio against bend orientation. Unexpectedly, it is found that, if equal core and cladding anisotropy are assumed in the theoretical analysis, then, unlike the ideal case of a straight anisotropic fibre, the differential losses virtually disappear in a bent bow-tie fibre with constant curvature.     

General solutions for stress-induced polarization in optical fibers

General solutions of thermal stresses and material birefringence in polarization-maintaining optical fibers are presented. The solutions give the stresses and birefringence at every point in the cross section of various fibers. The fundamental solution of the stress field is constructed by using the complex variable method in elasticity. When the cores, inner claddings, or stress-inducing regions are ellipses, circles, or bow-tie shapes, exact closed-form solutions can be found. The average core birefringence and center core birefringence of elliptical core, elliptical cladding, twin core, Panda, and bow-tie fibers are calculated and compared. Among these fibers, the bow-tie fiber shows the highest birefringence     

Design consideration for digital systems using multimode fibre with lossy cladding

Digital systems using multimode fibres with a step-index profile, a lossy cladding and a low-loss core are treated in the letter. The influence of fibre numerical aperture (n.a.) on repeater spacing is examined when the total core-cladding interface loss results from Fresnel reflection. It is shown, by way of example, that in this case there is no advantage in using low n.a. values, even at moderate modulation rates, owing to a diminishing receiver incident optical power with decreasing n.a.     

Si基SiO2波导芯层热应力的理论研究

结合弹性多层板热应力理论和应力集中效应给出了Si基SiO2波导芯层热应力的解析解,推导了芯层应力差的解析表达式.说明对于传统阵列波导光栅,芯层应力差来源于初始翘曲和波导各层热膨胀系数差;系统分析了波导各层材料对芯层应力差的影响,指出调节衬底的热膨胀系数、上包层的热膨胀系数、衬底的厚度和下包层的厚度都可以消除芯层应力差,但改变衬底和上包层热膨胀系数是调节芯层应力差的主要手段;讨论了几种常见金属应力板对芯层应力差的影响.结果表明,在阵列波导底部高温粘贴适当厚度的金属板可消除芯层应力差.     

Air guiding with honeycomb photonic bandgap fiber

We notice that honeycomb cladding was overlooked for air-guiding photonic bandgap fiber design. Air-silica honeycomb cladding is shown in this letter to be able to guide light in a slightly large-sized hollow core, with a reasonably wide bandwidth (/spl sim/200 nm). Noncircular air-hole shape is also explored for air-guiding honeycomb fiber design.     

领结型和椭圆型保偏光纤的应力和模式分析

应用有限元方法对领结型和椭圆型保偏光纤的热应力双折射特性和模式双折射进行了分析,给出了这两种保偏光纤的应力和双折射分布规律.对领结型保偏光纤,研究了纤芯的双折射和模式双折射随着应力区离纤芯距离的变化规律;对椭圆型保偏光纤,研究了纤芯的应力双折射和模式双折射随包层椭圆度的变化规律;分析了光纤应力区结构对纤芯双折射大小和均匀性的影响,并将应力诱导双折射与模式理论计算结果进行了比较.     

Analytical solution of bow-tie fibres

In the letter we attempt to treat theoretically the complicated profile structure of a polarising bow-tie fibre. The mathematical steps taken in our analysis include the use of a Fourier expansion of the index profile and an iterative technique to obtain the Green functions of the optical field. Numerical examples show that a bow-tie fibre with the usual profile structure has higher leaky losses than the predicted leaky losses of an anisotropic step-index model.     

非通视方位传递系统中保偏光纤分析

根据非通视方位传递系统对保偏光纤双折射的设计要求,重点分析了不同类型保偏光纤参数对双折射的影响。首先,从光纤应力应变、变分原理和应力光弹效应出发,推导建立保偏光纤应力光学耦合关系;然后,借助有限元分析软件研究了不同因素对保偏光纤双折射值的影响,并对两种保偏光纤（熊猫型保偏光纤和领结型保偏光纤）进行了对比分析。结果表明:在纤芯中心附近,减小纤芯与应力区的距离、增大应力区半径大小、固定纤芯尺寸时增大包层半径、提高保偏光纤参考温度等措施都能明显提高双折射值。同等条件下领结型保偏光纤产生的双折射值更大。研究成果能够为非通视方位传递系统中保偏光纤的设计选型提供一定的参考。     

Propagation characteristics of an optical waveguide with a diffused core boundary

When a unimode glass-fibre optical waveguide is produced by the double-crucible process, diffusion between core and cladding glasses is known to occur. The refractive-index profiles are determined theoretically in the letter for different rates of diffusion, under the assumption of infinite cladding radius. Subsequently the propagation characteristics of the dominant HE11 mode have been computed by means of a stratification approximation to the continuous refractive-index profile. Only at high diffusion rates is the predicted increase in electromagnetic dispersion unacceptable.     

Linearly birefringent fibres with zero polarisation-mode dispersion

It is found that in a single-mode optical fibre, if the material anisotropy in the cladding can be made substantially larger than that in the core, the group delays of the two orthogonally polarised modes can be equalised at some particular frequencies. Numerical results are presented for circular and elliptical fibres.     

A Highly Sensitive Fiber-Optic Refractive Index Sensor Based on an Edge-Written Long-Period Fiber Grating

A novel long-period fiber grating (LPFG) is edge-written without any destructive damage on the fiber based on the thermal shock and rapid cooling effects of the high-frequency CO₂ laser pulses exposure method in this letter. The refractive index disturbance induced by high-frequency CO₂ laser pulses mainly occurs in the edge region of the fiber cladding rather than in the fiber core. An edge-written LPFG with a resonant peak of ~18 dB is obtained using a standard telecommunication fiber. The experimental results show that such a novel LPFG structure has much higher refractive index sensitivity over conventional LPFGs side-written, which can be used as a refractive index sensor, or a tunable filter and modulator by controlling the refractive index sensitive film which is coated on the cladding of such an LPFG.     

国产化大模场掺镱光子晶体光纤的设计与制备

文章作者设计了大模场双包层光子晶体光纤(PCF)的波导结构,并采用自主知识产权的专利技术,在国内首次制备出了高数值孔径大模场掺镱双包层光子晶体光纤.该光子晶体光纤的内包层数值孔径为0.65,纤芯数值孔径为0.06,有效模场面积为1465.7/μm2     

Analytical solutions for thermal stresses in buried channel waveguides

Thermal stresses are important factors in determining the birefringence of optical waveguides. Analytical solutions are developed to estimate the thermal stresses in the core of a buried rectangular channel waveguide. The closed-form solutions for various situations are obtained. The results show that the thermal expansion mismatch between the upper cladding and substrate plays significant role in determining the anisotropy of the thermal stresses in the core. The analytical solutions developed in this paper are consistent with finite element simulations and experimental results.     

Micro-structured fiber Bragg gratings. Part I: Spectral characteristics

In this paper, the authors present a detailed theoretical and numerical analysis of an in-fiber photonic device based on micro-structured fiber Bragg gratings for sensing and telecommunication applications. The investigated structure consists in a fiber Bragg grating with a localized stripping of the cladding layer. The thinning of the cladding layer changes the core propagation features and thus it induces a phase delay on the guided mode, leading to the formation of a defect state inside the original grating band-gap in agreement with the phase-shift grating theory. The behaviour of the defect state is ruled by three main parameters identified in the length and depth of the stripped region and the surrounding refractive index. This special characteristic leads to the possibility to realize a new class of active and passive photonic devices. In particular, here, the theoretical and numerical analysis of the spectral behaviour exhibited by the device and its dependence on the perturbation features have been carried out. Particular emphasis has been focused on the effect of the SRI on the spectral response, providing an exhaustive analysis of the potentiality of the device both for sensing and telecommunication applications. In addition, the investigation of standard uniform and strong fiber Bragg gratings has been carried out. Finally, the case of real devices realized by wet chemical etching in hydrofluoric acid solutions was analysed and a spectral equivalence was found to extend the design rules identified for the ideal case.     

Ultrahigh Birefringent Photonic Crystal Fiber With Ultralow Confinement Loss

A photonic crystal fiber (PCF) with circular air holes in the fiber cladding and elliptical air holes in the fiber core is proposed. According to calculation, both ultrahigh birefringence (larger than 0.01) and ultralow confinement loss (less than 0.001dB/km) can be achieved simultaneously over a large wavelength range for a PCF with only four rings of circular air holes in the fiber cladding. The confinement loss in this PCF can be effectively reduced while the birefringence almost remains the same. The proposed design of the PCF is a solution to the tradeoff between the birefringence and the confinement loss for the originally reported highly birefringent elliptical-hole PCF. Moreover, an approach to modify the effective index of fiber core is also suggested in this letter     

Acousto-optic-induced cladding-mode reflection in a blazed-superstructure fiber grating

The coupling phenomenon between the cladding mode and the core mode in a blazed-superstructure fiber grating is experimentally demonstrated in this letter by applying both transverse and longitudinal acoustic waves. This is based on the difference between the cladding-mode wave vector and the core-mode wave vector, which is equivalent to the acoustic wave vector. By this means, a reflectivity-tunable multiwavelength switchable comb filter would be developed for the applications in fiber optics.     

Explicit birefringence analysis for anisotropic fibers

A hyper-perturbation approach is used to find the Taylor expansion formulas for the dispersion curves of arbitrarily indexed optical fibers. These formulas are then used to give analytically the birefringence characteristics of optical fibers with biaxially anisotropic core and cladding. The dependence of the fiber birefringence on the index difference, index anisotropy, and the normalized frequency is explicitly determined. Simple, accurate analytic formulas have been presented for anisotropic fibers with step- and parabolic-index profiles. Numerical results are in good agreement with the literature results     

Guided optical waves in fibers with negative dielectric constant

We classify and discuss all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber. The cases of a metallic core with a dielectric cladding and a dielectric core with a metallic cladding are considered. In each case, the intensity profile of the allowed guided modes is shown. The geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers     

AMM cladding fiber for coupled plasmonic propagation and core guidance

Dual effect of surface plasmon propagation and classical core guidance is made possible in this work with the integration of metamaterial in a fiber. Here, the alternate layers of \(\hbox {TiO}_{2}\) and Ag form anisotropic metamaterial that helps in plasmonic propagation. The designed fiber with the proposed metamaterial as a cladding supports both normal core guidance and surface guidance at different wavelength based on their anisotropy. Parameters such as dispersion and confinement loss are analyzed at different wavelength. Simulation results are obtained to confirm the proper propagation of modes.     

Acoustooptic Coherent Mode Coupling in Polarization-Maintaining Fiber and Its Application as a Variable-Polarization-Dependent Loss Element

In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP₀₁ core mode to LP_1m cladding mode of a polarization-maintaining fiber (PMF) induced by two acoustic gratings. We show narrowband variable attenuation along either birefringent axis of the PMF by varying the relative phase between the two acoustic gratings. By launching two acoustic gratings at different frequencies, variable attenuation and low crosstalk along either birefringent axis within the acoustooptic coupling bandwidth are demonstrated