Very large mode area optical fiber with complex ring cores

A novel kind of single-mode large-mode-area optical fiber is presented in this paper. The proposed fiber core is composed of high-index central rod and the surrounding multilayer rings. The mode characteristics are discussed considering the fiber structure parameters. The calculation results show that the proposed fiber possesses extreme large mode area of 2975 μm² with single mode operation at the wavelength of 1.08 μm. Even larger mode area of the complex ring core fiber with single mode output can be achieved by coiling the fiber, due to the significant difference of bending loss between the fundamental mode and the higher-order transverse modes. Such fibers are expected to find applications in the field of fiber lasers and amplifiers.     

单模双包层光纤之间的耦合

本文求得了单模双包层光纤之间耦合系数精确的解析表达式.计算了上升内包层、匹配包层和凹陷内包层光纤耦合系数随归一化频率V的关系曲线.也给出了不同V值的耦合系数随归一化距离(D/α)的关系曲线.该公式不但能够计算x偏振模的耦合系数,而且也能计算y偏振模的耦合系数.它可用于分析折射率差较大的光纤之间能量耦合以及耦合器的偏振特性.     

Single mode optical fiber characterisation by the LP11 mode radiation pattern

We describe a method for determining the V-value from the far-field radiation pattern of dual mode optical fibers (this method is an extension of the work by Gambling et al. for the LP₁₁ modes). The method involves measurements of the maximum radiation angle and the angle of the first minimum. Curves are presented which can be used with dual mode fibers, determining the core diameter, the refractive index difference between core and cladding, as well as the V-value in the single mode regime for longer wavelength operation with the help of dispersion curves of the dopant materials.     

大芯径大数值孔径阶跃光纤传导模特征方程求解

导出了相邻阶第二类变型贝塞尔函数的比值在小宗量的逼近形式,利用Lentz—Thompson方法计算相邻阶第一类贝塞尔函数的比值在小宗量的值,计算精度为浮点数系统的最低有效位。从而解决了求解大芯径、大数值孔径阶跃光纤的特征方程时．贝塞尔函数溢出双精度浮点数表示范围的问题。分别对多模石英光纤和大芯径大数值孔径的聚合物光纤的传导模特征方程进行了求解,石英光纤的传导模特征值计算结果与Optiwave公司的软件一致;对于聚合物光纤,算法给出了所有模式的计算结果,其中模式角向序数小于70的计算结果与Optiwave公司的软件一致,而Optiwave公司的软件不能计算角向序数大于70的模式。     

Power of entanglement in quantum communication

A communication channel is a physical system that transfers information from one place to another. Examples of communication channels include wires, optical fibers, and chains of spins that propagate spin waves through a medium. This Letter shows that the power-limited communication capacity of a multimode optical fiber or a set of parallel spin chains can be enhanced by introducing nonlinear couplings between the modes or chains. In particular, M coupled, entangled modes can send M bits in the same time it takes a single mode to send a single bit, and in the same time it takes M uncoupled, unentangled modes to send sqrt[M] bits.     

All propagation modes of large-core multimode optical fibers with an arbitrary core profile

Plastic optical fibers are typical large-core multimode optical fibers. High-bandwidth graded-index plastic optical fibers, which support a great number of propagation modes compared with conventional silica-glass multimode optical fibers, were developed in the 1990s. However, because they support a great number of propagation modes, their modal analyses have been limited to the WKB analysis. We obtain all the propagation modes of a convex-index large-core multimode optical fiber by use of the finite-element method, which has a strong advantage for arbitrary core profiles.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

Loss measurements in graded-index compound-glass fibers

The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.     

双芯光子晶体光纤中的模式干涉

应用全矢量超格子叠加模型分析了双芯光子晶体光纤(PCF)的模式特征.双芯PCF的基模和二次模分别由偏振方向不同的两个偶模和两个奇模组成，讨论了这四个模式的模式电场奇偶性.基于对双芯PCF模式电场奇偶性的认识，讨论了光纤中的矢量模式干涉问题.分析表明，不同偏振态模式的干涉不会引起芯间的功率转移，双芯PCF的芯间功率耦合是由相同偏振模式的干涉引起的.在相同偏振模式干涉过程中，光功率在两个芯子中呈余弦振荡，光纤的两个芯子一般存在约π／2的相位差.还讨论了两个不同偏振方向的耦合系数与波长的关系. 关键词： 光子晶体光纤 双芯光纤 模式 干涉 耦合     

Bragg guiding of domainlike nonlinear modes and kink arrays in lower-index core structures

We introduce a novel class of stable nonlinear modes trapped in a lower-index film core sandwiched between two optical lattices, or in the cylindrical core of a radial lattice, imprinted in defocusing media. Such a family of nonlinear modes transforms into defect lattice solitons when the core width is sufficiently small or into an array of kinks when the width is large enough. We find that higher-order modes with multiple zeros inside the guiding core can be stable in one-dimensional settings.     

Investigation on characteristics of W-type fiber with an inner cladding made of negative refractive index materials

A doubly clad optical fiber with an inner cladding made of negative refractive index material was proposed and the modes distribution, dispersion and time delay were investigated by fully vectorial numerical method. The results indicate that HE₁₂ mode can be operated as a single mode, the cut-off frequency of modes shifts to low normalized frequency with increasing thickness of inner cladding made of negative index materials, large negative dispersion and time delay can be obtained and tailored by the inner cladding. These results provide theoretical basis for designing novel fibers or optical devices.     

Photonic crystal fiber with a flattened fundamental mode for the fiber lasers

The 1-hole-missing and 7-hole-missing photonic crystal fibers (PCFs) with flattened fundamental modes (FMs) are proposed by introducing a layer of up-doped silica into the core of the PCFs. The transverse mode competitions are compared between the 7-hole-missing PCF lasers with and without flattened-FMs. The numerical results show that the flattened-FM PCF lasers can support the single transverse mode operation, even for a large value of the ratio of air hole diameter to the spacing between holes (up to 0.53).     

传像光纤束单丝芯径大小对光纤像面全息图的影响

光纤像面全息可以获得较高衍射效率的全息图和再现较大的物像。文中分析了相干光激励下传像光纤束单丝芯径的大小对其输出光场的影响，以及输出光场中低阶模与高阶模对全息照相的不同作用，在此基础上，提出缩小传像束单丝芯径，增多单丝根数，可以提高光纤像面全息图衍射效率和分辨率的观点，并通过实验进行了验证。     

Design principles of multifiber resonant directional couplers with hollow Bragg fibers: example of a 3 x 3 coupler

When several hollow photonic crystal fibers (HPCFs) are placed in proximity to each other, radiation-induced interfiber coupling between their core guided modes is observed. Under certain conditions coupling between the core modes of two touching collinear fibers can have a resonant increase via excitation of a low-quality intermirror cavity resonant state. Such coupling, however, decreases dramatically within the first micrometer of intermirror separation. Moreover, when fibers are touching, in the frequency domain a large number of accidental degeneracies with fiber surface and mirror states complicate the design of a stable 2 x 2 coupler. To alleviate these problems we consider coupling among three hollow Bragg fibers. When placed in the vertices of an isosceles triangle, even for a finite separation between fibers, triangular interfiber cavity forms a high-quality resonator that can be tuned via additional structural elements to a particular frequency of interest. Interfiber surface states are suppressed by keeping the fiber separation finite, thus allowing stable coupling conditions in a 3 x 3 HPCF coupler configuration.     

Designing optical-fiber modulators by using magnetic fluids

To reduce interface loss between optical fibers and devices in telecommunication systems, the development of an optical-fiber-based device that can be fused directly with fibers is important. A novel optical modulator consisting of a bare fiber core surrounded by magnetic fluids instead of by a SiO2 cladding layer is proposed. Applying a magnetic field raises the refractive index of the magnetic fluid. Thus we can control the occurrence of total reflection at the interface between the fiber core and the magnetic fluid when light propagates along the fiber. As a result, the intensity of the outgoing light is modulated by variation in field strength. Details of the design, fabrication, and working properties of such a modulator are presented.     

Large-mode-area fused-fiber combiners, with nearly lowest-mode brightness conservation

We show the feasibility of producing a low-mode all-fiber combiner fabricated from a large core and extremely small NA fibers. Although these fibers support multiple modes, the combiner that we produce can be operated nearly at the single mode regime while preserving the brightness of the combined beam almost perfectly with respect to the inputs. The M-square parameter of the combined beam was 2.3 and the power transfer efficiency was close to 100%. Such an all-fiber beam combining device is a rugged solution for high-brightness, high-efficiency beam delivery.     

Analysis of air-guiding photonic bandgap fibers

We present what is to our knowledge the first theoretical analysis of air-guiding photonic bandgap fibers. The fibers are characterized by a large hollow core and a microstructured cladding exhibiting photonic bandgap effects. Using an efficient, full-vectorial numerical method, we explain the operational principle of the fibers and obtain detailed information about the properties of the air-guided modes. This information includes accurate determination of the modes' spectral extent, cutoff properties, and mode-field distributions.     

Waveguide modes of hollow photonic-crystal fibers

Waveguide modes of microstructure fibers with a hollow core and a two-dimensionally periodic cladding are studied experimentally and theoretically. The spectrum of modes guided in the hollow core of these fibers displays isolated maxima, indicating that waveguiding is supported due to the high reflectivity of the fiber cladding within photonic band gaps. The main properties of the spectrum of modes guided in a hollow core of a photonic-crystal fiber and radiation intensity distribution in these modes are qualitatively explained in terms of the model of a periodic coaxial waveguide.     

热光系数与长周期光纤光栅的温度灵敏度研究

利用受温度影响的光纤的本征方程和相位匹配条件,从理论上研究了长周期光纤光栅(LPFGs)的温度响应特性,给出了LPFGs的温度灵敏度的解析表达式。对利用低模序包层模的LPFG进行了实验研究。结果表明,利用不同包层模的LPFGs具有不同的温度灵敏度。分析了光纤的材料热光系数和模的热光系数的差别。单模光纤导模的热光系数接近纤芯的材料热光系数,而包层模的热光系数比包层的材料热光系数大,模序越大,其值越大。适当调整纤芯和包层的热光系数,并选用不同的包层模,可以得到对温度灵敏或不灵敏的LPFGs。     

多孔芯光子晶体光纤及其偏振特性

强激光与气体的长距离相互作用能产生许多新奇的物理效应,而自由空间光束的自聚焦、衍射、散射等问题限制了该科技领域的发展。本文提出了一种新型多孔芯光子晶体光纤,纤芯亚波长、低折射率空气孔可以传光,具有宽带、低损耗、单模传输特性。利用倏逝波耦合效应,研究了纤芯亚波长空气孔束缚光的原理。根据光波传输的电磁场理论,分析了低折射率空气孔中的光强增大效应。强光在空气孔中长距离传输,为光与物质的相互作用提供了新条件,可以用于气体传感、非线性光学、高集成光子技术、原子操控等。由于纤芯空气孔可以传光,改变空气孔的大小,直接影响模场分布,进而可以获得很高的结构双折射。通过光纤结构参数的合理设计,分别获得了B=4×10^-2的高双折射、纤芯直径5 μm的大模场高双折射、大模面积单偏振单模特性,在光纤偏振器、光纤滤波器、光开关及光纤传感等领域有广泛的应用前景,为新型光场调控提供了新方法。